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Java 8 接口里的默认方法特性

这些年Java进化升级了很多，在Java库中引入的接口需要添加新的功能。在没有默认方法特性时，当你往接口中添加新方法时，接口内部所有实现的类都要历经一些修改。这将导致上千行的代码修改工作量。为了避免这点，Java 8引入了默认对象方法。亦即，如果你想要往现存的接口中添加任何功能，你只需添加默认方法特性而不会影响接口实现形式。

让我们看一些例子来更好的理解它。例如，我声明了一个具有打开和读取功能的接口“BookIterface”。接口的类需要实现打开和读取方法。

package org.smarttechie; /** * The interface is intended to open and read. The implementors should implement the methods to open and read. * @author Siva Prasad Rao Janapati * */ public interface BookInterface { /** * The method opens the book */ public void openTheBook(); /** * The method reads the book */ public void readTheBook(); }

现在，我们提供上面接口的实现代码。

package org.smarttechie; /** * The JavaBookImpl is the implementation of BookInterface * @author Siva Prasad Rao Janapati * */ public class JavaBookImpl implements BookInterface { /** * This opens the book */ @Override public void openTheBook() { System.out.println("The Java book is opened"); }  /** * This reads the book */ @Override public void readTheBook() { System.out.println("Reading the Java book");  } }

现在，我们想要给接口提供一个关闭功能。如果你直接添加关闭功能到book接口中，现存的实现类需要历经一些修改。有了默认方法特性后，我们能给book接口直接添加关闭功能。默认方法对所有实现都可用。

package org.smarttechie; /** * The interface is intended to open and read. The implementors should implement the methods to open and read. * @author Siva Prasad Rao Janapati * */ public interface BookInterface { /** * The method opens the book */ public void openTheBook(); /** * The method reads the book */ public void readTheBook(); /** * The default method implementation */ public default void closeTheBook() { System.out.println("Closting the book");  } }

package org.smarttechie; /**  * The JavaBookImpl is the implementation of BookInterface  * @author Siva Prasad Rao Janapati  *  */ public class JavaBookImpl implements BookInterface {  /**  * This opens the book  */  @Override  public void openTheBook() {  System.out.println("The Java book is opened");  }  /**  * This reads the book  */  @Override  public void readTheBook() {  System.out.println("Reading the Java book");  }  public static void main (String[] args) {  BookInterface bookInter = new JavaBookImpl();  //Call the default method declared in BookInterface  bookInter.closeTheBook();  JavaBookImpl book = new JavaBookImpl();  book.closeTheBook();  } }

下面给出了上述调用方法的字节码。从字节码中，我们可以认为默认方法是一种“虚方法”。

如果你想，你可以重载实现类中的默认方法：

package org.smarttechie; /**  * The JavaBookImpl is the implementation of BookInterface  * @author Siva Prasad Rao Janapati  *  */ public class JavaBookImpl implements BookInterface {  /**  * This opens the book  */  @Override  public void openTheBook() {  System.out.println("The Java book is opened");  }  /**  * This reads the book  */  @Override  public void readTheBook() {  System.out.println("Reading the Java book");  }  /*  * This closes the book  */  public void closeTheBook() {  System.out.println("Closing the JAVA book");  }  public static void main (String[] args) {  BookInterface book = new JavaBookImpl();  book.closeTheBook();  } }

到这会儿，你可能有一个疑问，如果我们实现了两个具有同样默认方法签名的接口会怎样？在那种情况下，调用实现会得到下面的编译错误提示。

“用参数()和()复制名为closedTheBook的默认方法是继承于TheBookInterface和BookInterface的类型。”

package org.smarttechie; public interface TechBookInterface { /** * The default method implementation */ public default void closeTheBook() { System.out.println("Closing the book");  } }

package org.smarttechie; /** * The JavaBookImpl is the implementation of BookInterface * @author Siva Prasad Rao Janapati * */ public class JavaBookImpl implements BookInterface, TechBookInterface { /** * This opens the book */ @Override public void openTheBook() { System.out.println("The Java book is opened"); } /** * This reads the book */ @Override public void readTheBook() { System.out.println("Reading the Java book"); } public static void main (String[] args) { BookInterface book = new JavaBookImpl(); book.closeTheBook();  } }

为了避免这个编译错误，我们需要在实现类中显式地定义那个具有同样签名的方法。

package org.smarttechie; /** * The JavaBookImpl is the implementation of BookInterface * @author Siva Prasad Rao Janapati * */ public class JavaBookImpl implements BookInterface, TechBookInterface { /** * This opens the book */ @Override public void openTheBook() { System.out.println("The Java book is opened"); } /** * This reads the book */ @Override public void readTheBook() { System.out.println("Reading the Java book"); } public void closeTheBook() { System.out.println("Closing the JAVA book"); } public static void main (String[] args) { BookInterface book = new JavaBookImpl(); book.closeTheBook();  } }

更深入的了解阅读，可以参考下面的链接：

原文 http://www.topthink.com/topic/23474.html

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