effective_java 第34条：用接口模拟可伸缩的枚举

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;
    }
}