"Operation" Enum

public enum Operation {
    PLUS {
          double eval(double x, double y) { return x + y; }
    },
      MINUS {
        double eval(double x, double y) { return x - y; }
      },
    TIMES {
          double eval(double x, double y) { return x * y; }
    },
      DIVIDED_BY {
        double eval(double x, double y) { return x / y; }
      };
    // Perform the arithmetic operation represented by this constant
      abstract double eval(double x, double y);
    public static void main(String args[]) {
          double x = 2.0;
        double y = 4.0;
  
        for (Operation op : Operation.values())
            System.out.println(x + " " + op + " " + y + " = " + op.eval(x, y));
      }
}

The document says it is anticipated that the need for this will be rare and it it a bit tricky.
  How people think it is tricky?


Reference

1. Java Lang Spec