接口是常量值和方法定义的集合。接口是一种特殊的抽象类。
java类是单继承的。classB Extends classA
java接口可以多继承。Interface3 Extends Interface0, Interface1, interface……
以下是spring ApplicationContext 接口的代码，同时继承了多个接口
public interface ApplicationContext extends EnvironmentCapable, ListableBeanFactory, HierarchicalBeanFactory,MessageSource, ApplicationEventPublisher, ResourcePatternResolver {/*** Return the unique id of this application context.* @return the unique id of the context, or {@code null} if none*/String getId(); /*** Return a name for the deployed application that this context belongs to.* @return a name for the deployed application, or the empty String by default*/String getApplicationName(); /*** Return a friendly name for this context.* @return a display name for this context (never {@code null})*/String getDisplayName(); /*** Return the timestamp when this context was first loaded.* @return the timestamp (ms) when this context was first loaded*/long getStartupDate(); /*** Return the parent context, or {@code null} if there is no parent* and this is the root of the context hierarchy.* @return the parent context, or {@code null} if there is no parent*/ApplicationContext getParent(); /*** Expose AutowireCapableBeanFactory functionality for this context.* <p>This is not typically used by application code, except for the purpose of* initializing bean instances that live outside of the application context,* applying the Spring bean lifecycle (fully or partly) to them.* <p>Alternatively, the internal BeanFactory exposed by the* {@link ConfigurableApplicationContext} interface offers access to the* {@link AutowireCapableBeanFactory} interface too. The present method mainly* serves as a convenient, specific facility on the ApplicationContext interface.* <p><b>NOTE: As of 4.2, this method will consistently throw IllegalStateException* after the application context has been closed.</b> In current Spring Framework* versions, only refreshable application contexts behave that way; as of 4.2,* all application context implementations will be required to comply.* @return the AutowireCapableBeanFactory for this context* @throws IllegalStateException if the context does not support the* {@link AutowireCapableBeanFactory} interface, or does not hold an* autowire-capable bean factory yet (e.g. if {@code refresh()} has* never been called), or if the context has been closed already* @see ConfigurableApplicationContext#refresh()* @see ConfigurableApplicationContext#getBeanFactory()*/AutowireCapableBeanFactory getAutowireCapableBeanFactory() throws IllegalStateException; }
不允许类多重继承的主要原因是，如果A同时继承B和C，而B和C同时有一个D方法，A如何决定该继承那一个呢?
但接口不存在这样的问题，接口全都是抽象方法继承谁都无所谓，所以接口可以继承多个接口。
注意：
1.一个类如果实现了一个接口，则要实现该接口的所有方法。
2.方法的名字、返回类型、参数必须与接口中完全一致。如果方法的返回类型不是void，则方法体必须至少有一条return语句。
3.因为接口的方法默认是public类型的，所以在实现的时候一定要用public来修饰(否则默认为protected类型，缩小了方法的使用范围)。
