第六章：枚举和注解。ITEM34：用接口模拟可伸缩的枚举。

1 package com.twoslow.cha6;
2
3 public interface OperationInterface {
4
5     double apply(double x , double y) ;
6 }

 1 package com.twoslow.cha6;
 2
 3 public enum BaseOperation implements OperationInterface{
 4
 5     PLUS("+"){
 6         public double apply(double x , double y ) {return x + y ;}
 7     },
 8     MINUS("-"){
 9         public double apply(double x , double y ) {return x - y ;}
10     },
11     TIMES("*"){
12         public double apply(double x , double y ) {return x * y ;}
13     },
14     DIVIDE("/"){
15         public double apply(double x , double y ) {return x / y ;}
16     } ;
17
18     private final String symbol ;
19
20     BaseOperation(String symbol) {
21         this.symbol = symbol ;
22     }
23 }

 1 package com.twoslow.cha6;
 2
 3 public enum ExtendedOperation implements OperationInterface{
 4
 5     EXP("^") {
 6         public double apply(double x, double y){return Math.pow(x, y) ;};
 7     } ,
 8
 9     REMAINDER("%") {
10         public double apply(double x , double y ) {return x % y ;} ;
11     } ;
12
13
14     private final String symbol ;
15
16     ExtendedOperation(String symbol) {
17         this.symbol = symbol ;
18     }
19 }

 1 package com.twoslow.cha6;
 2
 3 import java.util.Arrays;
 4 import java.util.Collection;
 5
 6 public class MainTest {
 7
 8     public static void main(String[] args) {
 9 //        test(ExtendedOperation.class , 3,2) ;
10
11         test2(Arrays.asList(BaseOperation.values()),3,2) ;
12     }
13
14     //泛型声明确保对象既表示枚举又表示Operation的子类型
15     private static <T extends Enum<T> & OperationInterface> void test(Class<T> opSet , double x , double y ) {
16         for(OperationInterface op : opSet.getEnumConstants()) {
17             System.out.println(op.apply(x, y));
18         }
19     }
20
21     private static  void test2(Collection<? extends OperationInterface> opSet , double x , double y ) {
22         for(OperationInterface op : opSet) {
23             System.out.println(op.apply(x, y));
24         }
25     }
26 }