`equals` method is overloaded but not overriddenJAVA-E0098

This class defines a version of the equals method which does not take an Object as its parameter, meaning it overloads the original Object.equals(Object) method instead of overriding it. Standard library collections will behave unexpectedly when handling values of this type.

Bad Practice

class MyObj {

    int val;

    public MyObj(int val) {
        this.val = val;
    }

    // Parameter is of type MyObj, not Object.
    public boolean equals(MyObj other) {
        // ...
    }
}

This can lead to subtle logic errors, because other code, such as standard library containers may use only the default equals(Object) function and not the class specific overload. For example, the following code will not work as expected:

List<MyObj> myObjs = Arrays.asList(new MyObj(42));

assert(myObjs.contains(new MyObj(42)) == true); // Error!

It seems obvious that myObjs contains the same value, and that the above code should have run successfully. However, List.contains will not take the overloaded equals method into account, and will use the default java.lang.Object.equals implementation which simply compares reference addresses instead.

Recommended

There are rarely any cases where an overloaded equals method alone has any advantages over just overriding the default implementation. Just override equals(Object) instead. If you believe it is better to have an overload specific to a particular class, consider override equals(Object) and calling your overriding method from it to ensure that equals always behaves predictably.

public boolean equals(MyObj other) {
    // ...
}


public boolean equals(Object other) {

    if (other instanceof MyObj) return this.equals((MyObj) other);
    else // ...
}

References

• Best practices regarding equals - stackoverflow
• SpotBugs - EQ_OTHER_USE_OBJECT