单例设计模式

/*
单例设计模式。


*/
//饿汉式。
/*
class Single
{
    private static final Single s = new Single();
    private Single(){}
    public static Single getInstance()
    {
        return s;
    }
}
*/


//懒汉式

class Single
{
    private static Single s = null;
    private Single(){}


    public static  Single getInstance()
    {
        if(s==null)
        {
            synchronized(Single.class)
            {
                if(s==null)
                    //--->A;
                    s = new Single();
            }
        }
        return s;
    }
}

class SingleDemo 
{
    public static void main(String[] args) 
    {
        System.out.println("Hello World!");
    }
}