单例模块（singleton）:表示一个类只会生成唯一的对象；
代码示例：
public class Test{

    public static void main(String[] args) {
        Singleton singleton1 = Singleton.getInstance();
        Singleton singleton2 = Singleton.getInstance();
        System.out.println(singleton1 == singleton2);
    }

}

class Singleton{
   
    private static Singleton singleton = new Singleton();
    private Singleton(){      
    }
   
    public static Singleton getInstance(){
        return singleton;
    }
}