Java一个枚举类的2种实现。详解编程语言

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); 
    } 
}

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); 
}