接口和抽象类 : 接口 « 类定义 « Java 教程

1. The interface should be regarded as a contract between a service provider and its clients.
2. An abstract class is a class that cannot be instantiated
3. An abstract class must be implemented by a subclass.
4. In Java, the interface is a type.

Follow this format to write an interface:

1. The Printable interface has a method, print.
2. print is public even though there is no public keyword.