Description

Checks that array initialization contains a trailing comma.

int[] a = new int[]
{
  1,
  2,
  3,
};
        

By default, the check demands a comma at the end if neither left nor right curly braces are on the same line as the last element of the array.

return new int[] { 0 };
return new int[] { 0
  };
return new int[] {
  0 };
        

Rationale: Putting this comma in makes it easier to change the order of the elements or add new elements on the end. Main benefit of a trailing comma is that when you add new entry to an array, no surrounding lines are changed.

{
  100000000000000000000,
  200000000000000000000, // OK
}

{
  100000000000000000000,
  200000000000000000000,
  300000000000000000000,  // Just this line added, no other changes
}
        

If closing brace is on the same line as trailing comma, this benefit is gone (as the check does not demand a certain location of curly braces the following two cases will not produce a violation):

{100000000000000000000,
 200000000000000000000,} // Trailing comma not needed, line needs to be modified anyway

{100000000000000000000,
 200000000000000000000, // Modified line
 300000000000000000000,} // Added line
        

If opening brace is on the same line as trailing comma there's also (more arguable) problem:

{100000000000000000000, // Line cannot be just duplicated to slightly modify entry
}

{100000000000000000000,
 100000000000000000001, // More work needed to duplicate
}
        

Properties

Examples

To configure the check:

<module name="ArrayTrailingComma"/>
        

Which results in the following violations:

int[] numbers = {1, 2, 3};        //no violation
boolean[] bools = {
true,
true,
false
};        //violation

String[][] text = {{},{},};        //no violation

double[][] decimals = {
{0.5, 2.3, 1.1,},        //no violation
{1.7, 1.9, 0.6},
{0.8, 7.4, 6.5}
};        // violation as previous line misses a comma

char[] chars = {'a', 'b', 'c'
  };        / /no violation

String[] letters = {
  "a", "b", "c"};        // no violation

int[] a1 = new int[]{
  1,
  2
  ,
};        // no violation

int[] a2 = new int[]{
  1,
  2
  ,};        // no violation
        

To configure check to always validate trailing comma:

<module name="ArrayTrailingComma">
    <property name="alwaysDemandTrailingComma" value="true"/>
</module>
        

Example:

int[] numbers = {1, 2, 3}; // violation
boolean[] bools = {
true,
true,
false // violation
};

String[][] text = {{},{},}; // OK

double[][] decimals = {
{0.5, 2.3, 1.1,}, // OK
{1.7, 1.9, 0.6}, // violation
{0.8, 7.4, 6.5} // violation
}; // violation, previous line misses a comma

char[] chars = {'a', 'b', 'c'  // violation
  };

String[] letters = {
  "a", "b", "c"}; // violation

int[] a1 = new int[]{
  1,
  2
  ,
}; // OK

int[] a2 = new int[]{
  1,
  2
  ,}; // OK
        

Example of Usage

• Checkstyle Style

Violation Messages

• array.trailing.comma

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Detects double brace initialization.

Rationale: Double brace initialization (set of Instance Initializers in class body) may look cool, but it is considered as anti-pattern and should be avoided. This is also can lead to a hard-to-detect memory leak, if the anonymous class instance is returned outside and other object(s) hold reference to it. Created anonymous class is not static, it holds an implicit reference to the outer class instance. See this blog post and article for more details. Check ignores any comments and semicolons in class body.

Examples

To configure the check:

<module name="AvoidDoubleBraceInitialization"/>
        

Which results in the following violations:

class MyClass {
    List<Integer> list1 = new ArrayList<>() { // violation
        {
            add(1);
        }
    };
    List<String> list2 = new ArrayList<>() { // violation
        ;
        // comments and semicolons are ignored
        {
            add("foo");
        }
    };
}
        

Check only looks for double brace initialization and it ignores cases where the anonymous class has fields or methods. Though these might create the same memory issues as double brace, the extra class members can produce side effects if changed incorrectly.

class MyClass {
    List<Object> list = new ArrayList<>() { // OK, not pure double brace pattern
        private int field;
        {
            add(new Object());
        }
    };
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• avoid.double.brace.init

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Detects inline conditionals. Here is one example of an inline conditional:

String a = getParameter("a");
String b = (a==null || a.length()<1) ? null : a.substring(1);
        

Rationale: Some developers find inline conditionals hard to read, so their employer's coding standards forbid them.

Examples

To configure the check:

<module name="AvoidInlineConditionals"/>
        

Example:

int x = 5;
boolean foobar = (x == 5); // OK

String text;
text = (text == null) ? "" : text; // violation

String b;
if (a != null && a.length() >= 1) { // OK
    b = a.substring(1);
} else {
    b = null;
}

b = (a != null && a.length() >= 1) ? a.substring(1) : null; // violation
        

Example of Usage

• Checkstyle Style

Violation Messages

• inline.conditional.avoid

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks if call to superclass constructor without arguments is present. Such invocation is redundant because constructor body implicitly begins with a superclass constructor invocation super(); See specification for detailed information.

Examples

To configure the check:

<module name="AvoidNoArgumentSuperConstructorCall"/>
        

Example of violations

class MyClass extends SomeOtherClass {
    MyClass() {
        super(); // violation
    }

    MyClass(int arg) {
        super(arg); // OK, call with argument have to be explicit
    }

    MyClass(long arg) {
        // OK, call is implicit
    }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• super.constructor.call

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that classes and records which define a covariant equals() method also override method equals(Object).

Covariant equals() - method that is similar to equals(Object), but with a covariant parameter type (any subtype of Object).

Notice: the enums are also checked, even though they cannot override equals(Object). The reason is to point out that implementing equals() in enums is considered an awful practice: it may cause having two different enum values that are equal using covariant enum method, and not equal when compared normally.

Inspired by Finding Bugs is Easy, chapter '4.5 Bad Covariant Definition of Equals (Eq)':

Java classes and records may override the equals(Object) method to define a predicate for object equality. This method is used by many of the Java runtime library classes; for example, to implement generic containers.

Programmers sometimes mistakenly use the type of their class Foo as the type of the parameter to equals():

public boolean equals(Foo obj) {...}
        

This covariant version of equals() does not override the version in the Object class, and it may lead to unexpected behavior at runtime, especially if the class is used with one of the standard collection classes which expect that the standard equals(Object) method is overridden.

This kind of bug is not obvious because it looks correct, and in circumstances where the class is accessed through the references of the class type (rather than a supertype), it will work correctly. However, the first time it is used in a container, the behavior might be mysterious. For these reasons, this type of bug can elude testing and code inspections.

Examples

To configure the check:

<module name="CovariantEquals"/>
        

For example:

class Test {
  public boolean equals(Test i) {  // violation
    return false;
  }
}
        

The same class without violations:

class Test {
  public boolean equals(Test i) {  // no violation
    return false;
  }

  public boolean equals(Object i) {
    return false;
  }
}
        

Another example:

record Test(String str) {
    public boolean equals(Test r) {  // violation
        return false;
    }
}
        

The same record without violations:

record Test(String str) {
    public boolean equals(Test r) {  // no violation
        return false;
    }

    public boolean equals(Object r) {
        return false;
    }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• covariant.equals

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that the parts of a class, record, or interface declaration appear in the order suggested by the Code Conventions for the Java Programming Language.

According to Code Conventions for the Java Programming Language , the parts of a class or interface declaration should appear in the following order:

1. Class (static) variables. First the public class variables, then protected, then package level (no access modifier), and then private.
2. Instance variables. First the public class variables, then protected, then package level (no access modifier), and then private.
3. Constructors
4. Methods

Purpose of ignore* option is to ignore related violations, however it still impacts on other class members.

ATTENTION: the check skips class fields which have forward references from validation due to the fact that we have Checkstyle's limitations to clearly detect user intention of fields location and grouping. For example:

public class A {
  private double x = 1.0;
  private double y = 2.0;
  public double slope = x / y; // will be skipped from validation due to forward reference
}
        

Properties

Examples

To configure the check:

<module name="DeclarationOrder"/>
        

With default options:

class K {
  int a;
  void m(){}
  K(){}  <-- "Constructor definition in wrong order"
  int b; <-- "Instance variable definition in wrong order"
}
        

With ignoreConstructors option:

class K {
  int a;
  void m(){}
  K(){}
  int b; <-- "Instance variable definition in wrong order"
}
        

With ignoreConstructors option and without a method definition in a source class:

class K {
  int a;
  K(){}
  int b; <-- "Instance variable definition in wrong order"
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• declaration.order.access
• declaration.order.constructor
• declaration.order.instance
• declaration.order.static

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Check that the default is after all the cases in a switch statement.

Rationale: Java allows default anywhere within the switch statement. But it is more readable if it comes after the last case.

Properties

Examples

To configure the check:

<module name="DefaultComesLast"/>
        

Example:

switch (i) {
  case 1:
    break;
  case 2:
    break;
  default: // OK
    break;
}

switch (i) {
  case 1:
    break;
  case 2:
    break; // OK, no default
}

switch (i) {
  case 1:
    break;
  default: // violation, 'default' before 'case'
    break;
  case 2:
    break;
}

switch (i) {
  case 1:
  default: // violation, 'default' before 'case'
    break;
  case 2:
    break;
}
        

To configure the check to allow default label to be not last if it is shared with case:

<module name="DefaultComesLast">
  <property name="skipIfLastAndSharedWithCase" value="true"/>
</module>
        

Example:

switch (i) {
  case 1:
    break;
  case 2:
  default: // OK
    break;
  case 3:
    break;
}

switch (i) {
  case 1:
    break;
  default: // violation
  case 2:
    break;
}

// Switch rules are not subject to fall through, so this is still a violation:
switch (i) {
case 1 -> x = 9;
default -> x = 10; // violation
case 2 -> x = 32;
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• default.comes.last
• default.comes.last.in.casegroup

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Detects empty statements (standalone ";" semicolon). Empty statements often introduce bugs that are hard to spot

Examples

To configure the check:

<module name="EmptyStatement"/>
        

Example:

public void foo() {
  int i = 5;
  if (i > 3); // violation, ";" right after if statement
    i++;
  for (i = 0; i < 5; i++); // violation
    i++;
  while (i > 10) // OK
    i++;
}
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• empty.statement

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that any combination of String literals is on the left side of an equals() comparison. Also checks for String literals assigned to some field (such as someString.equals(anotherString = "text")).

