Java static和final的巧妙应用

Java static和final的巧妙应用

监听当前对象被创建了几个..我们可以用一个static来确定个数.(此处说明下为什么要用static:一个类中静态变量只会被初始化一次,不管你实例化多少个类,里面始终只有一个).

下面是使用计数来跟踪仍旧访问着共享对象的对象数量:

package com.glacier.demo; 
 
class Shared { 
    private int refcount = 0; 
    private long counter = 0; 
    private final long id = counter++; 
 
    public Shared() { 
        System.out.println("Creating " + this); 
    } 
 
    public void addRef() { 
        refcount++; 
    } 
 
    protected void dispose() { 
        if (refcount == 0) { 
            System.out.println("Disposing " + this); 
        } 
    } 
 
    public String toString() { 
        return "Shared " + id; 
    } 
} 
 
class Composing { 
    private Shared shared; 
    private static long counter = 0; 
    private final long id = counter++; 
 
    public Composing(Shared shared) { 
        System.out.println("Creating " + this); 
        this.shared = shared; 
        this.shared.addRef(); 
    } 
 
    protected void dispose() { 
        System.out.println("disposing " + this); 
        shared.dispose(); 
    } 
 
    public String toString() { 
        return "Compsing " + id; 
    } 
} 
 
public class Demo { 
 
    /**
    * @param args
    */ 
    public static void main(String[] args) { 
        // TODO Auto-generated method stub 
 
        Shared shared = new Shared(); 
        Composing[] composing = { new Composing(shared), new Composing(shared), 
                new Composing(shared), new Composing(shared), 
                new Composing(shared) }; 
        for (Composing c : composing) { 
            c.dispose(); 
        } 
    } 
 
} 

static long counter 跟踪所创建的shared的实例的数量,还可以为id提供数值.counter的类型是long而不是int,这样可以防止溢出.id是final类型的,我们不希望它在生命周期中改变.

在将一个共享对象附到类上时,必须记住调用addRef(),但是dispose()方法将跟踪引用数,并决定何时执行清理.使用这种技巧需要加倍细心.

看下运行结果:

Creating Shared 0
Creating Compsing 0
Creating Compsing 1
Creating Compsing 2
Creating Compsing 3
Creating Compsing 4
disposing Compsing 0
disposing Compsing 1
disposing Compsing 2
disposing Compsing 3
disposing Compsing 4