Enum method overriding

Question

I've found Enums defined like the following:

public Enum MyEnum {

   ONE
   {
      @Override
      public int getSomething() {
         return 1;
      } 
   },

   TWO
   {
      @Override
      public int getSomething() {
        return 2;
      }
   }

   int getSomething()
   {
      return 0;
   }
}

Somehow I feel some type of discomfort with this implementation because I would think that ideally a field should be defined for this purpose and the class should look something like:

public Enum MyEnum{

   ONE(1),
   TWO(2)

   private int theSomething;

   private MyEnum(int something) {
      theSomething = something;
   }

   int getSomething()
   {
      return theSomething;
   }
}

The problem is that apart from personal discomfort I cannot find any good reason to change this code. Do any exists?

Solution 1

(moved from comment)

Your first example is used commonly to implement a finite state machine in Java. It eliminates the need for every method having to have a if (state == FOO) {} else if (state == BAR) etc

class MyFSM {

    enum State {
        FIRST_STATE {
            @Override
            void start(MyFSM fsm) {
                fsm.doStart();
            }
            @Override
            void stop(MyFSM fsm) {
                throw new IllegalStateException("Not Started!");
            }
        },
        SECOND_STATE {
            @Override
            void start(MyFSM fsm) {
                throw new IllegalStateException("Already Started!");
            }
            @Override
            void stop(MyFSM fsm) {
                fsm.doStop();
            }
        };

        abstract void start(MyFSM fsm);
        abstract void stop(MyFSM fsm);       
    }

    private volatile State state = State.FIRST_STATE;

    public synchronized void start() {
        state.start(this);
    }

    private void doStart() {
        state = SECOND_STATE;
    }

    public synchronized void stop() {
        state.stop(this);
    }

    private void doStop() {
        state = FIRST_STATE;
    }
}

Solution 2

The first pattern is slightly better for "default" methods that don't all need to be overridden.

public enum Modes {
    MODE_ONE {
        @Override public boolean canDoA() {
            return true;
        }
    },
    MODE_TWO {
        @Override public boolean canDoB() {
            return true;
        }
    },
    MODE_THREE {
        @Override public boolean canDoC() {
            return true;
        }
    };

    public boolean canDoA() {
        return false;
    }

    public boolean canDoB() {
        return false;
    }

    public boolean canDoC() {
        return false;
    }

}

Comments

[Tom]

The second form makes it more readable and more extendable, go for that one!

[Brian Roach]

It's a state machine in your first example. Not implemented quite right (getSomething() should be abstract ) ... but that's why you use that type of structure. And the methods are generally far more complex in a real example (they actually do things rather than returning static values) and/or throw IllegalStateException when they shouldn't be called in a current state.

[Louis Wasserman]

Both of these are valid approaches.

[Sednus]

The case is using a simple enum, just getter methods. But good to know this, thanks!

[Brian Roach]

Then it's overkill, IMHO. Valid ... but overkill. The above is the only reason I'd use it.