Rationale: Calling the equals() method on String literals will avoid a potential NullPointerException. Also, it is pretty common to see null checks right before equals comparisons but following this rule such checks are not required.

Properties

Examples

To configure the check:

<module name="EqualsAvoidNull"/>
        

Example:

String nullString = null;
nullString.equals("My_Sweet_String");            // violation
"My_Sweet_String".equals(nullString);            // OK
nullString.equalsIgnoreCase("My_Sweet_String");  // violation
"My_Sweet_String".equalsIgnoreCase(nullString);  // OK
        

To configure the check to allow ignoreEqualsIgnoreCase:

<module name="EqualsAvoidNull">
  <property name="ignoreEqualsIgnoreCase" value="true"/>
</module>
        

Example:

String nullString = null;
nullString.equals("My_Sweet_String");            // violation
"My_Sweet_String".equals(nullString);            // OK
nullString.equalsIgnoreCase("My_Sweet_String");  // OK
"My_Sweet_String".equalsIgnoreCase(nullString);  // OK
        

Example of Usage

• Checkstyle Style

Violation Messages

• equals.avoid.null
• equalsIgnoreCase.avoid.null

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that classes that either override equals() or hashCode() also overrides the other. This check only verifies that the method declarations match Object.equals(Object) and Object.hashCode() exactly to be considered an override. This check does not verify invalid method names, parameters other than Object, or anything else.

Rationale: The contract of equals() and hashCode() requires that equal objects have the same hashCode. Therefore, whenever you override equals() you must override hashCode() to ensure that your class can be used in hash-based collections.

Examples

To configure the check:

<module name="EqualsHashCode"/>
        

Example:

public static class Example1 {
    public int hashCode() {
        // code
    }
    public boolean equals(String o) { // violation, overloaded implementation of 'equals'
        // code
    }
}
public static class Example2 {
    public boolean equals(Object o) { // violation, no 'hashCode'
        // code
    }
    public boolean equals(String o) {
        // code
    }
}
public static class Example3 {
    public int hashCode() {
        // code
    }
    public boolean equals(Object o) { // OK
        // code
    }
    public boolean equals(String o) {
        // code
    }
}
public static class Example4 {
    public int hashCode() {
        // code
    }
    public boolean equals(java.lang.Object o) { // OK
        // code
   }
}
public static class Example5 {
    public static int hashCode(int i) {
        // code
    }
    public boolean equals(Object o) { // violation, overloaded implementation of 'hashCode'
        // code
    }
}
public static class Example6 {
    public int hashCode() { // violation, overloaded implementation of 'equals'
        // code
    }
    public static boolean equals(Object o, Object o2) {
        // code
    }
}
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• equals.noEquals
• equals.noHashCode

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks if any class or object member is explicitly initialized to default for its type value (null for object references, zero for numeric types and char and false for boolean.

Rationale: Each instance variable gets initialized twice, to the same value. Java initializes each instance variable to its default value (0 or null) before performing any initialization specified in the code. So there is a minor inefficiency.

Properties

Examples

To configure the check:

<module name="ExplicitInitialization"/>
        

Example:

public class Test {
  private int intField1 = 0; // violation
  private int intField2 = 1;
  private int intField3;

  private char charField1 = '\0'; // violation
  private char charField2 = 'b';
  private char charField3;

  private boolean boolField1 = false; // violation
  private boolean boolField2 = true;
  private boolean boolField3;

  private Obj objField1 = null; // violation
  private Obj objField2 = new Obj();
  private Obj objField3;

  private int arrField1[] = null; // violation
  private int arrField2[] = new int[10];
  private int arrField3[];
}
        

To configure the check so that it only checks for objects that explicitly initialize to null:

<module name="ExplicitInitialization">
  <property name="onlyObjectReferences" value="true"/>
</module>
        

Example:

public class Test {
  private int intField1 = 0; // ignored
  private int intField2 = 1;
  private int intField3;

  private char charField1 = '\0'; // ignored
  private char charField2 = 'b';
  private char charField3;

  private boolean boolField1 = false; // ignored
  private boolean boolField2 = true;
  private boolean boolField3;

  private Obj objField1 = null; // violation
  private Obj objField2 = new Obj();
  private Obj objField3;

  private int arrField1[] = null; // violation
  private int arrField2[] = new int[10];
  private int arrField3[];
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• explicit.init

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks for fall-through in switch statements. Finds locations where a case contains Java code but lacks a break, return, throw or continue statement.

The check honors special comments to suppress the warning. By default the texts "fallthru", "fall thru", "fall-thru", "fallthrough", "fall through", "fall-through" "fallsthrough", "falls through", "falls-through" (case sensitive). The comment containing these words must be all on one line, and must be on the last non-empty line before the case triggering the warning or on the same line before the case (ugly, but possible).

Note: The check assumes that there is no unreachable code in the case.

Properties

Examples

To configure the check:

<module name="FallThrough"/>
        

Example:

public void foo() throws Exception {
  int i = 0;
  while (i >= 0) {
    switch (i) {
      case 1:
        i++;
      case 2: // violation, previous case contains code but lacks
              // break, return, throw or continue statement
        i++;
        break;
      case 3: // OK
        i++;
        return;
      case 4: // OK
        i++;
        throw new Exception();
      case 5: // OK
        i++;
        continue;
      case 6: // OK
      case 7: // Previous case: OK, case does not contain code
              // This case: OK, by default the last case might not have statement
              // that transfer control
        i++;

    }
  }
}
        

Example how to suppress violations by comment:

switch (i) {
  case 1:
    i++; // fall through

  case 2: // OK
    i++;
    // fallthru
  case 3: { // OK
    i++;
  }
  /* fall-thru */
  case 4: // OK
    i++;
    // Fallthru
  case 5: // violation, "Fallthru" in case 4 should be "fallthru"
    i++;
    // fall through
    i++;
  case 6: // violation, the comment must be on the last non-empty line before 'case'
    i++;
  /* fall through */case 7: // OK, comment can appear on the same line but before 'case'
    i++;
}
        

To configure the check to enable check for last case group:

<module name="FallThrough">
   <property name="checkLastCaseGroup" value="true"/>
</module>
        

Example:

switch (i) {
  case 1:
    break;
  case 2: // Previous case: OK
          // This case: violation, last case must have statement that transfer control
    i++;
}
        

To configure the check with custom relief pattern:

<module name="FallThrough">
   <property name="reliefPattern" value="FALL?[ -]?THROUGH"/>
</module>
        

Example:

switch (i) {
  case 1:
    i++;
    // FALL-THROUGH
  case 2: // OK, "FALL-THROUGH" matches the regular expression "FALL?[ -]?THROUGH"
    i++;
    // fall-through
  case 3: // violation, "fall-through" doesn't match
    break;
}
        

Example of Usage

• Google Style
• Checkstyle Style

Violation Messages

• fall.through
• fall.through.last

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that local variables that never have their values changed are declared final. The check can be configured to also check that unchanged parameters are declared final.

Notes

When configured to check parameters, the check ignores parameters of interface methods and abstract methods.

Properties

Examples

To configure the check:

<module name="FinalLocalVariable"/>
        

To configure the check so that it checks local variables and parameters:

<module name="FinalLocalVariable">
  <property name="tokens" value="VARIABLE_DEF,PARAMETER_DEF"/>
</module>
        

By default, this Check skip final validation on Enhanced For-Loop.

Option 'validateEnhancedForLoopVariable' could be used to make Check to validate even variable from Enhanced For Loop.

An example of how to configure the check so that it also validates enhanced For Loop Variable is:

<module name="FinalLocalVariable">
  <property name="tokens" value="VARIABLE_DEF"/>
  <property name="validateEnhancedForLoopVariable" value="true"/>
</module>
        

Example:

for (int number : myNumbers) { // violation
  System.out.println(number);
}
        

An example of how to configure check on local variables and parameters but do not validate loop variables:

<module name="FinalLocalVariable">
  <property name="tokens" value="VARIABLE_DEF,PARAMETER_DEF"/>
  <property name="validateEnhancedForLoopVariable" value="false"/>
</module>
        

Example:

public class MyClass {
  static int foo(int x, int y) { //violations, parameters should be final
    return x+y;
  }
  public static void main (String []args) { //violation, parameters should be final
    for (String i : args) {
      System.out.println(i);
    }
    int result=foo(1,2); // violation
  }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• final.variable

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that a local variable or a parameter does not shadow a field that is defined in the same class.

Notes

It is possible to configure the check to ignore all property setter methods.

A method is recognized as a setter if it is in the following form

${returnType} set${Name}(${anyType} ${name}) { ... }
        

where ${anyType} is any primitive type, class or interface name; ${name} is name of the variable that is being set and ${Name} its capitalized form that appears in the method name. By default it is expected that setter returns void, i.e. ${returnType} is 'void'. For example

void setTime(long time) { ... }
        

Any other return types will not let method match a setter pattern. However, by setting setterCanReturnItsClass property to true definition of a setter is expanded, so that setter return type can also be a class in which setter is declared. For example

class PageBuilder {
  PageBuilder setName(String name) { ... }
}
        

Such methods are known as chain-setters and a common when Builder-pattern is used. Property setterCanReturnItsClass has effect only if ignoreSetter is set to true.

