Why is compareTo on an Enum final in Java?

Question

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

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