Mock test question about Parent and Child constructors.

[Gabriela Rios]

Hello, I'm taking some mock test on skill-guru.com and I found this question:

class Base
   {  
      int  x=10;
         Base()
          {    show();  }
 
        void show()
   	{   System.out.print ("Base Show " +x + "  "); 	}
   }

class Child extends Base
   { 
       int x=20;
          Child()
            {  show();  }
   
        void show()
         {    System.out.print("Child Show " + x +"  ")
         }
       public static void main( String s[ ] )
           {   Base obj = new Child(); 
	   }
  }

What is the Output of the Program ? Select the one Correct answer

The answer is "Child Show 0 Child Show 20" but I don't quite understand why x is printing cero and why is it calling the show method from Chil while in the constructor's parent.

I appreciate if you can explain a little bit about why this is happening.


Regards,

[dennis deems]

The variable x in the class Child hides the variable x in the class Base. (Base.x is still there, we just can't see it unless we use the super keyword.)

You know that there is an implicit call to the superclass constructor. But realize that when this superclass call is made the varlable Child.x has not yet been initialized. Until we return from the superclass constructor, its value will remain the default value of zero. Only then will the initialization in the subclass occur.

The subclass version of show is called because even though the superclass constructor is called, it is a Child instance that has been created, and it is that implementation that can be executed.

[John Jai]

The base class constructor is called from the implicitly placed super() in the child's constructor. I hope you understand this.

When the super class constructor, the object in creation is the child class object. So in the super class constructor, when you try to call an overridden method, the method present in the actual object will be called.

Try marking the super class method as either final or private and note the difference.

We had a recent discussion on this topic. I will get that thread for you.

[John Jai]

See the example given by Rob https://coderanch.com/t/561068/java/java/Constructor#2547318

[O. Ziggy]

I thought a constructor is not able to access instance methods unless its super constructor has completed. This does not appear to be the case here.

[John Jai]

The super constructor is first run and it's the code in the super constructor that calls the child's instance method. Code in child's constructor runs only after the call to super() finishes. Your thought was correct Ziggy.

[Gabriela Rios]

Thanks! It all makes sense now.