Package com.puppycrawl.tools.checkstyle.checks

Class TrailingCommentCheck

  • All Implemented Interfaces:
    Configurable, Contextualizable
    public class TrailingCommentCheck
extends AbstractCheck

    The check to ensure that requires that comments be the only thing on a line. 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.

    • Property format - Specify pattern for strings allowed before the comment. Type is java.util.regex.Pattern. Default value is "^[\s});]*$".
    • Property legalComment - Define pattern for text allowed in trailing comments. (This pattern will not be applied to multiline comments and the text of the comment will be trimmed before matching.) Type is java.util.regex.Pattern. Default value is null.

    To configure the check: 

     <module name="TrailingComment"/>

    To configure the check so it enforces only comment on a line: 

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

    Parent is com.puppycrawl.tools.checkstyle.TreeWalker

    Violation Message Keys:

    • trailing.comments
    Since:
    3.4

    • Field Detail

      • MSG_KEY

        public static final java.lang.String MSG_KEY
        A key is pointing to the warning message text in "messages.properties" file.
        See Also:
        Constant Field Values

    • Constructor Detail

      • TrailingCommentCheck

        public TrailingCommentCheck()

    • Method Detail

      • setLegalComment

        public void setLegalComment​(java.util.regex.Pattern legalComment)
        Setter to define pattern for text allowed in trailing comments. (This pattern will not be applied to multiline comments and the text of the comment will be trimmed before matching.)
        Parameters:
        legalComment - pattern to set.

      • setFormat

        public final void setFormat​(java.util.regex.Pattern pattern)
        Setter to specify pattern for strings allowed before the comment.
        Parameters:
        pattern - a pattern

      • getDefaultTokens

        public int[] getDefaultTokens()
        Description copied from class: AbstractCheck
        Returns the default token a check is interested in. Only used if the configuration for a check does not define the tokens.
        Specified by:
        getDefaultTokens in class AbstractCheck
        Returns:
        the default tokens
        See Also:
        TokenTypes

      • getAcceptableTokens

        public int[] getAcceptableTokens()
        Description copied from class: AbstractCheck
        The configurable token set. Used to protect Checks against malicious users who specify an unacceptable token set in the configuration file. The default implementation returns the check's default tokens.
        Specified by:
        getAcceptableTokens in class AbstractCheck
        Returns:
        the token set this check is designed for.
        See Also:
        TokenTypes

      • getRequiredTokens

        public int[] getRequiredTokens()
        Description copied from class: AbstractCheck
        The tokens that this check must be registered for.
        Specified by:
        getRequiredTokens in class AbstractCheck
        Returns:
        the token set this must be registered for.
        See Also:
        TokenTypes

      • beginTree

        public void beginTree​(DetailAST rootAST)
        Description copied from class: AbstractCheck
        Called before the starting to process a tree. Ideal place to initialize information that is to be collected whilst processing a tree.
        Overrides:
        beginTree in class AbstractCheck
        Parameters:
        rootAST - the root of the tree