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，Java8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。那么java 8 接口默认方法有哪些特性，跟yjbys小编一起来来看看吧!

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();

}

}