Properties

Examples

To configure the check:

<module name="HiddenField"/>
        

To configure the check so that it checks local variables but not parameters:

<module name="HiddenField">
  <property name="tokens" value="VARIABLE_DEF"/>
</module>
        

To configure the check so that it ignores the variables and parameters named "test":

<module name="HiddenField">
  <property name="ignoreFormat" value="^test$"/>
</module>
        

class SomeClass
{
  private List<String> test;

  private void addTest(List<String> test) // no violation
  {
    this.test.addAll(test);
  }

  private void foo()
  {
    final List<String> test = new ArrayList<>(); // no violation
    ...
  }
}
        

To configure the check so that it ignores constructor parameters:

<module name="HiddenField">
  <property name="ignoreConstructorParameter" value="true"/>
</module>
        

To configure the check so that it ignores the parameter of setter methods:

<module name="HiddenField">
  <property name="ignoreSetter" value="true"/>
</module>
        

To configure the check so that it ignores the parameter of setter methods recognizing setter as returning either void or a class in which it is declared:

<module name="HiddenField">
  <property name="ignoreSetter" value="true"/>
  <property name="setterCanReturnItsClass" value="true"/>
</module>
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• hidden.field

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that certain exception types do not appear in a catch statement.

Rationale: catching java.lang.Exception, java.lang.Error or java.lang.RuntimeException is almost never acceptable. Novice developers often simply catch Exception in an attempt to handle multiple exception classes. This unfortunately leads to code that inadvertently catches NullPointerException, OutOfMemoryError, etc.

Properties

Examples

To configure the check:

<module name="IllegalCatch"/>
        

Example:

try {
    // some code here
} catch (Exception e) { // violation

}

try {
    // some code here
} catch (ArithmeticException e) { // OK

} catch (Exception e) { // violation, catching Exception is illegal
                          and order of catch blocks doesn't matter
}

try {
    // some code here
} catch (ArithmeticException | Exception e) { // violation, catching Exception is illegal

}

try {
    // some code here
} catch (ArithmeticException e) { // OK

}
        

To configure the check to override the default list with ArithmeticException and OutOfMemoryError:

<module name="IllegalCatch">
  <property name="illegalClassNames" value="ArithmeticException,
              OutOfMemoryError"/>
</module>
        

Example:

try {
    // some code here
} catch (OutOfMemoryError e) { // violation

}

try {
    // some code here
} catch (ArithmeticException e) { // violation

}

try {
    // some code here
} catch (NullPointerException e) { // OK

} catch (OutOfMemoryError e) { // violation

}

try {
    // some code here
} catch (ArithmeticException | Exception e) {  // violation

}

try {
    // some code here
} catch (Exception e) { // OK

}
        

Example of Usage

• Checkstyle Style

Violation Messages

• illegal.catch

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks for illegal instantiations where a factory method is preferred.

Rationale: Depending on the project, for some classes it might be preferable to create instances through factory methods rather than calling the constructor.

A simple example is the java.lang.Boolean class. For performance reasons, it is preferable to use the predefined constants TRUE and FALSE. Constructor invocations should be replaced by calls to Boolean.valueOf().

Some extremely performance sensitive projects may require the use of factory methods for other classes as well, to enforce the usage of number caches or object pools.

Notes

There is a limitation that it is currently not possible to specify array classes.

Properties

Examples

To configure the check to find instantiations of java.lang.Boolean:

<module name="IllegalInstantiation">
  <property name="classes" value="java.lang.Boolean"/>
</module>
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• instantiation.avoid

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that specified types are not declared to be thrown. Declaring that a method throws java.lang.Error or java.lang.RuntimeException is almost never acceptable.

Properties

Examples

To configure the check:

<module name="IllegalThrows"/>
        

Example:

public class Test {
  public void func1() throws RuntimeException {} // violation
  public void func2() throws Exception {}  // ok
  public void func3() throws Error {}  // violation
  public void func4() throws Throwable {} // violation
  public void func5() throws NullPointerException {} // ok
  @Override
  public void toString() throws Error {} // ok
}
        

To configure the check rejecting throws NullPointerException from methods:

<module name="IllegalThrows">
  <property name="illegalClassNames" value="NullPointerException"/>
</module>
        

Example:

public class Test {
  public void func1() throws RuntimeException {} // ok
  public void func2() throws Exception {}  // ok
  public void func3() throws Error {}  // ok
  public void func4() throws Throwable {} // ok
  public void func5() throws NullPointerException {} // violation
  @Override
  public void toString() throws Error {} // ok
}
        

To configure the check ignoring method named "func1()":

<module name="IllegalThrows">
  <property name="ignoredMethodNames" value="func1"/>
</module>
        

Example:

public class Test {
  public void func1() throws RuntimeException {} // ok
  public void func2() throws Exception {}  // ok
  public void func3() throws Error {}  // violation
  public void func4() throws Throwable {} // violation
  public void func5() throws NullPointerException {} // ok
  @Override
  public void toString() throws Error {} // ok
}
        

To configure the check to warn on overridden methods:

<module name="IllegalThrows">
  <property name="ignoreOverriddenMethods" value="false"/>
</module>
        

Example:

public class Test {
  public void func1() throws RuntimeException {} // violation
  public void func2() throws Exception {}  // ok
  public void func3() throws Error {}  // violation
  public void func4() throws Throwable {} // violation
  public void func5() throws NullPointerException {} // ok
  @Override
  public void toString() throws Error {} // violation
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• illegal.throw

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks for illegal tokens. By default labels are prohibited.

Rationale: Certain language features can harm readability, lead to confusion or are not obvious to novice developers. Other features may be discouraged in certain frameworks, such as not having native methods in Enterprise JavaBeans components.

Properties

Examples

To configure the check:

<module name="IllegalToken"/>
        

Example:

public void myTest() {
    outer: // violation
    for (int i = 0; i < 5; i++) {
        if (i == 1) {
            break outer;
        }
    }
}
          

To configure the check to report violation on token LITERAL_NATIVE:

<module name="IllegalToken">
  <property name="tokens" value="LITERAL_NATIVE"/>
</module>
        

Example:

public native void myTest(); // violation
          

Example of Usage

• Checkstyle Style

Violation Messages

• illegal.token

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks specified tokens text for matching an illegal pattern. By default no tokens are specified.

Properties

Examples

To configure the check to forbid String literals containing "a href":

<module name="IllegalTokenText">
  <property name="tokens" value="STRING_LITERAL"/>
  <property name="format" value="a href"/>
</module>
        

Example:

public void myTest() {
    String test = "a href"; // violation
    String test2 = "A href"; // OK, case is sensitive
}
        

To configure the check to forbid String literals containing "a href" for the ignoreCase mode:

<module name="IllegalTokenText">
  <property name="tokens" value="STRING_LITERAL"/>
  <property name="format" value="a href"/>
  <property name="ignoreCase" value="true"/>
</module>
        

Example:

public void myTest() {
    String test = "a href"; // violation
    String test2 = "A href"; // violation, case is ignored
}
        

To configure the check to forbid string literal text blocks containing """:

<module name="IllegalTokenText">
    <property name="tokens" value="TEXT_BLOCK_CONTENT"/>
    <property name="format" value='"'/>
</module>
        

Example:

public void myTest() {
    final String quote = """
               \""""; // violation
}
        

To configure the check to forbid leading zeros in an integer literal, other than zero and a hex literal:

<module name="IllegalTokenText">
  <property name="tokens" value="NUM_INT,NUM_LONG"/>
  <property name="format" value="^0[^lx]"/>
  <property name="ignoreCase" value="true"/>
</module>
        

Example:

public void myTest() {
    int test1 = 0; // OK
    int test2 = 0x111; // OK
    int test3 = 0X111; // OK, case is ignored
    int test4 = 010; // violation
    long test5 = 0L; // OK
    long test6 = 010L; // violation
}
        

Example of Usage

• Google Style
• Checkstyle Style

Violation Messages

• illegal.token.text

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that particular classes or interfaces are never used.

Rationale: Helps reduce coupling on concrete classes.

For additional restriction of type usage see also: IllegalInstantiation, IllegalImport

Properties

Notes

It is possible to set illegal class names via short or canonical name. Specifying illegal type invokes analyzing imports and Check puts violations at corresponding declarations (of variables, methods or parameters). This helps to avoid ambiguous cases, e.g.: java.awt.List was set as illegal class name, then, code like:

import java.util.List;
...
List list; //No violation here
          

will be ok.

In most cases it's justified to put following classes to illegalClassNames:

• GregorianCalendar
• Hashtable
• ArrayList
• LinkedList
• Vector

as methods that are differ from interface methods are rarely used, so in most cases user will benefit from checking for them.

