Pay Attention to the Reference of Java

   

     今天调试了个这样的一个问题，在Java中很容易就会忽略了引用的问题，一不小心就被会这实质为指针的引用给隐藏了臭虫，若没调试我就中了这招。

我用以下的例子来展现这样的问题：

我们定义两个类：Drawer和Apple

/**
 * Object Apple
 */
public class Apple {
	private String color;

	public String getColor() {
		return color;
	}

	public void setColor(String color) {
		this.color = color;
	}
}

 

/**
 * Object Drawer that is used to put apples.
 * Actually, it is used to put appleBox which is used to put apples.
 */
public class Drawer {
	private List<Apple> appleBox;

	public Drawer() {
		appleBox=new ArrayList<Apple>();
	}

	public Drawer(List<Apple> appleBox) {
		this.appleBox = appleBox;
	}
	
	public void putAppleToBox(Apple apple){
		this.appleBox.add(apple);
	}
	
	public void setAppleBox(List<Apple> appleBox) {
		this.appleBox = appleBox;
	}

	public List<Apple> getAppleBox(){
		return appleBox;
	}
}

      Our test case is :

    public void testDrawer(){		
		Drawer drawer=new Drawer();
		Apple apple=new Apple();
		apple.setColor("Red");
		drawer.putAppleToBox(apple);
		System.out.println(drawer.getAppleBox().get(0).getColor());
		System.out.println("---------");
		
		Drawer drawer2=new Drawer();
		drawer2.setAppleBox(drawer.getAppleBox());//关键所在	
		Apple apple2=new Apple();
		apple2.setColor("Yellow");
		drawer2.putAppleToBox(apple2);
		System.out.println(drawer.getAppleBox().size());
		System.out.println(drawer2.getAppleBox().size());
		System.out.println("---------");
		drawer2.getAppleBox().get(0).setColor("Green");
		System.out.println(drawer.getAppleBox().get(0).getColor());
		System.out.println(drawer.getAppleBox().get(1).getColor());
		System.out.println(drawer2.getAppleBox().get(0).getColor());
		System.out.println(drawer2.getAppleBox().get(1).getColor());		
	}

 The test result will be:

Red

---------

2

2

---------

Green

Yellow

Green

Yellow

 经验丰富的程序员可能一眼就看出来这样一个实质问题：这里的AppleBox的ArrayList实例只有一个。两个抽屉里面的appleBox引用指向的是同一个ArrayList实例。

这里有个误区，就是以为new了两个抽屉的实例就以为万事大吉了。

当使用drawer2.setAppleBox(drawer.getAppleBox());时，只是将drawer2的appleBox引用指向drawer的appleBox指向的实例。

我们再简化上面的程序：

Apple不变，讲抽屉简化只能放一个苹果，去掉AppleBox这个类：

/**
 * Object Drawer that is used to put one apple.
 */
public class Drawer {
	private Apple apple;

	public Drawer() {		
	}

	public void putApple(Apple apple){
		this.apple=apple;
	}
	
	public Apple getApple(){
		return apple;
	}
}

   Our Test case :

    public void testDrawer(){

		Drawer drawer=new Drawer();
		Apple apple=new Apple();
		apple.setColor("Red");
		drawer.putApple(apple);
		System.out.println(drawer.getApple().getColor());
		
		Drawer drawer2=new Drawer();
		drawer2.putApple(drawer.getApple());
		drawer2.getApple().setColor("Green");
		System.out.println(drawer.getApple().getColor());
		System.out.println(drawer2.getApple().getColor());			
    }

 The test result will be:

Red

Green

Green

其实实质是一样的，这里的Apple实例只有一个，两个抽屉的apple引用指向同一个Apple实例。

总结：

1. Java中的Reference实质就是指针；

2. 使用他，你得特别地注意了。