Why is compareTo on an Enum final in Java?

comparable, compareto, enums, java

Solution

For consistency I guess... when you see an `enum` type, you know for a fact that its natural ordering is the order in which the constants are declared.

To workaround this, you can easily create your own `Comparator<MyEnum>` and use it whenever you need a different ordering:

enum MyEnum
{
    DOG("woof"),
    CAT("meow");

    String sound;    
    MyEnum(String s) { sound = s; }
}

class MyEnumComparator implements Comparator<MyEnum>
{
    public int compare(MyEnum o1, MyEnum o2)
    {
        return -o1.compareTo(o2); // this flips the order
        return o1.sound.length() - o2.sound.length(); // this compares length
    }
}

You can use the `Comparator` directly:

MyEnumComparator comparator = new MyEnumComparator();
int order = comparator.compare(MyEnum.CAT, MyEnum.DOG);

or use it in collections or arrays:

NavigableSet<MyEnum> set = new TreeSet<MyEnum>(comparator);
MyEnum[] array = MyEnum.values();
Arrays.sort(array, comparator);

Further information:

- The Java Tutorial on Enum Types

- Sun's Guide to Enums

- Class Enum API

Problem

An enum in Java implements the `Comparable` interface. It would have been nice to override `Comparable`'s `compareTo` method, but here it's marked as final. The default natural order on `Enum`'s `compareTo` is the listed order. Does anyone know why a Java enums have this restriction?

Original source