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Java 8 接口里的默认方法特性研究


 

   

  这篇文章我们将要探讨Java 8中接口里的默认方法特性。Java8指出“默认方法使得新功能被添加到库中的接口里面,同时又能保证与这些接口老版本代码的二进制兼容性。

  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”。

  Java 8 接口里的默认方法特性

  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();

  }

  }