比较好的的单例模式

enum Single {
    Single;

    private () {
        
    }

    public void print(){
        System.out.println("hello world");
    }
}

public class SingleDemo {
	public static void main(String[] args) {
    	Single a = Single.Single;
    	a.print();
}

public class Singleton
{
    private Singleton(){}
   
    private static class SingletonHandler {
        private static Singleton singleton = new Singleton();
    }
    
    public static Singleton getInstance(){
        return SingletonHandler.singleton;
    }
    
    public void someMethod(){
        System.out.println("This's display!");
    }
}

public class Singleton
{
    private Singleton(){}

	private static Singleton singleton = new Singleton();
    
    public static Singleton getInstance(){
        return singleton;
    }
    
    public void someMethod(){
        System.out.println("This's display!");
    }
}