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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. * 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. *
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. *
NOTE: As of 4.2, this method will consistently throw IllegalStateException * after the application context has been closed. 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）。
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