java设计模式-单例模式

public class Hungry {

    private Hungry(){}
  
    private static final Hungry hungry = new Hungry();

    public static Hungry getInstance(){

        return  hungry;
    }

}

 //在外部需要使用的时候才进行实例化
public class LazyOne {
    private LazyOne(){}


    //静态块，公共内存区域
    private static LazyOne lazy = null;

    public static LazyOne getInstance(){

        //调用方法之前，先判断
        //如果没有初始化，将其进行初始化,并且赋值
        //将该实例缓存好
        if(lazy == null){
            lazy = new LazyOne();
        }
        //如果已经初始化，直接返回之前已经保存好的结果

        return lazy;

    }

}

public class LazyThree {

    //默认使用LazyThree的时候，会先初始化内部类
    //如果没使用的话，内部类是不加载的

    private LazyThree(){}

    //static 是为了使单例的空间共享
    //保证这个方法不会被重写，重载
    public static final LazyThree getInstance(){
        //在返回结果以前，一定会先加载内部类
        return LazyHolder.LAZY;
    }


    //默认不加载
    private static class LazyHolder{
        private static final LazyThree LAZY = new LazyThree();
    }


}

//注册式单例
public class RegisterMap_bxc {
    //相当于本地缓存
    private static Map<String,Object> regist = new HashMap<String,Object>();

    public static RegisterMap_bxc getInstance(String name)
    {
        if(null == name)
        {
            name = RegisterMap_bxc.class.getName();
        }
        synchronized (regist)
        {
            if(!regist.containsKey(name))
            {
                regist.put(RegisterMap_bxc.class.getName(),new RegisterMap_bxc());
            }
            return (RegisterMap_bxc)regist.get(name);
        }

    }

}