Constructors, super(), this()

[Gautham Muralidharan]

This is SuperDuper Class

public class SuperDuper {
	
	public String name;
	public int age=50;
		
	public SuperDuper()
	{
		System.out.println("SuperDuper's Constructor");
		
	}
	public SuperDuper(int age)
	{
		this();
		this.age=age;
		System.out.println("SuperDuper's parameterized Constructor");
	}
}	

This is Super Class

public class Super extends SuperDuper {
	public String name;
	public int age=40;
	

	public Super()
	{
		System.out.println("Super's Constructor");
		
	}
	public Super(int age)
	{
		super();
		this.age=age;
		System.out.println("Super's parameterized Constructor");
	}
}

This is Sub Class

public class Sub extends Super{
	public String name;
	public int age=30;
	
	public Sub()
	{
		
		System.out.println("Sub's Constructor");
		
	}
	public Sub(int age)
	{
		super();
		this.age=age;
		System.out.println("Sub's parameterized Constructor");
	}
}

This is class Runners2

public class Runners2 {

	public static void main(String[] args) {
		
		System.out.println("The output is follows\n");
		SuperDuper one = new SuperDuper(10);
		System.out.println(one.age);
		System.out.println("");
		
		SuperDuper two = new Super(10);
		System.out.println(two.age);
		System.out.println("");
		
		SuperDuper three = new Sub(10);
		System.out.println(three.age);

	}

}

I get the output:

The output is follows

SuperDuper's Constructor
SuperDuper's parameterized Constructor
10

SuperDuper's Constructor
Super's parameterized Constructor
50

SuperDuper's Constructor
Super's Constructor
Sub's parameterized Constructor
50

I have used three parameterized constructors in the class Runners2.

SuperDuper one = new SuperDuper(10);

This changes the age value from 50 to 10.
Why does

SuperDuper two = new Super(10);

and

SuperDuper three = new Sub(10);

doesn't change the age value to 10?

What should I do to get the output as 10 instead of 50 at line 9 & 14 in the output?
Replacing super() by this() in the Super class and Sub class also produces the same result. Why?

[Sergiu Dobozi]

I think if you set two as a Super object, the value of 10 will then show up. But since you set two and three SuperDuper objects, it will always show the age of 50.

[Henry Wong]

First of all, you do know that instances of your Super class has 2 different age instance variables right? And instances of your Sub class has 3 different age instance variables?

Due to all the hiding, the constructor of the Super class and Sub class only changes the one that is declared in their respective classes. And since, your Runner2 class only displays the one that is in the SuperDuper class, your expected result is not seen.


So, how to fix? Well, it depends. If you didn't intend to have three copies of the variables, then you should fix that. And if you did intend it, but expect them to be equal, then you should fix that (by setting them all). For the latter, have the constructor call their superclass' constructors that takes an age.

Henry

[Gautham Muralidharan]

Sergiu Dobozi wrote:I think if you set two as a Super object, the value of 10 will then show up. But since you set two and three SuperDuper objects, it will always show the age of 50.

Thank You so much.
Is there any way I can do it without setting two and three as Super object and Sub object respectively?

[Stephan van Hulst]

Don't hide your fields in sub-classes.

SuperDuper already has an age field, why are you redeclaring it in Super and Sub?

If you want to change the default value of the existing field in SuperDuper, you should do that in the default constructors of your sub-classes. Preferably, you should also make the field private and final:

class SuperDuper {

  private final int age;

  SuperDuper() {
    this.age = 50;
    System.out.println("SuperDuper's constructor");
  }

  SuperDuper(int age) {
    this.age = age;
    System.out.println("SuperDuper's parameterized constructor");
  }

  int age() {
    return age;
  }
}

class Super extends SuperDuper {

  Super() {
    super(40);
    System.out.println("Super's constructor");
  }

  Super(int age) {
    super(age);
    System.out.println("Super's parameterized constructor");
  }
}

class Sub extends Super {
  
  Sub() {
    super(30);
    System.out.println("Sub's constructor");
  }

  Sub(int age) {
    super(age);
    System.out.println("Sub's parameterized constructor");
  }
}

[Junilu Lacar]

Fields do not participate in polymorphism, only instance methods do. You have declared all the variables in your main() method to be of type SuperDuper, so the value of one.age, two.age, three.age are all resolved to SuperDuper.age at compile time. Try declaring variable two as type Super and variable three as type Sub.

[Gautham Muralidharan]

Thank You, Stephan, Junilu, Henry and Sergiu. Now I've got it. Learnt more about polymorphism, method hiding and super(). Thanks again.