How to implement singleton pattern using Java enumerations?

1. Implementation steps

(1) The internal class uses enumeration

(2) Initialize in the constructor of the enumeration class

(3) Get the singleton method provided by the outside

(4) Get the singleton object, return and test

2, instance

public class SingletonObj {
    //内部类使用枚举
    private enum SingletonEnum {
        INSTANCE;
 
        private SingletonObj singletonObj;
//在枚举类的构造器里初始化singletonObj
        SingletonEnum() {
            singletonObj = new SingletonObj();
        }
 
        private SingletonObj getSingletonObj() {
            return singletonObj;
        }
    }
 
    //对外部提供的获取单例的方法
    public static SingletonObj getInstance() {
        //获取单例对象，返回
        return SingletonEnum.INSTANCE.getSingletonObj();
    }
 
    //测试
    public static void main(String[] args) {
        SingletonObj a = SingletonObj.getInstance();
        SingletonObj b = SingletonObj.getInstance();
        System.out.println(a == b);//true
    }
}

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