Description

Rationale: The Java Language Specification strongly discourages the usage of public and abstract for method declarations in interface definitions as a matter of style.

The check validates:

1. Interface and annotation definitions.
2. Final modifier on methods of final and anonymous classes.
3. Type declarations nested under interfaces that are declared as public or static.
4. Class constructors.
5. Nested enum definitions that are declared as static.
6. record definitions that are declared as final and nested record definitions that are declared as static.
7. strictfp modifier when using JDK 17 or later. See reason at JEP 306
8. final modifier on unnamed variables when using JDK 22 or later.

interfaces by definition are abstract so the abstract modifier is redundant on them.

Type declarations nested under interfaces by definition are public and static, so the public and static modifiers on nested type declarations are redundant. On the other hand, classes inside of interfaces can be abstract or non abstract. So, abstract modifier is allowed.

Fields in interfaces and annotations are automatically public, static and final, so these modifiers are redundant as well.

As annotations are a form of interface, their fields are also automatically public, static and final just as their annotation fields are automatically public and abstract.

A record class is implicitly final and cannot be abstract, these restrictions emphasize that the API of a record class is defined solely by its state description, and cannot be enhanced later by another class. Nested records are implicitly static. This avoids an immediately enclosing instance which would silently add state to the record class. See JEP 395 for more info.

Enums by definition are static implicit subclasses of java.lang.Enum<E>. So, the static modifier on the enums is redundant. In addition, if enum is inside of interface, public modifier is also redundant.

Enums can also contain abstract methods and methods which can be overridden by the declared enumeration fields. See the following example:

public enum EnumClass {
  FIELD_1,
  FIELD_2 {
    @Override
    public final void method1() {} // violation expected
  };

  public void method1() {}
  public final void method2() {} // no violation expected
}
        

Since these methods can be overridden in these situations, the final methods are not marked as redundant even though they can't be extended by other classes/enums.

Nested enum types are always static by default.

Final classes by definition cannot be extended so the final modifier on the method of a final class is redundant.

Public modifier for constructors in non-public non-protected classes is always obsolete:

public class PublicClass {
  public PublicClass() {} // OK
}

class PackagePrivateClass {
  public PackagePrivateClass() {} // violation expected
}
        

There is no violation in the following example, because removing public modifier from ProtectedInnerClass constructor will make this code not compiling:

package a;
public class ClassExample {
  protected class ProtectedInnerClass {
    public ProtectedInnerClass () {}
  }
}

package b;
import a.ClassExample;
public class ClassExtending extends ClassExample {
  ProtectedInnerClass pc = new ProtectedInnerClass();
}
        

Properties

Examples

To configure the check:

<module name="Checker">
  <module name="TreeWalker">
    <module name="RedundantModifier"/>
  </module>
</module>
        

Example:

public class Example1 {

  void test() {
    // violation below, 'Redundant 'final' modifier'
    try (final var a = lock()) {

    } catch (Exception e) {

    }
  }

  // violation below, 'Redundant 'abstract' modifier'
  abstract interface I {
    public abstract void m();
    // 2 violations above:
    //    'Redundant 'public' modifier'
    //    'Redundant 'abstract' modifier'
    public int x = 0; // violation, 'Redundant 'public' modifier'
  }

  static enum E { // violation, 'Redundant 'static' modifier'
        A, B, C
  }

  // violation below, 'Redundant 'strictfp' modifier'
  public strictfp class Test { }

  AutoCloseable lock() {
    return null;
  }

}
        

To configure the check to check only methods:

<module name="Checker">
  <module name="TreeWalker">
    <module name="RedundantModifier">
      <property name="tokens" value="METHOD_DEF"/>
    </module>
  </module>
</module>
        

Example:

public class Example2 {

  void test() {
    try (final var a = lock()) {

    } catch (Exception e) {

    }
  }

  abstract interface I {
    public abstract void m();
    // 2 violations above:
    //    'Redundant 'public' modifier'
    //    'Redundant 'abstract' modifier'
    public int x = 0;
  }

  static enum E {
        A, B, C
  }

  public strictfp class Test { }

  AutoCloseable lock() {
    return null;
  }
}
        

To configure the check if you are using JDK 11:

<module name="Checker">
  <module name="TreeWalker">
    <module name="RedundantModifier">
        <property name="jdkVersion" value="11"/>
    </module>
  </module>
</module>
        

Example:

public class Example3 {

  void test() {
    // violation below, 'Redundant 'final' modifier'
    try (final var a = lock()) {

    } catch (Exception e) {

    }
  }

  // violation below, 'Redundant 'abstract' modifier'
  abstract interface I {
    public abstract void m();
    // 2 violations above:
    //    'Redundant 'public' modifier'
    //    'Redundant 'abstract' modifier'
    public int x = 0; // violation, 'Redundant 'public' modifier'
  }

  static enum E { // violation, 'Redundant 'static' modifier'
        A, B, C
  }

  // ok below, 'strictfp' is not redundant before JDK 17
  public strictfp class Test { }

  AutoCloseable lock() {
    return null;
  }

}
        

Example of Usage

• Sun Style
• Checkstyle Style

Violation Messages

• redundantModifier

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.modifier

Parent Module

TreeWalker