只生成唯一的对象。
有以下几种实现方式，推荐第一种。

public class Singleton {
    //利用静态内部类的静态变量来初始化唯一对象，且在调用的时候才加载SingleHolder类，实现懒加载的效果，也保证了线程安全
    private static class SingleHolder{
        private static Singleton instance = new Singleton();
    }
    
    public static Singleton getInstance(){
        return SingleHolder.instance;
    }
    
    public static void main(String[] args) {
        Singleton instance01 = Singleton.getInstance();
        Singleton instance02 = Singleton.getInstance();
        System.out.println(instance01 == instance02);
        SingleFive singleFive01 = SingleFive.INSTANCE;
        SingleFive singleFive02 = SingleFive.INSTANCE;
        System.out.println(singleFive01 == singleFive02);
    }
}

//饿汉式单例

class SingleOne{
    private static SingleOne instance = new SingleOne();
    private SingleOne(){}
    public static SingleOne getInstance(){
        return instance;
    }
}

//懒加载，也叫懒汉式单例，致命的是在多线程不能正常工作

class SingleTwo{
    private static SingleTwo instance = null;
    private SingleTwo(){}
    public static SingleTwo getInstance(){
        if(instance == null){
            instance = new SingleTwo();
        }
        return instance;
    }
}

//懒加载，遗憾的是，效率很低，99%情况下不需要同步

class SingleThree{
    private static SingleThree instance;
    private SingleThree(){}
    public static synchronized SingleThree getInstance(){
        if(instance == null){
            instance = new SingleThree();
        }
        return instance;
    }
}

//懒加载，线程安全,双重校验锁法

class SingleFour{
    private volatile static SingleFour instance;
    private SingleFour(){}
    public static SingleFour getInstance(){
        if(instance == null){
            synchronized(SingleFour.class){
                if(instance == null){
                    instance = new SingleFour();
                }
            }
        }
        return instance;
    }
}

//枚举,Effective Java作者Josh Bloch 提倡的方式，它不仅能避免多线程同步问题，而且还能防止反序列化重新创建新的对象

enum SingleFive {  
    INSTANCE;  
}