Singleton设计模式及其子类的默认构造函数

Question

In my coding, I use Singleton class with Singleton Design Pattern . Question is why its sub class does not allowed to use default constructor?

I get compile time error :

Implicit super constructor Singleton() is not visible. Must explicitly invoke another constructor

Singleton.java

public class Singleton {
    private static Singleton singleton;

    private Singleton() {
        System.out.println("I am user class");
    }

    public static Singleton getInstance() {
        if(singleton == null) {
            singleton = new Singleton();
        }
        return singleton;
    }

}

SubClass.java

public class SubClass extends Singleton {
    public SubClass(){
        System.out.println("I am sub class");
    }
}

Answer 1

When you create an instance of SubClass then it automatically invokes the constructor of its SuperClass to initialize its fields, and that further invokes all the superclass constructors in the inheritance hierarchy

Now since your SuperClass constructor is private, so it cannot invoke that. So, you are getting that exception..

But it doesn't make sense to subclass a singleton class, because in that case, your class will no longer be singleton .

You should re-think about your design and what you are trying to do. And change your design accordingly.

Answer 2

SingleTon class are not supposed to be inherited so below ensures that

private Singleton() {

Below code

public SubClass(){
    System.out.println("I am sub class");
}

Is same as :

public SubClass(){
    super(); //Error here as super class constructor is private
    System.out.println("I am sub class");
}

Answer 3

Because your Singleton class no-arg constructor is private . When you instantiate sub-class it tries to instantiate super class also. But your super class has private constructor. It fails.

 private Singleton() {
        System.out.println("I am user class");
    }

It doesn't make sense to subclass Singleton class. You may need to re-think about your design.

Answer 4

First of all, subclassing a singleton class is breaking the basics of Singleton Pattern. Secondly, due to the chain of calling the constructors through the inheritance tree, you get that exception since the constructor of the parent class is private .

You will probably add a public constructor with some parameters to make the child inherit from its parent. Then you will have maybe many instances of SubClass . Any Subclass instance IS-A Singleton instance, too. And this is against the purposes of Singleton pattern. That's why you should avoid subclassing a singleton class.