Design Pattern —— Singleton

public class Singleton
{
    private static Singleton instance = null;
    
    private static String key = "key";
    
    private Singleton()
    {
    }
    
    public static Singleton getInstance()
    {
        if(instance == null)
        {
            synchronized (key)
            {
                if(instance == null)
                {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
    
    public void say(){
        System.out.println("Hello world!");
    }
}

public class SingletonTest
{
    public static void main(String[] args)
    {
        Singleton singleton = Singleton.getInstance();
        for (int i = 0; i < 5; i++) {
            singleton.say();
        }
    }
}