Enumeration class implements singleton mode

1. The singleton pattern

I believe everyone is familiar with the singleton mode , Here is an example of using enumeration classes to implement the singleton pattern .

1.1 Hungry Han style single case mode

public class YYSingletonDemo {
    
    private YYSingleton() {
    
    }

    private enum YYSingleton {
    
        INSTANCE; 
    }

    public static YYSingleton getInstance() {
    
        return YYSingleton.INSTANCE;
    }
}

1.2 Lazy singleton pattern

public class YYSingleton {
    

    private YYSingleton() {
    

    }

    private enum YYSingletonHolder {
    
        INSTANCE;
        private  YYSingleton instance = null;
        private YYSingleton getYYSingleton() {
    
            instance = new YYSingleton();
            return instance;
        }
    }

    public static YYSingleton getInstance() {
    
        return YYSingletonHolder.INSTANCE.instance;
    }
}

2. Why is using enum classes thread safe singleton mode ？

Due to the particularity of enumeration classes , Ensure the uniqueness of enumeration types . Because the enumeration class will be loaded at compile time , Therefore, it belongs to the starving Han style singleton mode . But if you treat it as a member variable , Only when in use will it be loaded , It belongs to lazy singleton mode .