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# 任务6：提炼计算函数

好，走到这里，我们已经离一开始前进了许多：奇怪的命名不见了，魔法字符串已经被更表意的枚举类型`Unit`替代，表示英寸的`f`也被很轻松地改成了`Unit.FOOT`，收获不小！

接下来的任务，是消除一个不太明显的味道，也就是在`as()`方法中重复的`new Length`创建逻辑。仔细观察，其实`Length`对象的创建过程，唯一区别是在计算不同的`value`值。不过，要消除重复的对象创建代码，我们还得做点准备工作，那就是把这部分关于`value`的计算逻辑先剥离出来。

![](/files/-Lr9XDRLzUiQlC5pzeoo)

## 你的任务

Java：

```
git checkout task-6
./gradlew test
```

JavaScript：

```
git checkout task-6
npm test
```

1. 动手之前，先花几分钟思考一下重构步骤：如何做到平滑替换？
2. 严格按照重构步骤执行重构，频繁运行测试
3. `git checkout final`看一下参考步骤，找出与你设定步骤的异同
4. 多练几遍

## 参考资料

* 《重构 2》6.1 提炼函数（Extract Function）
* 《重构 2》10.3 以卫语句取代嵌套条件表达式（Replace Nested Conditional with Guard Clauses）


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