从PHP V 4迁移到PHP v5时要利用的下一个重要特性是支持通过接口、抽象类和方法进行契约编程。

The next important feature to take advantage of when migrating from PHP V4 to V5 is support for contract programming through interfaces and abstract classes and methods.

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Authenticator 是一个抽象类，所以需要从它派生出子类，并实现 getPasswordAuthentication() 方法来存储用户名和密码（见清单 13）。

The Authenticator class is abstract, so you'll need to subclass it and implement the getPasswordAuthentication() method to store the username and password (see Listing 13).

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我们不能实例化一个抽象类，即使它们的字段和方法都是具体的。

We can't instantiate instances of abstract classes, even when all their field and method members are concrete.

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