Examples

To configure the default check:

<module name="IllegalType"/>
        

public class Test extends TreeSet { // violation
  public <T extends java.util.HashSet> void method() { // violation

    LinkedHashMap<Integer, String> lhmap =
        new LinkedHashMap<Integer, String>(); // violation
    TreeMap<Integer, String> treemap = new TreeMap<Integer, String>(); // violation
    Test t; // OK
    HashMap<String, String> hmap; // violation
    Queue<Integer> intqueue; // OK

    java.lang.IllegalArgumentException illegalex; // OK
    java.util.TreeSet treeset; // violation
  }

}
        

To configure the Check so that particular tokens are checked:

<module name="IllegalType">
  <property name="tokens" value="METHOD_DEF"/>
</module>
        

public class Test extends TreeSet { // OK
  public <T extends java.util.HashSet> void method() { // violation

    LinkedHashMap<Integer, String> lhmap = new LinkedHashMap<Integer, String>(); // OK

    java.lang.IllegalArgumentException illegalex; // OK
    java.util.TreeSet treeset; // Ok
  }

  public <T extends java.util.HashSet> void typeParam(T t) {} // violation

  public void fullName(TreeSet a) {} // OK

}
        

To configure the Check so that it ignores function() methods:

<module name="IllegalType">
  <property name="ignoredMethodNames" value="function"/>
</module>
        

public class Test {
  public HashMap<String, String> function() { // OK
    // code
  }

  public HashMap<String, String> function1() { // violation
    // code
  }
}
        

To configure the Check so that it validates abstract class names:

<module name="IllegalType">
  <property name="validateAbstractClassNames" value="true"/>
  <property name="illegalAbstractClassNameFormat" value="Gitt"/>
</module>
        

class Test extends Gitter { // violation
}

class Test1 extends Github { // OK
}
        

To configure the Check so that it verifies only public, protected or static methods and fields:

<module name="IllegalType">
  <property name="memberModifiers" value="LITERAL_PUBLIC,
    LITERAL_PROTECTED, LITERAL_STATIC"/>
</module>
        

public class Test {
  public HashMap<String, String> function1() { // violation
    // code
  }

  private HashMap<String, String> function2() { // OK
    // code
  }

  protected HashMap<Integer, String> function3() { // violation
    // code
  }

  public static TreeMap<Integer, String> function4() { // violation
    // code
  }

}
        

To configure the check so that it verifies usage of types Boolean and Foo:

<module name="IllegalType">
  <property name="illegalClassNames" value="Boolean, Foo"/>
</module>
        

public class Test {

  public Set<Boolean> set; // violation
  public java.util.List<Map<Boolean, Foo>> list; // violation

  private void method(List<Foo> list, Boolean value) { // violation
    SomeType.<Boolean>foo(); // violation
    final Consumer<Foo> consumer = Foo<Boolean>::foo; // violation
  }

  public <T extends Boolean, U extends Serializable> void typeParam(T a) {} // violation

  public void fullName(java.util.ArrayList<? super Boolean> a) {} // violation

  public abstract Set<Boolean> shortName(Set<? super Boolean> a); // violation

  public Set<? extends Foo> typeArgument() { // violation
    return new TreeSet<Foo<Boolean>>();
  }

}
        

Example of Usage

• Checkstyle Style

Violation Messages

• illegal.type

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks for assignments in subexpressions, such as in String s = Integer.toString(i = 2);.

Rationale: With the exception of loop idioms, all assignments should occur in their own top-level statement to increase readability. With inner assignments like the one given above, it is difficult to see all places where a variable is set.

Note: Check allows usage of the popular assignments in loops:

String line;
while ((line = bufferedReader.readLine()) != null) { // OK
  // process the line
}

for (;(line = bufferedReader.readLine()) != null;) { // OK
  // process the line
}

do {
  // process the line
}
while ((line = bufferedReader.readLine()) != null); // OK
        

Assignment inside a condition is not a problem here, as the assignment is surrounded by an extra pair of parentheses. The comparison is != null and there is no chance that intention was to write line == reader.readLine().

Examples

To configure the check:

<module name="InnerAssignment"/>
        

Example:

class MyClass {

  void foo() {
    int a, b;
    a = b = 5; // violation, assignment to each variable should be in a separate statement
    a = b += 5; // violation

    a = 5; // OK
    b = 5; // OK
    a = 5; b = 5; // OK

    double myDouble;
    double[] doubleArray = new double[] {myDouble = 4.5, 15.5}; // violation

    String nameOne;
    List<String> myList = new ArrayList<String>();
    myList.add(nameOne = "tom"); // violation

    for (int k = 0; k < 10; k = k + 2) { // OK
      // some code
    }

    boolean someVal;
    if (someVal = true) { // violation
      // some code
    }

    while (someVal = false) {} // violation

    InputStream is = new FileInputStream("textFile.txt");
    while ((b = is.read()) != -1) { // OK, this is a common idiom
      // some code
    }
  }

  boolean testMethod() {
    boolean val;
    return val = true; // violation
  }
}
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• assignment.inner.avoid

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that there are no "magic numbers" where a magic number is a numeric literal that is not defined as a constant. By default, -1, 0, 1, and 2 are not considered to be magic numbers.

Constant definition is any variable/field that has 'final' modifier. It is fine to have one constant defining multiple numeric literals within one expression:

static final int SECONDS_PER_DAY = 24 * 60 * 60;
static final double SPECIAL_RATIO = 4.0 / 3.0;
static final double SPECIAL_SUM = 1 + Math.E;
static final double SPECIAL_DIFFERENCE = 4 - Math.PI;
static final Border STANDARD_BORDER = BorderFactory.createEmptyBorder(3, 3, 3, 3);
static final Integer ANSWER_TO_THE_ULTIMATE_QUESTION_OF_LIFE = new Integer(42);
        

Properties

Examples

To configure the check with default configuration:

<module name="MagicNumber"/>
        

results is following violations:

@MyAnnotation(6) // violation
class MyClass {
  private field = 7; // violation

  void foo() {
    int i = i + 1; // no violation
    int j = j + 8; // violation
  }

  public int hashCode() {
    return 10;    // violation
  }
}
@interface anno {
  int value() default 10; // no violation
}
        

To configure the check so that it checks floating-point numbers that are not 0, 0.5, or 1:

<module name="MagicNumber">
  <property name="tokens" value="NUM_DOUBLE, NUM_FLOAT"/>
  <property name="ignoreNumbers" value="0, 0.5, 1"/>
  <property name="ignoreFieldDeclaration" value="true"/>
  <property name="ignoreAnnotation" value="true"/>
</module>
        

results is following violations:

@MyAnnotation(6) // no violation
class MyClass {
  private field = 7; // no violation

  void foo() {
    int i = i + 1; // no violation
    int j = j + 8; // violation
  }
}
        

To configure the check so that it ignores magic numbers in field declarations:

<module name="MagicNumber">
    <property name="ignoreFieldDeclaration" value="false"/>
</module>
        

results in the following violations:

public record MyRecord() {
    private static int myInt = 7; // ok, field declaration

    void foo() {
        int i = myInt + 1; // no violation, 1 is defined as non-magic
        int j = myInt + 8; // violation
    }
}
        

To configure the check to check annotation element defaults:

<module name="MagicNumber">
  <property name="ignoreAnnotationElementDefaults" value="false"/>
</module>
        

results in following violations:

@interface anno {
  int value() default 10; // violation
  int[] value2() default {10}; // violation
}
        

Config example of constantWaiverParentToken option:

<module name="MagicNumber">
  <property name="constantWaiverParentToken" value="ASSIGN,ARRAY_INIT,EXPR,
  UNARY_PLUS, UNARY_MINUS, TYPECAST, ELIST, DIV, PLUS "/>
</module>
        

result is following violation:

class TestMethodCall {
  public void method2() {
    final TestMethodCall dummyObject = new TestMethodCall(62);    //violation
    final int a = 3;        // ok as waiver is ASSIGN
    final int [] b = {4, 5} // ok as waiver is ARRAY_INIT
    final int c = -3;       // ok as waiver is UNARY_MINUS
    final int d = +4;       // ok as waiver is UNARY_PLUS
    final int e = method(1, 2) // ELIST is there but violation due to METHOD_CALL
    final int x = 3 * 4;    // violation
    final int y = 3 / 4;    // ok as waiver is DIV
    final int z = 3 + 4;    // ok as waiver is PLUS
    final int w = 3 - 4;    // violation
    final int x = (int)(3.4);    //ok as waiver is TYPECAST
  }
}
        

Config example of ignoreHashCodeMethod option:

<module name="MagicNumber">
    <property name="ignoreHashCodeMethod" value="true"/>
</module>
        

result is no violation:

class TestHashCode {
  public int hashCode() {
    return 10;              // OK
  }
}
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• magic.number

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Evaluates Xpath query and report violation on all matching AST nodes. This check allows user to implement custom checks using Xpath. If Xpath query is not specified explicitly, then the check does nothing.

It is recommended to define custom message for violation to explain what is not allowed and what to use instead, default message might be too abstract. To customize a message you need to add message element with matchxpath.match as key attribute and desired message as value attribute.

Please read more about Xpath syntax at Xpath Syntax. Information regarding Xpath functions can be found at XSLT/XPath Reference. Note, that @text attribute can used only with token types that are listed in XpathUtil.

Properties

Examples

Checkstyle provides command line tool and GUI application with options to show AST and to ease usage of Xpath queries.

-T option prints AST tree of the checked file.

$ java -jar checkstyle-X.XX-all.jar -T Main.java
CLASS_DEF -> CLASS_DEF [1:0]
|--MODIFIERS -> MODIFIERS [1:0]
|   `--LITERAL_PUBLIC -> public [1:0]
|--LITERAL_CLASS -> class [1:7]
|--IDENT -> Main [1:13]
`--OBJBLOCK -> OBJBLOCK [1:18]
|--LCURLY -> { [1:18]
|--METHOD_DEF -> METHOD_DEF [2:4]
|   |--MODIFIERS -> MODIFIERS [2:4]
|   |   `--LITERAL_PUBLIC -> public [2:4]
|   |--TYPE -> TYPE [2:11]
|   |   `--IDENT -> String [2:11]
|   |--IDENT -> sayHello [2:18]
|   |--LPAREN -> ( [2:26]
|   |--PARAMETERS -> PARAMETERS [2:27]
|   |   `--PARAMETER_DEF -> PARAMETER_DEF [2:27]
|   |       |--MODIFIERS -> MODIFIERS [2:27]
|   |       |--TYPE -> TYPE [2:27]
|   |       |   `--IDENT -> String [2:27]
|   |       `--IDENT -> name [2:34]
|   |--RPAREN -> ) [2:38]
|   `--SLIST -> { [2:40]
|       |--LITERAL_RETURN -> return [3:8]
|       |   |--EXPR -> EXPR [3:25]
|       |   |   `--PLUS -> + [3:25]
|       |   |       |--STRING_LITERAL -> "Hello, " [3:15]
|       |   |       `--IDENT -> name [3:27]
|       |   `--SEMI -> ; [3:31]
|       `--RCURLY -> } [4:4]
`--RCURLY -> } [5:0]
        

-b option shows AST nodes that match given Xpath query. This command can be used to validate accuracy of Xpath query against given file.

