多态性:为什么要使用“List list = new ArrayList"?而不是“ArrayList list = new ArrayList"? [英] Polymorphism: Why use "List list = new ArrayList" instead of "ArrayList list = new ArrayList"?

问题描述

可能重复:
为什么要首选 Java 类的接口?

我应该什么时候使用

List<Object> list = new ArrayList<Object>();

ArrayList 继承自 List，所以如果 ArrayList 中的某些功能不在 List 中，那么我会失去 ArrayList 的一些特性，对吧?并且编译器在尝试访问这些方法时会注意到错误?

ArrayList inherits from List, so if some features in ArrayList aren't in List, then I will have lost some of the features of ArrayList, right? And the compiler will notice an error when trying to access these methods?

推荐答案

这样做的主要原因是将代码与接口的特定实现分离.当您像这样编写代码时:

The main reason you'd do this is to decouple your code from a specific implementation of the interface. When you write your code like this:

List list = new ArrayList();  

您的其余代码只知道数据是 List 类型的，这是更可取的，因为它允许您轻松地在 List 接口的不同实现之间切换.

the rest of your code only knows that data is of type List, which is preferable because it allows you to switch between different implementations of the List interface with ease.

例如，假设您正在编写一个相当大的第 3 方库，并假设您决定使用 LinkedList 来实现库的核心.如果你的库严重依赖访问这些列表中的元素，那么最终你会发现你做出了一个糟糕的设计决定；你会意识到你应该使用 ArrayList (它给 O(1) 访问时间)而不是一个 LinkedList (它给 O(n) 访问时间).假设您一直在对接口进行编程，那么进行这样的更改很容易.您只需将 List 的实例从，

For instance, say you were writing a fairly large 3rd party library, and say that you decided to implement the core of your library with a LinkedList. If your library relies heavily on accessing elements in these lists, then eventually you'll find that you've made a poor design decision; you'll realize that you should have used an ArrayList (which gives O(1) access time) instead of a LinkedList (which gives O(n) access time). Assuming you have been programming to an interface, making such a change is easy. You would simply change the instance of List from,

List list = new LinkedList();

到

List list = new ArrayList();  

并且您知道这会起作用，因为您已经编写了代码以遵循 List 接口提供的合同.

and you know that this will work because you have written your code to follow the contract provided by the List interface.

另一方面，如果您使用 LinkedList list = new LinkedList() 实现了库的核心，那么进行这样的更改就不会那么容易了，因为无法保证您的其余代码不使用特定于 LinkedList 类的方法.

On the other hand, if you had implemented the core of your library using LinkedList list = new LinkedList(), making such a change wouldn't be as easy, as there is no guarantee that the rest of your code doesn't make use of methods specific to the LinkedList class.

总而言之，选择只是设计问题...但是这种设计非常重要(尤其是在处理大型项目时)，因为它允许您稍后进行特定于实现的更改而不会破坏现有的代码.

All in all, the choice is simply a matter of design... but this kind of design is very important (especially when working on large projects), as it will allow you to make implementation-specific changes later without breaking existing code.

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