testa.java

Singleton的java实现！

/**
 *  A test for SingletonA
 */
public class Test  {
    public static void main(String[] args) {
        // Can not create a instance !
        //SingletonA instance1 = new SingletonA();  
        Singleton instance2 = Singleton.getInstance();
        if(instance2 != null) {
            instance2.setNum();
            System.out.println("i is:" + instance2.getNum());
        }
        Singleton instance3 = Singleton.getInstance();
        if(instance3 == null) {
            System.out.println("Can not get instance twice !");   
        }
    }
}