$ java -jar checkstyle-X.XX-all.jar Main.java -b "//METHOD_DEF[./IDENT[@text='sayHello']]"
CLASS_DEF -> CLASS_DEF [1:0]
`--OBJBLOCK -> OBJBLOCK [1:18]
|--METHOD_DEF -> METHOD_DEF [2:4]
        

The following example demonstrates validation of methods order, so that public methods should come before the private ones:

<module name="MatchXpath">
  <property name="query" value="//METHOD_DEF[.//LITERAL_PRIVATE and
          following-sibling::METHOD_DEF[.//LITERAL_PUBLIC]]"/>
  <message key="matchxpath.match"
          value="Private methods must appear after public methods"/>
</module>
        

Example:

public class Test {
  public void method1() { }
  private void method2() { } // violation
  public void method3() { }
  private void method4() { } // violation
  public void method5() { }
  private void method6() { } // ok
}
        

To violate if there are any parametrized constructors

<module name="MatchXpath">
  <property name="query" value="//CTOR_DEF[count(./PARAMETERS/*)
          > 0]"/>
  <message key="matchxpath.match"
          value="Parameterized constructors are not allowed, there should be only default
          ctor"/>
</module>
        

Example:

public class Test {
  public Test(Object c) { } // violation
  public Test(int a, HashMap<String, Integer> b) { } // violation
  public Test() { } // ok
}
        

To violate if method name is 'test' or 'foo'

<module name="MatchXpath">
  <property name="query" value="//METHOD_DEF[./IDENT[@text='test'
          or @text='foo']]"/>
  <message key="matchxpath.match"
          value="Method name should not be 'test' or 'foo'"/>
</module>
        

Example:

public class Test {
  public void test() {} // violation
  public void getName() {} // ok
  public void foo() {} // violation
  public void sayHello() {} // ok
}
        

To violate if new instance creation was done without var type

<module name="MatchXpath">
  <property name="query" value="//VARIABLE_DEF[./ASSIGN/EXPR/LITERAL_NEW
          and not(./TYPE/IDENT[@text='var'])]"/>
  <message key="matchxpath.match"
          value="New instances should be created via 'var' keyword to avoid duplication of
          type reference in statement"/>
</module>
        

Example:

public class Test {
  public void foo() {
    SomeObject a = new SomeObject(); // violation
    var b = new SomeObject(); // OK
  }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• matchxpath.match

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that classes (except abstract ones) define a constructor and don't rely on the default one.

Examples

To configure the check:

<module name="MissingCtor"/>
        

Example:

class ExampleOk { // OK
  private int a;
  ExampleOk(int a) {
    this.a = a;
  }
}
class ExampleDefaultCtor { // OK
  private String s;
  ExampleDefaultCtor() {
    s = "foobar";
  }
}
class InvalidExample { // violation, class must have a constructor.
  public void test() {}
}
abstract class AbstractExample { // OK
  public abstract void test() {}
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• missing.ctor

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that switch statement has a default clause.

Rationale: It's usually a good idea to introduce a default case in every switch statement. Even if the developer is sure that all currently possible cases are covered, this should be expressed in the default branch, e.g. by using an assertion. This way the code is protected against later changes, e.g. introduction of new types in an enumeration type. Note that the compiler requires switch expressions to be exhaustive, so this check does not enforce default branches on such expressions.

Examples

To configure the check:

<module name="MissingSwitchDefault"/>
        

Example of violation:

switch (i) {    // violation
  case 1:
    break;
  case 2:
    break;
}
        

Example of correct code:

switch (i) {
  case 1:
    break;
  case 2:
    break;
  default: // OK
    break;
}
        

Example of Usage

• Google Style
• Sun Style
• Checkstyle Style

Violation Messages

• missing.switch.default

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that for loop control variables are not modified inside the for block. An example is:

for (int i = 0; i < 1; i++) {
  i++; //violation
}
        

Rationale: If the control variable is modified inside the loop body, the program flow becomes more difficult to follow. See FOR statement specification for more details.

Such loop would be suppressed:

for (int i = 0; i < 10;) {
  i++;
}
        

Properties

Examples

To configure the check:

<module name="ModifiedControlVariable"/>
        

By default, This Check validates Enhanced For-Loop.

Option 'skipEnhancedForLoopVariable' could be used to skip check of variable from Enhanced For Loop.

An example of how to configure the check so that it skips enhanced For Loop Variable is:

<module name="ModifiedControlVariable">
  <property name="skipEnhancedForLoopVariable" value="true"/>
</module>
        

Example:

for (String line: lines) {
  line = line.trim();   // it will skip this violation
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• modified.control.variable

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks for multiple occurrences of the same string literal within a single file.

Rationale: Code duplication makes maintenance more difficult, so it can be better to replace the multiple occurrences with a constant.

Properties

Examples

To configure the check:

<module name="MultipleStringLiterals"/>
        

To configure the check so that it allows two occurrences of each string:

<module name="MultipleStringLiterals">
  <property name="allowedDuplicates" value="2"/>
</module>
        

To configure the check so that it ignores ", " and empty strings:

<module name="MultipleStringLiterals">
  <property name="ignoreStringsRegexp"
    value='^(("")|(", "))$'/>
</module>
        

To configure the check so that it flags duplicate strings in all syntactical contexts, even in annotations like @SuppressWarnings("unchecked"):

<module name="MultipleStringLiterals">
  <property name="ignoreOccurrenceContext" value=""/>
</module>
        

Example of Usage

• Checkstyle Style

Violation Messages

• multiple.string.literal

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that each variable declaration is in its own statement and on its own line.

Rationale: the Java code conventions chapter 6.1 recommends that declarations should be one per line/statement.

Examples

To configure the check:

<module name="MultipleVariableDeclarations"/>
        

Example:

public class Test {
  public void myTest() {
    int mid;
    int high;
    // ...

    int lower, higher; // violation
    // ...

    int value,
        index; // violation
    // ...

    int place = mid, number = high;  // violation
  }
}
        

Example of Usage

• Google Style
• Sun Style
• Checkstyle Style

Violation Messages

• multiple.variable.declarations
• multiple.variable.declarations.comma

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Restricts nested for blocks to a specified depth.

Properties

Examples

To configure the check:

<module name="NestedForDepth"/>
        

Example:

for(int i=0; i<10; i++) {
    for(int j=0; j<i; j++) {
        for(int k=0; k<j; k++) { // violation, max allowed nested loop number is 1
        }
    }
}

for(int i=0; i<10; i++) {
    for(int j=0; j<i; j++) { // ok
    }
}
        

To configure the check to allow nesting depth 2:

<module name="NestedForDepth">
  <property name="max" value="2"/>
</module>
        

Example:

for(int i=0; i<10; i++) {
    for(int j=0; j<i; j++) {
        for(int k=0; k<j; k++) {
            for(int l=0; l<k; l++) { // violation, max allowed nested loop number is 2
            }
        }
    }
}

for(int i=0; i<10; i++) {
    for(int j=0; j<i; j++) {
        for(int k=0; k<j; k++) { // ok
        }
    }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• nested.for.depth

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Restricts nested if-else blocks to a specified depth.

Properties

Examples

To configure the check:

<module name="NestedIfDepth"/>
        

Valid code example:

if (true) {
    if (true) {} // OK
    else {}
}
        

Invalid code example:

if (true) {
    if (true) {
        if (true) { // violation, nested if-else depth is 2 (max allowed is 1)
        }
    }
}
        

To configure the check to allow nesting depth 3:

<module name="NestedIfDepth">
  <property name="max" value="3"/>
</module>
        

Valid code example:

if (true) {
   if (true) {
      if (true) {
         if (true) {} // OK
         else {}
      }
   }
}
        

Invalid code example:

if (true) {
   if (true) {
      if (true) {
         if (true) {
            if (true) { // violation, nested if-else depth is 4 (max allowed is 3)
               if (true) {} // violation, nested if-else depth is 5 (max allowed is 3)
               else {}
            }
         }
      }
   }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• nested.if.depth

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Restricts nested try-catch-finally blocks to a specified depth.

Properties

Examples

To configure the check:

<module name="NestedTryDepth"/>
        

case 1: Example of code with violation:

try {
    try {
        try { // violation, current depth is 2, default max allowed depth is 1
        } catch (Exception e) {
        }
    } catch (Exception e) {
    }
} catch (Exception e) {
}
        

case 1: Example of compliant code:

try {
    try { // OK, current depth is 1, default max allowed depth is also 1
    } catch (Exception e) {
    }
} catch (Exception e) {
}
        

case 2: Example of code for handling unique and general exceptions

try {
    try { // OK, current depth is 1, default max allowed depth is also 1
            // any more nesting could cause code violation!
            throw ArithmeticException();
    } catch (ArithmeticException e) { // catches arithmetic exceptions
    } catch (NumberFormatException e) { // catches number-format exceptions
    } catch (Exception e) { // catches general exceptions other than stated above
    }
} catch (
  ArithmeticException
  | NumberFormatException
  | ArrayIndexOutOfBoundsException e) { // catches any of the 3 exception
} catch (Exception e) { // catches general exception
} finally { // do something when try-catch block finished execution
}
        

To configure the check to allow nesting depth 3:

<module name="NestedTryDepth">
  <property name="max" value="3"/>
</module>
        

Example of code with violation:

try {
    try {
        try {
            try {
                try { // violation, current depth is 4, max allowed depth is 3
                } catch (Exception e) {
                }
            } catch (Exception e) {
            }
        } catch (Exception e) {
        }
    } catch (Exception e) {
    }
} catch (Exception e) {
}
        

Example of compliant code:

try {
    try {
        try {
            try { // OK, current depth is 3, max allowed depth is also 3
            } catch (Exception e) {
            }
        } catch (Exception e) {
        }
    } catch (Exception e) {
    }
} catch (Exception e) {
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• nested.try.depth

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that array initialization do not contain a trailing comma. Rationale: JLS allows trailing commas in arrays and enumerations, but does not allow them in other locations. To unify the coding style, the use of trailing commas should be prohibited.

int[] foo = new int[] {
  1,
  2
};
        

The check demands that there should not be any comma after the last element of an array.

