TrailingComment

Since Checkstyle 3.4

Description

The check to ensure that lines with code do not end with comment. For the case of // comments that means that the only thing that should precede it is whitespace. It doesn't check comments if they do not end a line; for example, it accepts the following: Thread.sleep( 10 /*some comment here*/ ); Format property is intended to deal with the } // while example.

Rationale: Steve McConnell in Code Complete suggests that endline comments are a bad practice. An end line comment would be one that is on the same line as actual code. For example: 

a = b + c;      // Some insightful comment
d = e / f;        // Another comment for this line

Quoting Code Complete for the justification:

  • "The comments have to be aligned so that they do not interfere with the visual structure of the code. If you don't align them neatly, they'll make your listing look like it's been through a washing machine."
  • "Endline comments tend to be hard to format...It takes time to align them. Such time is not spent learning more about the code; it's dedicated solely to the tedious task of pressing the spacebar or tab key."
  • "Endline comments are also hard to maintain. If the code on any line containing an endline comment grows, it bumps the comment farther out, and all the other endline comments will have to bumped out to match. Styles that are hard to maintain aren't maintained...."
  • "Endline comments also tend to be cryptic. The right side of the line doesn't offer much room and the desire to keep the comment on one line means the comment must be short. Work then goes into making the line as short as possible instead of as clear as possible. The comment usually ends up as cryptic as possible...."
  • "A systemic problem with endline comments is that it's hard to write a meaningful comment for one line of code. Most endline comments just repeat the line of code, which hurts more than it helps."

McConnell's comments on being hard to maintain when the size of the line changes are even more important in the age of automated refactorings.

Properties

name description type default value since
format Specify pattern for strings allowed before the comment. Pattern "^[\s});]*$" 3.4
legalComment Define pattern for text allowed in trailing comments. This pattern will not be applied to multiline comments. Pattern null 4.2

Examples

To configure the check: 

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

Example: 

public class Example1 {
  public static void main(String[] args) {
    int x = 10;
    // OK
    if (/* OK */ x > 5) {}
    int a = 5; // violation
    doSomething(
            "param1"
    ); // OK, by default such trailing of method/code-block ending is allowed

  }

  private static void doSomething(String param) {
  }
}

To configure the check to enforce only comment on a line: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="TrailingComment">
      <property name="format" value="^\s*$"/>
    </module>
  </module>
</module>

Example: 

public class Example2 {
  public static void main(String[] args) {
    int x = new Random().nextInt(100);
    // OK
    if (/* OK, this comment does not end the line */ x > 5) {}
    int a = 5; // violation, line content before comment should match pattern "^\s*$"
    doSomething(
            "param1"
    ); // violation, line content before comment should match pattern "^\s*$"
  }

  private static void doSomething(String param) {
  }
}

To configure check so that trailing comment with exact comments like "SUPPRESS CHECKSTYLE", "NOPMD", "NOSONAR" are suppressed: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="TrailingComment"/>
    <module name="SuppressionXpathSingleFilter">
      <property name="checks" value="TrailingCommentCheck"/>
      <property name="query" value="//SINGLE_LINE_COMMENT
        [./COMMENT_CONTENT[@text=' NOSONAR\n' or @text=' NOPMD\n'
        or @text=' SUPPRESS CHECKSTYLE\n']]"/>
    </module>
  </module>
</module>

Example for trailing comments check to suppress specific trailing comment: 

public class Example3 {
  int a; // SUPPRESS CHECKSTYLE
  int b; // NOPMD
  int c; // NOSONAR
  int d; // violation, not suppressed
}

To configure check so that trailing comment starting with "SUPPRESS CHECKSTYLE", "NOPMD", "NOSONAR" are suppressed: 

<module name="Checker">
  <module name="TreeWalker">
    <module name="TrailingComment"/>
    <module name="SuppressionXpathSingleFilter">
      <property name="checks" value="TrailingCommentCheck"/>
      <property name="query" value="//SINGLE_LINE_COMMENT
        [./COMMENT_CONTENT[starts-with(@text, ' NOPMD')]]"/>
      <property name="query" value="//SINGLE_LINE_COMMENT
        [./COMMENT_CONTENT[starts-with(@text, ' SUPPRESS CHECKSTYLE')]]"/>
      <property name="query" value="//SINGLE_LINE_COMMENT
        [./COMMENT_CONTENT[starts-with(@text, ' NOSONAR')]]"/>
    </module>
  </module>
</module>

Example: 

public class Example4 {
  int a; // SUPPRESS CHECKSTYLE - OK, comment starts with " SUPPRESS CHECKSTYLE"
  int b; // NOPMD - OK, comment starts with " NOPMD"
  int c; // NOSONAR - OK, comment starts with " NOSONAR"
  int d; // violation, not suppressed
}

Example of Usage

Violation Messages

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

Parent Module

TreeWalker