Package com.puppycrawl.tools.checkstyle.checks.coding

Class CovariantEqualsCheck

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

    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.

    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;
   }
 }

    Parent is com.puppycrawl.tools.checkstyle.TreeWalker

    Violation Message Keys:

    • covariant.equals
    Since:
    3.2

    • 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

      • CovariantEqualsCheck

        public CovariantEqualsCheck()

    • Method Detail

      • 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

      • 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

      • 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