java 接口可以多继承

叁歲伎倆 2022-08-01 00:28 236阅读 0赞

**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）。**