public enum Operation {
    PLUS, MINUS, TIMES, DIVIDE;

    double apply(double x, double y) {
        switch (this) {
            case PLUS:
                return x + y;
            case MINUS:
                return x - y;
            case TIMES:
                return x * y;
            
 
case DIVIDE:
                return x / y;
        }
        throw new AssertionError("Unknow op:" + this);
    }
}

實現方式二：

特定於常量的方法實現（constant-specific method implementation）

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);
}

Java一個枚舉類的2種實現。