`equals` method defined for enumerationJAVA-E0096

This enum defines an overload for the equals method using the enum's own class type. The equals() method of an enum is not meant to be overloaded (or overridden), and doing so may cause weird bugs to crop up when values of this enum are compared.

Equality on enumerations is defined using object identity, that is, the address of the object. This works because each variant of an enum is essentially a static final instance of that enum class with data corresponding to the declared variant. All usages of an `enum's variants point to the same set of static constants.

Overloading the equals method for an enum value is exceptionally bad practice since the more specialized overloaded method will always be chosen when comparing enum variants against each other. Unless the enum value is cast to Object before comparison, the original equals method will not be used.

Bad Practice

enum TestEnum {
    First("st"),
    Second("nd"),
    Third("rd"),
    Fourth("th"),
    Fifth("th"),
    Sixth("th"),
    Seventh("th"),
    Eighth("th"),
    Ninth("th");

    String notation;

    TestEnum(String n) {
        notation = n;
    }

    // Overload of equals(Object).
    public boolean equals(TestEnum other) {
        return this.notation == other.notation;
    }
}

// ...

assertTrue(TestEnum.Sixth.equals((Object)TestEnum.Ninth));  // False...
assertTrue(TestEnum.Sixth.equals(TestEnum.Ninth));          // True?!

Recommended

Remove the unnecessary equals method.

enum TestEnum {
    First("st"),
    Second("nd"),
    Third("rd"),
    Fourth("th"),
    Fifth("th"),
    Sixth("th"),
    Seventh("th"),
    Eighth("th"),
    Ninth("th");

    String notation;

    TestEnum(String n) {
        notation = n;
    }
}

References

• SpotBugs - EQ_DONT_DEFINE_EQUALS_FOR_ENUM