Java 8 接口里的默认方法特性

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”。

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：
· https://jcp.org/en/jsr/detail?id=335
· https://docs.oracle.com/javase/tutorial/java/IandI/defaultmethods.html