The reference must exist in the test and precede the conditional block.
这个引用必须存在于测试中并且在条件块前。
In most cases, a conditional block issues user input actions depending on the value of a reference.
大多数情况下，一个条件块依靠一个引用的值发出用户输入动作。
By arranging for objects of different types to implement common methods, we move the dispatch decision into the objects and out of an explicit conditional block.
通过安排不同类型的对象来执行公共方法，我们将分派决定移到对象中并从显式条件块中消除。
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