String[] foo = new String[] {
  "FOO",
  "BAR", //violation
}
        

Examples

To configure the check:

<module name="NoArrayTrailingComma"/>
        

Which results in the following violations:

String[] foo1 = {
  "FOO", // OK
  "BAR", // violation
};
String[] foo2 = { "FOO", "BAR", }; // violation
String[] foo3 = {
  "FOO", // OK
  "BAR" // OK
};
String[] foo4 = { "FOO", "BAR" }; // OK
        

Example of Usage

• Checkstyle Style

Violation Messages

• no.array.trailing.comma

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that the clone method is not overridden from the Object class.

This check is almost exactly the same as the NoFinalizerCheck.

See Object.clone()

Rationale: The clone method relies on strange, hard to follow rules that are difficult to get right and do not work in all situations. In some cases, either a copy constructor or a static factory method can be used instead of the clone method to return copies of an object. For more information on rules for the clone method and its issues, see Effective Java: Programming Language Guide First Edition by Joshua Bloch pages 45-52.

Below are some of the rules/reasons why the clone method should be avoided.

• Classes supporting the clone method should implement the Cloneable interface but the Cloneable interface does not include the clone method. As a result, it doesn't enforce the method override.
• The Cloneable interface forces the Object's clone method to work correctly. Without implementing it, the Object's clone method will throw a CloneNotSupportedException.
• Non-final classes must return the object returned from a call to super.clone().
• Final classes can use a constructor to create a clone which is different from non-final classes.
• If a super class implements the clone method incorrectly all subclasses calling super.clone() are doomed to failure.
• If a class has references to mutable objects then those object references must be replaced with copies in the clone method after calling super.clone().
• The clone method does not work correctly with final mutable object references because final references cannot be reassigned.
• If a super class overrides the clone method then all subclasses must provide a correct clone implementation.

Two alternatives to the clone method, in some cases, is a copy constructor or a static factory method to return copies of an object. Both of these approaches are simpler and do not conflict with final fields. They do not force the calling client to handle a CloneNotSupportedException. They also are typed therefore no casting is necessary. Finally, they are more flexible since they can take interface types rather than concrete classes.

Sometimes a copy constructor or static factory is not an acceptable alternative to the clone method. The example below highlights the limitation of a copy constructor (or static factory). Assume Square is a subclass for Shape.

Shape s1 = new Square();
System.out.println(s1 instanceof Square); //true
        

...assume at this point the code knows nothing of s1 being a Square that's the beauty of polymorphism but the code wants to copy the Square which is declared as a Shape, its super type...

Shape s2 = new Shape(s1); //using the copy constructor
System.out.println(s2 instanceof Square); //false
        

The working solution (without knowing about all subclasses and doing many casts) is to do the following (assuming correct clone implementation).

Shape s2 = s1.clone();
System.out.println(s2 instanceof Square); //true
        

Just keep in mind if this type of polymorphic cloning is required then a properly implemented clone method may be the best choice.

Much of this information was taken from Effective Java: Programming Language Guide First Edition by Joshua Bloch pages 45-52. Give Bloch credit for writing an excellent book.

Examples

To configure the check:

<module name="NoClone"/>
        

Example:

public class Foo {

 public Object clone() {return null;} // violation, overrides the clone method

 public Foo clone() {return null;} // violation, overrides the clone method

 public static Object clone(Object o) {return null;} // OK

 public static Foo clone(Foo o) {return null;} // OK

}
        

Example of Usage

• Checkstyle Style

Violation Messages

• avoid.clone.method

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that enum definition does not contain a trailing comma. Rationale: JLS allows trailing commas in arrays and enumerations, but does not allow them in other locations. To unify the coding style, the use of trailing commas should be prohibited.

enum Foo1 {
  FOO,
  BAR;
}
        

The check demands that there should not be any comma after last constant in enum definition.

enum Foo1 {
  FOO,
  BAR, //violation
}
        

Examples

To configure the check:

<module name="NoEnumTrailingComma"/>
        

Which results in the following violations:

enum Foo1 {
  FOO,
  BAR; //OK
}
enum Foo2 {
  FOO,
  BAR //OK
}
enum Foo3 {
  FOO,
  BAR, //violation
}
enum Foo4 {
  FOO,
  BAR, // violation
  ;
}
enum Foo5 {
  FOO,
  BAR,; // violation
}
enum Foo6 { FOO, BAR,; } // violation
enum Foo7 { FOO, BAR, } // violation
enum Foo8 {
  FOO,
  BAR // OK
  ;
}
enum Foo9 { FOO, BAR; } // OK
enum Foo10 { FOO, BAR } // OK
        

Example of Usage

• Checkstyle Style

Violation Messages

• no.enum.trailing.comma

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that there is no method finalize with zero parameters.

See Object.finalize()

Rationale: Finalizers are unpredictable, often dangerous, and generally unnecessary. Their use can cause erratic behavior, poor performance, and portability problems. For more information for the finalize method and its issues, see Effective Java: Programming Language Guide Third Edition by Joshua Bloch, §8.

Examples

To configure the check:

<module name="NoFinalizer"/>
        

Example:

public class Test {

    protected void finalize() throws Throwable { // violation
        try {
           System.out.println("overriding finalize()");
        } catch (Throwable t) {
           throw t;
        } finally {
           super.finalize();
        }
    }
}
        

Example of Usage

• Google Style
• Checkstyle Style

Violation Messages

• avoid.finalizer.method

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that there is only one statement per line.

Rationale: It's very difficult to read multiple statements on one line.

In the Java programming language, statements are the fundamental unit of execution. All statements except blocks are terminated by a semicolon. Blocks are denoted by open and close curly braces.

OneStatementPerLineCheck checks the following types of statements: variable declaration statements, empty statements, import statements, assignment statements, expression statements, increment statements, object creation statements, 'for loop' statements, 'break' statements, 'continue' statements, 'return' statements, resources statements (optional).

Properties

Examples

To configure the check:

<module name="OneStatementPerLine"/>
        

The following examples will be flagged as a violation:

//Each line causes violation:
int var1; int var2;
var1 = 1; var2 = 2;
int var1 = 1; int var2 = 2;
var1++; var2++;
Object obj1 = new Object(); Object obj2 = new Object();
import java.io.EOFException; import java.io.BufferedReader;
;; //two empty statements on the same line.

//Multi-line statements:
int var1 = 1
; var2 = 2; //violation here
int o = 1, p = 2,
r = 5; int t; //violation here
        

An example of how to configure the check to treat resources in a try statement as statements to require them on their own line:

<module name="OneStatementPerLine">
  <property name="treatTryResourcesAsStatement" value="true"/>
</module>
        

Note: resource declarations can contain variable definitions and variable references (from java9). When property "treatTryResourcesAsStatement" is enabled, this check is only applied to variable definitions. If there are one or more variable references and one variable definition on the same line in resources declaration, there is no violation. The following examples will illustrate difference:

OutputStream s1 = new PipedOutputStream();
OutputStream s2 = new PipedOutputStream();
// only one statement(variable definition) with two variable references
try (s1; s2; OutputStream s3 = new PipedOutputStream();) // OK
{}
// two statements with variable definitions
try (Reader r = new PipedReader(); s2; Reader s3 = new PipedReader() // violation
) {}
        

Example of Usage

• Google Style
• Checkstyle Style

Violation Messages

• multiple.statements.line

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that overloaded methods are grouped together. Overloaded methods have the same name but different signatures where the signature can differ by the number of input parameters or type of input parameters or both.

Examples

To configure the check:

<module name="OverloadMethodsDeclarationOrder"/>
        

Example of correct grouping of overloaded methods:

public void foo(int i) {}
public void foo(String s) {}
public void foo(String s, int i) {}
public void foo(int i, String s) {}
public void notFoo() {}
        

Example of incorrect grouping of overloaded methods:

public void foo(int i) {} // OK
public void foo(String s) {} // OK
public void notFoo() {} // violation. Have to be after foo(String s, int i)
public void foo(int i, String s) {}
public void foo(String s, int i) {}
        

Example of Usage

• Google Style
• Checkstyle Style

Violation Messages

• overload.methods.declaration

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Ensures that a class has a package declaration, and (optionally) whether the package name matches the directory name for the source file.

Rationale: Classes that live in the null package cannot be imported. Many novice developers are not aware of this.

Packages provide logical namespace to classes and should be stored in the form of directory levels to provide physical grouping to your classes. These directories are added to the classpath so that your classes are visible to JVM when it runs the code.

Properties

Examples

To configure the check:

<module name="PackageDeclaration"/>
        

Let us consider the class AnnotationLocationCheck which is in the directory /com/puppycrawl/tools/checkstyle/checks/annotations/

package com.puppycrawl.tools.checkstyle.checks; //Violation
public class AnnotationLocationCheck extends AbstractCheck {
  //...
}
        

Example of how the check works when matchDirectoryStructure option is set to false. Let us again consider the AnnotationLocationCheck class located at directory /com/puppycrawl/tools/checkstyle/checks/annotations/ along with the following setup,

<module name="PackageDeclaration">
<property name="matchDirectoryStructure" value="false"/>
</module>
        

package com.puppycrawl.tools.checkstyle.checks;  //No Violation

public class AnnotationLocationCheck extends AbstractCheck {
  //...
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• mismatch.package.directory
• missing.package.declaration

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Disallows assignment of parameters.

Rationale: Parameter assignment is often considered poor programming practice. Forcing developers to declare parameters as final is often onerous. Having a check ensure that parameters are never assigned would give the best of both worlds.

Examples

To configure the check:

<module name="ParameterAssignment"/>
        

Example of Usage

• Checkstyle Style

Violation Messages

• parameter.assignment

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that references to instance variables and methods of the present object are explicitly of the form "this.varName" or "this.methodName(args)" and that those references don't rely on the default behavior when "this." is absent.

