EqualsHashCode
Since Checkstyle 3.0
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="Checker">
<module name="TreeWalker">
<module name="EqualsHashCode"/>
</module>
</module>
Example:
class Example1 {
public int hashCode() { // violation, no valid 'equals'
return 0;
}
public boolean equals(String o) { return false; }
}
class ExampleNoHashCode {
public boolean equals(Object o) { // violation, no 'hashCode'
return false;
}
public boolean equals(String o) { return false; }
}
class ExampleBothMethods1 {
public int hashCode() { return 0; }
public boolean equals(Object o) { // ok, both methods exist
return false;
}
public boolean equals(String o) { return false; }
}
class ExampleBothMethods2 {
public int hashCode() { return 0; }
public boolean equals(java.lang.Object o) { // ok, both methods exist
return false;
}
}
class ExampleNoValidHashCode {
public static int hashCode(int i) { return 0; }
public boolean equals(Object o) { // violation, no valid 'hashCode'
return false;
}
}
class ExampleNoValidEquals {
public int hashCode() { // violation, no valid 'equals'
return 0;
}
public static boolean equals(Object o, Object o2) { return false; }
}
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.coding