class Single

{

    /**

    方式一、饿汉式

    开发建议使用

    */

    //1.将构造函数私有化

    private Single(){}

    //2.在类中创建对象

    private static Single s=new Single();

    //3.提供一个方法用于获取此类对象

    public static Single getInstance()

    {

        return s;

    }

    /**

    方式二、懒汉式

    */

    private Single(){}

    private static Single s=null;

    //线程不安全

    public static Single getInstance()

    {

        if(s==null)

            s=new Single();

        return s;

    }

    //加同步(效率低)

    public static synchronized Single getInstance()

    {

        if(s==null)

            s=new Single();

        return s;

    }

    //双重判断

    public static Single getInstance()

    {

        if(s==null)

        {

            synchronized(Single.class)

            {

                if(s==null)

                    s=new Single();

            }

        }

        return s;

    }

    private int num;

    public int getNum()

    {

        return num;

    }

    public void setNum(int num)

    {

        this.num=num;

    }

}

class SingleDemo

{

    public static void main (String[] args)

    {

        Single s1=Single.getInstance();

        Single s2=Single.getInstance();

        s1.setNum();

        System.out.println(s2.getNum());

    }

}