正文
effective_java 第34条：用接口模拟可伸缩的枚举
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例如：
/**
* 加减乘除枚举
* Created by yulinfeng on 8/20/17.
*/
public enum Operation {
PLUS {
double apply(double x, double y) {
return x + y;
}
},
MIUS {
double apply(double x, double y) {
return x - y;
}
},
TIMES {
double apply(double x, double y) {
return x * y;
}
},
DEVIDE {
double apply(double x, double y) {
return x / y;
}
}; abstract double apply(double x, double y);
}
同时提到当需要新增一个操作符时直接在源代码中新增即可，且不会忘记重写apply方法，但从软件开发的可扩展性来说这并不是一个好的方法，软件可扩展性并不是在原有代码上做修改，如果这段代码是在jar中的呢？这个时候就需要接口出场了，我们修改上述例子：
/**
* 操作符接口
* Created by yulinfeng on 8/20/17.
*/
public interface Operation {
double apply(double x, double y);
}
/**
* 基本操作符，实现自Operation接口
* Created by yulinfeng on 8/20/17.
*/
public enum BasicOperation implements Operation{
PLUS("+") {
public double apply(double x, double y) {
return x + y;
}
},
MIUS("-") {
public double apply(double x, double y) {
return x - y;
}
},
TIMES ("*") {
public double apply(double x, double y) {
return x * y;
}
},
DEVIDE ("/") {
public double apply(double x, double y) {
return x / y;
}
};
private final String symbol; BasicOperation(String symbol) {
this.symbol = symbol;
}
}
当我们需要扩展操作符枚举的时候只需要重新实现Operation接口即可：
/**
* 扩展操作符
* Created by yulinfeng on 8/20/17.
*/
public enum ExtendedOperation implements Operation {
EXP ("^") {
public double apply(double x, double y) {
return Math.pow(x, y);
}
},
REMAINDER ("%") {
public double apply(double x, double y) {
return x % y;
}
};
private final String symbol; ExtendedOperation(String symbol) {
this.symbol = symbol;
}
}
