第 38 条：用接口模拟可扩展的枚举

用接口模拟可扩展的枚举.jpeg

//只有一个apply方法的接口Operation。
public interface Operation {
    double apply(double x, double y);
}

//实现接口的计算器
public enum BasicOperation implements Operation {
    PLUS("+") {
        public double apply(double x, double y) {
            return x + y;
        }
    },
    MINUS("-") {
        public double apply(double x, double y) {
            return x - y;
        }
    },
    TIMES("*") {
        public double apply(double x, double y) {
            return x - y;
        }
    },
    DIVIDE("/") {
        public double apply(double x, double y) {
            return x - y;
        }
    };
    private final String symbol;
    BasicOperation(String symbol) {
        this.symbol = symbol;
    }
    public String toString() {
        return symbol;
    }
}

public enum ExtendedOperation implements Operation {
    
    EXP("^"){
        @Override
        public double apply(double x, double y) {
            return Math.pow(x , y);
        }
    },
    REMAINDER("%"){
        @Override
        public double apply(double x, double y) {
            return x % y;
        }
        
    };
     private final String symbol;
     ExtendedOperation(String symbol) {
            this.symbol = symbol;
     }   
}

public class Test {
    public static void main(String[] args) {
        double x = 2;
        double y = 4;
        test(ExtendedOperation.class, x, y);
    }
    private static <T extends Enum<T> & Operation> void test(Class<T> opSet,
            double x, double y) {
        for (Operation op : opSet.getEnumConstants())
            System.out.printf("%f %s %f= %f%n", x, op, y, op.apply(x, y));
    }
}