Warning: the Check is very controversial if 'validateOnlyOverlapping' option is set to 'false' and not that actual nowadays.

Rationale:

1. The same notation/habit for C++ and Java (C++ have global methods, so having "this." do make sense in it to distinguish call of method of class instead of global).
2. Non-IDE development (ease of refactoring, some clearness to distinguish static and non-static methods).

Notes

Limitations: Nothing is currently done about static variables or catch-blocks. Static methods invoked on a class name seem to be OK; both the class name and the method name have a DOT parent. Non-static methods invoked on either this or a variable name seem to be OK, likewise.

Properties

Examples

To configure the default check:

<module name="RequireThis"/>
        

To configure to check the this qualifier for fields only:

<module name="RequireThis">
  <property name="checkMethods" value="false"/>
</module>
        

Examples of how the check works if validateOnlyOverlapping option is set to true:

public static class A {
  private int field1;
  private int field2;

  public A(int field1) {
    // Overlapping by constructor argument.
    field1 = field1; // violation: Reference to instance variable "field1" needs "this".
    field2 = 0;
  }

  void foo3() {
    String field1 = "values";
    // Overlapping by local variable.
    field1 = field1; // violation:  Reference to instance variable "field1" needs "this".
  }
}

public static class B {
  private int field;

  public A(int f) {
    field = f;
  }

  String addSuffixToField(String field) {
    // Overlapping by method argument. Equal to "return field = field + "suffix";"
    return field += "suffix"; // violation: Reference to instance variable "field" needs "this".
  }
}

public static record C(int x) {
    void getTwoX(int x) {
        // Overlapping by method argument.
        return x += x; // violation: Reference to instance variable "x" needs "this".
    }
}
        

Please, be aware of the following logic, which is implemented in the check:

1) If you arrange 'this' in your code on your own, the check will not raise violation for variables which use 'this' to reference a class field, for example:

public class C {
  private int scale;
  private int x;
  public void foo(int scale) {
    scale = this.scale; // no violation
    if (scale > 0) {
      scale = -scale; // no violation
    }
    x *= scale;
  }
}
        

2) If method parameter is returned from the method, the check will not raise violation for returned variable/parameter, for example:

public class D {
  private String prefix;
  public String modifyPrefix(String prefix) {
    prefix = "^" + prefix + "$" // no violation (modification of parameter)
    return prefix; // modified method parameter is returned from the method
  }
}
        

Examples of how the check works if validateOnlyOverlapping option is set to false:

public static class A {
  private int field1;
  private int field2;

  public A(int field1) {
    field1 = field1; // violation: Reference to instance variable "field1" needs "this".
    field2 = 0; // violation: Reference to instance variable "field2" needs "this".
    String field2;
    field2 = "0"; // No violation. Local var allowed
  }

  void foo3() {
    String field1 = "values";
    field1 = field1; // violation:  Reference to instance variable "field1" needs "this".
  }
}

public static class B {
  private int field;

  public A(int f) {
    field = f; // violation:  Reference to instance variable "field" needs "this".
  }

  String addSuffixToField(String field) {
    return field += "suffix"; // violation: Reference to instance variable "field" needs "this".
  }
}

// If the variable is locally defined, there won't be a violation provided the variable
// doesn't overlap.
class C {
  private String s1 = "foo1";
  String s2 = "foo2";

  C() {
    s1 = "bar1"; // Violation. Reference to instance variable 's1' needs "this.".
    String s2;
    s2 = "bar2"; // No violation. Local var allowed.
    s2 += s2; // Violation. Overlapping. Reference to instance variable 's2' needs "this.".
  }
}

public static record D(int x) {
    public D {
        x = x; // violation: Reference to instance variable "x" needs "this".
    }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• require.this.method
• require.this.variable

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Restricts the number of return statements in methods, constructors and lambda expressions. Ignores specified methods (equals by default).

max property will only check returns in methods and lambdas that return a specific value (Ex: 'return 1;').

maxForVoid property will only check returns in methods, constructors, and lambdas that have no return type (IE 'return;'). It will only count visible return statements. Return statements not normally written, but implied, at the end of the method/constructor definition will not be taken into account. To disallow "return;" in void return type methods, use a value of 0.

Rationale: Too many return points can mean that code is attempting to do too much or may be difficult to understand.

Properties

Examples

To configure the check so that it doesn't allow more than three return statements per method (ignoring the equals() method):

<module name="ReturnCount">
  <property name="max" value="3"/>
</module>
        

To configure the check so that it doesn't allow any return statements per void method:

<module name="ReturnCount">
  <property name="maxForVoid" value="0"/>
</module>
        

To configure the check so that it doesn't allow more than 2 return statements per method (ignoring the equals() method) and more than 1 return statements per void method:

<module name="ReturnCount">
  <property name="max" value="2"/>
  <property name="maxForVoid" value="1"/>
</module>
        

To configure the check so that it doesn't allow more than three return statements per method for all methods:

<module name="ReturnCount">
  <property name="max" value="3"/>
  <property name="format" value="^$"/>
</module>
        

To configure the check so that it doesn't allow any return statements in constructors, more than one return statement in all lambda expressions and more than two return statements in methods:

<module name="ReturnCount">
  <property name="max" value="0"/>
  <property name="tokens" value="CTOR_DEF"/>
</module>
<module name="ReturnCount">
  <property name="max" value="1"/>
  <property name="tokens" value="LAMBDA"/>
</module>
<module name="ReturnCount">
  <property name="max" value="2"/>
  <property name="tokens" value="METHOD_DEF"/>
</module>
        

Example of Usage

• Checkstyle Style

Violation Messages

• return.count
• return.countVoid

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks for over-complicated boolean expressions. Currently finds code like if (b == true), b || true, !false, etc.

Rationale: Complex boolean logic makes code hard to understand and maintain.

Examples

To configure the check:

<module name="SimplifyBooleanExpression"/>
        

Example:

public class Test {

  public void bar() {

    boolean a, b;
    Foo c, d, e;

    if (!false) {}; // violation, can be simplified to true

    if (a == true) {}; // violation, can be simplified to a
    if (a == b) {}; // OK
    if (a == false) {}; // violation, can be simplified to !a
    if (!(a != true)) {}; // violation, can be simplified to a

    e = (a || b) ? c : d; // OK
    e = (a || false) ? c : d; // violation, can be simplified to a
    e = (a && b) ? c : d; // OK

  }

}
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• simplify.expression

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks for over-complicated boolean return statements. For example the following code

if (valid())
  return false;
else
  return true;
        

could be written as

return !valid();
        

The idea for this Check has been shamelessly stolen from the equivalent PMD rule.

Examples

To configure the check:

<module name="SimplifyBooleanReturn"/>
        

Example:

public class Test {

 private boolean cond;
 private Foo a;
 private Foo b;

 public boolean check1() {
  if (cond) { // violation, can be simplified
    return true;
  }
  else {
    return false;
  }
 }

 // Ok, simplified version of check1()
 public boolean check2() {
  return cond;
 }

 // violations, can be simplified
 public boolean check3() {
  if (cond == true) { // can be simplified to "if (cond)"
    return false;
  }
  else {
    return true; // can be simplified to "return !cond"
  }
 }

 // Ok, can be simplified but doesn't return a Boolean
 public Foo choose1() {
  if (cond) {
    return a;
  }
  else {
    return b;
  }
 }

 // Ok, simplified version of choose1()
 public Foo choose2() {
  return cond ? a: b;
 }

}
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• simplify.boolReturn

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that string literals are not used with == or !=. Since == will compare the object references, not the actual value of the strings, String.equals() should be used. More information can be found in this article.

Rationale: Novice Java programmers often use code like:

if (x == "something")
        

when they mean

if ("something".equals(x))
        

Examples

To configure the check:

<module name="StringLiteralEquality"/>
        

Examples of violations:

String status = "pending";

if (status == "done") {} // violation

while (status != "done") {} // violation

boolean flag = (status == "done"); // violation

boolean flag = (status.equals("done")); // OK

String name = "X";

if (name == getName()) {}
// OK, limitation that check cannot tell runtime type returned from method call
        

Example of Usage

• Checkstyle Style

Violation Messages

• string.literal.equality

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that an overriding clone() method invokes super.clone(). Does not check native methods, as they have no possible java defined implementation.

Reference: Object.clone().

Examples

To configure the check:

<module name="SuperClone"/>
        

Example:

class A {

 public Object clone() { // OK
  return super.clone();
 }
}

class B {
private int b;

 public B clone() { // violation, does not call super.clone()
  B other = new B();
  other.b = this.b;
  return other;
 }
}

class C {

 public C clone() { // OK
  return (C) super.clone();
 }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• missing.super.call

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks that an overriding finalize() method invokes super.finalize(). Does not check native methods, as they have no possible java defined implementation.

References: How to Handle Java Finalization's Memory-Retention Issues; 10 points on finalize method in Java.

Examples

To configure the check:

<module name="SuperFinalize"/>
        

Example of Usage

• Checkstyle Style

Violation Messages

• missing.super.call

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks if unnecessary parentheses are used in a statement or expression. The check will flag the following with warnings:

return (x);          // parens around identifier
return (x + 1);      // parens around return value
int x = (y / 2 + 1); // parens around assignment rhs
for (int i = (0); i < 10; i++) {  // parens around literal
t -= (z + 1);        // parens around assignment rhs
        

Notes

The check is not "type aware", that is to say, it can't tell if parentheses are unnecessary based on the types in an expression. It also doesn't know about operator precedence and associativity; therefore it won't catch something like

int x = (a + b) + c;
        

In the above case, given that a, b, and c are all int variables, the parentheses around a + b are not needed.

