Possible Duplicate:
Why should the interface for a Java class be prefered?
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，那么我将失go ArrayList的一些功能，对吗？当试图访问这些方法时，编译器会注意到一个错误吗？
Possible Duplicate:
Why should the interface for a Java class be prefered?
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，那么我将失go ArrayList的一些功能，对吗？当试图访问这些方法时，编译器会注意到一个错误吗？
这样做的主要原因是将代码与接口的特定实现解耦.当您像这样编写代码时:
List list = new ArrayList();
代码的其余部分只知道数据类型为List，这更可取，因为它允许您轻松地在List接口的不同实现之间切换.
例如，假设您正在编写一个相当大的第三方库，并说您决定用LinkedList实现库的核心.如果你的库严重依赖于访问这些列表中的元素，那么最终你会发现你做了一个糟糕的设计决定；你会意识到你应该使用ArrayList(它给出了O(1)访问时间)而不是LinkedList(它给出了O(n)访问时间).假设你一直在编程一个接口，做这样的改变很容易.您只需将List的实例从，
List list = new LinkedList();
到
List list = new ArrayList();
and you know that this will work because you have written your code 到 follow the contract provided by the List interface.
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 到 the LinkedList class.
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 到 make implementation-specific changes later without breaking existing code.