enum type with constant-specific method implementations

public enum Operation {
   PLUS   { double apply(double x, double y) {return x + y;} },
   MINUS  { double apply(double x, double y) {return x - y;} },
   TIMES  { double apply(double x, double y) {return x * y;} },
   DIVIDE { double apply(double x, double y) {return x / y;} };

   abstract double apply(double x, double y);
}

References
- Effective Java 2nd Edition