Java 8的default方法详解

作者：chszs，转载需注明。博客主页：http://blog.csdn.net/chszs

Java 8新增了default方法，它可以在接口添加新功能特性，而且还不影响接口的实现类。下面我们通过例子来说明这一点。

public class MyClass implements InterfaceA {
	public static void main(String[] args){
	}
	
	@Override
	public void saySomething() {
		// TODO Auto-generated method stub
	}
}

interface InterfaceA{
	public void saySomething();
}

Default方法是非常有用的，通过在接口定义的方法的访问修饰符前加上关键字default，那么实现类就无需提供该方法的实现了。比如：

public class MyClass implements InterfaceA {
	public static void main(String[] args){
	}
	
	@Override
	public void saySomething() {
		// TODO Auto-generated method stub
	}
}

interface InterfaceA{
	public void saySomething();
	default public void sayHi(){
		System.out.println("Hi");
	}
}

那么，多接口存在冲突该怎么办？

由于Java类可以实现多个接口，那么就可能存在这样的情况：两个或多个接口都有一个同名的default接口方法，从而造成冲突。因为Java虚拟机在程序运行时，并不清楚你要使用哪一个default方法。这会导致编译错误。

让我们来看看下面的例子。

public class MyClass implements InterfaceA, InterfaceB {
	public static void main(String[] args){
		MyClass mc = new MyClass();
		mc.sayHi();
	}
	
	@Override
	public void saySomething() {
		// TODO Auto-generated method stub
	}
}

interface InterfaceA{
	public void saySomething();
	default public void sayHi(){
		System.out.println("Hi from InterfaceA");
	}
}

interface InterfaceB{
	default public void sayHi(){
		System.out.println("Hi from InterfaceB");
	}
}

“Duplicate default methods named sayHi with the parameters () and () are inherited from the types InterfaceB and InterfaceA.”

除非在MyClass类中重写了sayHi()方法：

public class MyClass implements InterfaceA, InterfaceB {
	public static void main(String[] args){
		MyClass mc = new MyClass();
		mc.sayHi();
	}
	
	@Override
	public void saySomething() {
		// TODO Auto-generated method stub
	}
	
	@Override
	public void sayHi(){
		System.out.println("implemetation of sayHi() in MyClass");
	}
}

interface InterfaceA{
	public void saySomething();
	default public void sayHi(){
		System.out.println("Hi from InterfaceA");
	}
}

interface InterfaceB{
	default public void sayHi(){
		System.out.println("Hi from InterfaceB");
	}
}

public class MyClass implements InterfaceA, InterfaceB {
	public static void main(String[] args){
		MyClass mc = new MyClass();
		mc.sayHi();
	}
	
	@Override
	public void saySomething() {
		// TODO Auto-generated method stub
	}
	
	@Override
	public void sayHi(){
		InterfaceA.super.sayHi();
	}
}

interface InterfaceA{
	public void saySomething();
	default public void sayHi(){
		System.out.println("Hi from InterfaceA");
	}
}

interface InterfaceB{
	default public void sayHi(){
		System.out.println("Hi from InterfaceB");
	}
}