Properties

Examples

To configure the check:

<module name="UnnecessaryParentheses"/>
        

Which results in the following violations:

public int square(int a, int b){
  int square = (a * b); //violation
  return (square); //violation
}
int sumOfSquares = 0;
for(int i=(0); i<10; i++){ //violation
  int x = (i + 1); //violation
  sumOfSquares += (square(x * x)); //violation
}
double num = (10.0); //violation
List<String> list = Arrays.asList("a1", "b1", "c1");
myList.stream()
  .filter((s) -> s.startsWith("c")) //violation
  .forEach(System.out::println);
        

Example of Usage

• Checkstyle Style

Violation Messages

• unnecessary.paren.assign
• unnecessary.paren.expr
• unnecessary.paren.ident
• unnecessary.paren.lambda
• unnecessary.paren.literal
• unnecessary.paren.return
• unnecessary.paren.string

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks if unnecessary semicolon is used after type declaration.

Notes

This check is not applicable to nested type declarations, UnnecessarySemicolonAfterTypeMemberDeclaration is responsible for it.

Properties

Examples

To configure the check:

<module name="UnnecessarySemicolonAfterOuterTypeDeclaration"/>
        

Example:

class A {

   class Nested {

   }; // OK, nested type declarations are ignored

}; // violation

interface B {

}; // violation

enum C {

}; // violation

{@literal @}interface D {

}; // violation
        

To configure the check to detect unnecessary semicolon only after top level class definitions:

<module name="UnnecessarySemicolonAfterOuterTypeDeclaration">
  <property name="tokens" value="CLASS_DEF"/>
</module>
        

Example:

class A {

}; // violation

interface B {

}; // OK

enum C {

}; // OK

{@literal @}interface D {

}; // OK
        

Example of Usage

• Checkstyle Style

Violation Messages

• unnecessary.semicolon

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks if unnecessary semicolon is used after type member declaration.

Notes

This check is not applicable to empty statements (unnecessary semicolons inside methods or init blocks), EmptyStatement is responsible for it.

Properties

Examples

To configure the check:

<module name="UnnecessarySemicolonAfterTypeMemberDeclaration"/>
        

Results in following:

class A {
    ; // violation, standalone semicolon
    {}; // violation, extra semicolon after init block
    static {}; // violation, extra semicolon after static init block
    A(){}; // violation, extra semicolon after constructor definition
    void method() {}; // violation, extra semicolon after method definition
    int field = 10;; // violation, extra semicolon after field declaration

    {
        ; // no violation, it is empty statement inside init block
    }

    static {
        ; // no violation, it is empty statement inside static init block
    }

    void anotherMethod() {
        ; // no violation, it is empty statement
        if(true); // no violation, it is empty statement
    }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• unnecessary.semicolon

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks if unnecessary semicolon is in enum definitions. Semicolon is not needed if enum body contains only enum constants.

Examples

To configure the check:

<module name="UnnecessarySemicolonInEnumeration"/>
        

Example of violations

enum One {
    A,B; // violation
}
enum Two {
    A,B,; // violation
}
enum Three {
    A,B(); // violation
}
enum Four {
    A,B{}; // violation
}
enum Five {
    A,
    B
    ; // violation
}
        

Example of good cases

enum Normal {
    A,
    B,
    ; // required ";", no violation
    Normal(){}
}
enum NoSemicolon {
    A, B // only enum constants, no semicolon required
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• unnecessary.semicolon

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks if unnecessary semicolon is used in last resource declaration.

Properties

Examples

To configure the check:

<module name="UnnecessarySemicolonInTryWithResources"/>
        

Example of violations

class A {
    void method() throws IOException {
        try(Reader r1 = new PipedReader();){} // violation
        try(Reader r4 = new PipedReader();Reader r5 = new PipedReader()
        ;){} // violation
        try(Reader r6 = new PipedReader();
            Reader r7
                   = new PipedReader();
        ){}
    }
}
        

To configure the check to detect unnecessary semicolon if closing paren is not on same line

<module name="UnnecessarySemicolonInTryWithResources">
  <property name="allowWhenNoBraceAfterSemicolon" value="false"/>
</module>
        

Example of exclusion

class A {
    void method() throws IOException {
        try(Reader r1 = new PipedReader();){} // violation
        try(Reader r6 = new PipedReader();
            Reader r7 = new PipedReader(); // violation
        ){}
    }
}
        

Example of Usage

• Checkstyle Style

Violation Messages

• unnecessary.semicolon

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker

Description

Checks the distance between declaration of variable and its first usage. Note : Variable declaration/initialization statements are not counted while calculating length.

Properties

Examples

To configure the check with default config:

<module name="VariableDeclarationUsageDistance"/>
        

Example:

public class Test {

  public void foo1() {
    int num;        // violation, distance = 4
    final int PI;   // OK, final variables not checked
    System.out.println("Statement 1");
    System.out.println("Statement 2");
    System.out.println("Statement 3");
    num = 1;
    PI = 3.14;
  }

  public void foo2() {
    int a;          // OK, used in different scope
    int b;          // OK, used in different scope
    int count = 0;  // OK, used in different scope

    {
      System.out.println("Inside inner scope");
      a = 1;
      b = 2;
      count++;
    }
  }
}
        

Check can detect a block of initialization methods. If a variable is used in such a block and there are no other statements after variable declaration, then distance = 1.

Case #1:

int minutes = 5;
Calendar cal = Calendar.getInstance();
cal.setTimeInMillis(timeNow);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
cal.set(Calendar.HOUR_OF_DAY, hh);
cal.set(Calendar.MINUTE, minutes);
        

The distance for the variable "minutes" is 1 even though this variable is used in the fifth method's call.

Case #2:

int minutes = 5;
Calendar cal = Calendar.getInstance();
cal.setTimeInMillis(timeNow);
cal.set(Calendar.SECOND, 0);
cal.set(Calendar.MILLISECOND, 0);
System.out.println(cal);
cal.set(Calendar.HOUR_OF_DAY, hh);
cal.set(Calendar.MINUTE, minutes);
        

The distance for the variable "minutes" is 6 because there is one more expression (except the initialization block) between the declaration of this variable and its usage.

To configure the check to set allowed distance:

<module name="VariableDeclarationUsageDistance">
  <property name="allowedDistance" value="4"/>
</module>
        

Example:

public class Test {

  public void foo1() {
    int num;        // OK, distance = 4
    final int PI;   // OK, final variables not checked
    System.out.println("Statement 1");
    System.out.println("Statement 2");
    System.out.println("Statement 3");
    num = 1;
    PI = 3.14;
  }

  public void foo2() {
    int a;          // OK, used in different scope
    int b;          // OK, used in different scope
    int count = 0;  // OK, used in different scope

    {
      System.out.println("Inside inner scope");
      a = 1;
      b = 2;
      count++;
    }
  }
}
        

To configure the check to ignore certain variables:

<module name="VariableDeclarationUsageDistance">
  <property name="ignoreVariablePattern" value="^num$"/>
</module>
        

This configuration ignores variables named "num".

Example:

public class Test {

  public void foo1() {
    int num;        // OK, variable ignored
    final int PI;   // OK, final variables not checked
    System.out.println("Statement 1");
    System.out.println("Statement 2");
    System.out.println("Statement 3");
    num = 1;
    PI = 3.14;
  }

  public void foo2() {
    int a;          // OK, used in different scope
    int b;          // OK, used in different scope
    int count = 0;  // OK, used in different scope

    {
      System.out.println("Inside inner scope");
      a = 1;
      b = 2;
      count++;
    }
  }
}
        

To configure the check to force validation between scopes:

<module name="VariableDeclarationUsageDistance">
  <property name="validateBetweenScopes" value="true"/>
</module>
        

Example:

public class Test {

  public void foo1() {
    int num;        // violation, distance = 4
    final int PI;   // OK, final variables not checked
    System.out.println("Statement 1");
    System.out.println("Statement 2");
    System.out.println("Statement 3");
    num = 1;
    PI = 3.14;
  }

  public void foo2() {
    int a;          // OK, distance = 2
    int b;          // OK, distance = 3
    int count = 0;  // violation, distance = 4

    {
      System.out.println("Inside inner scope");
      a = 1;
      b = 2;
      count++;
    }
  }
}
        

To configure the check to check final variables:

<module name="VariableDeclarationUsageDistance">
  <property name="ignoreFinal" value="false"/>
</module>
        

Example:

public class Test {

  public void foo1() {
    int num;        // violation, distance = 4
    final int PI;   // violation, distance = 5
    System.out.println("Statement 1");
    System.out.println("Statement 2");
    System.out.println("Statement 3");
    num = 1;
    PI = 3.14;
  }

  public void foo2() {
    int a;          // OK, used in different scope
    int b;          // OK, used in different scope
    int count = 0;  // OK, used in different scope

    {
      System.out.println("Inside inner scope");
      a = 1;
      b = 2;
      count++;
    }
  }
}
        

Notes

ATTENTION! (Unsupported cases)

Case #1:

{
  int c;
  int a = 3;
  int b = 2;
  {
    a = a + b;
    c = b;
  }
}
        

Distance for variable 'a' = 1; Distance for variable 'b' = 1; Distance for variable 'c' = 2.

As distance by default is 1 the check doesn't raise warning for variables 'a' and 'b' to move them into the block.

Case #2:

int sum = 0;
for (int i = 0; i < 20; i++) {
  a++;
  b--;
  sum++;
  if (sum > 10) {
    res = true;
  }
}
        

Distance for variable 'sum' = 3.

As the distance is more than the default (=1), the check raises warning for variable 'sum' to move it into the 'for(...)' block. But there is situation when variable 'sum' hasn't to be 0 within each iteration. So, to avoid such warnings you can use the Suppression Filter, provided by Checkstyle, for the whole class.

Example of Usage

• Google Style
• Checkstyle Style

Violation Messages

• variable.declaration.usage.distance
• variable.declaration.usage.distance.extend

All messages can be customized if the default message doesn't suit you. Please see the documentation to learn how to.

Package

com.puppycrawl.tools.checkstyle.checks.coding

Parent Module

TreeWalker