generic type variable

基本解释

So far, when dealing with collections, you've seen for/in used with a generic variable type, like Object.
迄今为止，在处理集合的时候，您已经看到 for/in 使用通用的变量类型，例如 Object。
As you can see the type of the class variables is generic, but you can improve the generator to understand if a variable should contain a number or a function.
如你所见，类的成员变量的类型是通用类型，但是你可以改进产生器，使之知道一个变量应该包含一个数字或一个函数。
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Because we'd like our Lists to be generic in the type of element they contain, we'd like our static factory method to take an argument of type variable T and return an instance of List.
因为我们希望 List 成为其所包含的元素类型的泛型，所以希望静态 factory 方法带有类型变量 T 这一参数并返回 List 的实例。