单例模式

2008-03-19

[原]单例模式

关键字: 单例模式

下面代码中的理解:
Singleton类之所以是private型构造方法,就是为了防止其它类通过new来创建实例,即如此,那我们就必须用一个static的方法来创建一个实例(为什么要用static的方法?因为既然其它类不能通过new来创建实例,那么就无法获取其对象,那么只用有类的方法来获取了)

class Singleton {
     private static Singleton instance;
     private Singleton(){}
     public static Singleton getInstance(){
         if(instance==null){
             instance = new Singleton();
         }
         return instance;
     }
     public void say(){
         System.out.println("单例模式");
     }
}

public class SingletonDemo{
    public static void main(String[] args) {
        Singleton s1 = Singleton.getInstance();
        //再次getInstance()的时候,instance已经被实例化了
        //所以不会再new,即s2指向刚初始化的实例
        Singleton s2 = Singleton.getInstance();
        System.out.println(s1==s2);
        s1.say();
        s2.say();
    }
}

class Singleton {
	 private static Singleton instance;
	 private Singleton(){}
	 public static Singleton getInstance(){
		 if(instance==null){
			 instance = new Singleton();
		 }
		 return instance;
	 }
	 public void say(){
		 System.out.println("单例模式");
	 }
}

public class SingletonDemo{
	public static void main(String[] args) {
		Singleton s1 = Singleton.getInstance();
		//再次getInstance()的时候,instance已经被实例化了
		//所以不会再new,即s2指向刚初始化的实例
		Singleton s2 = Singleton.getInstance();
		System.out.println(s1==s2);
		s1.say();
		s2.say();
	}
}

另一种写法:
注意下面注释掉的部分没有final,在测试中发现一个问题就是有final与没有final最后的测试输出都为ture,不解,后来问题的一个高人,经指点,得知单态的情况下.有final与没有final在于后面的修改,而不是new与不new的问题.final在这个例子中是表示那个instance在后面是不可以修改的

class Singleton {
    //private static Singleton instance= new Singleton();
    private static final Singleton instance= new Singleton();
     private Singleton(){}
     public static Singleton getInstance(){
         return instance;
     }
     public void say(){
         System.out.println("单例模式");
     }
}

public class SingletonDemo{
    public static void main(String[] args) {
        Singleton s1 = Singleton.getInstance();
        Singleton s2 = Singleton.getInstance();
        System.out.println(s1==s2);
        s1.say();
        s2.say();
    }
}

class Singleton {
	//private static Singleton instance= new Singleton();
	private static final Singleton instance= new Singleton();
	 private Singleton(){}
	 public static Singleton getInstance(){
		 return instance;
	 }
	 public void say(){
		 System.out.println("单例模式");
	 }
}

public class SingletonDemo{
	public static void main(String[] args) {
		Singleton s1 = Singleton.getInstance();
		Singleton s2 = Singleton.getInstance();
		System.out.println(s1==s2);
		s1.say();
		s2.say();
	}
}

转载自  http://singlewolf.iteye.com