Overloaded constructor question

[shuba gopal]

class Building {
Building() { System.out.print("b "); }
Building(String name) {
this(); System.out.print("bn " + name);
}
}
public class House extends Building {
House() { System.out.print("h "); }
House(String name) {
this(); System.out.print("hn " + name);
}
public static void main(String[] args) { new House("x "); }
}

Hi, the output for this code is "b h hn x". I expected the output to be "b bn h hn x". I understand that the superclass constructor will be invoked first. I expected that String constructor will also be invoked.

[Stuart James]

This() calls the default constructor on the sub class.

You *probably* want to call the non-default super constructor instead - if I understand your question. Though this will not give you your expected output either.

This and Super calls cannot appear in the same constructor.

[Jesper de Jong]

Welcome to the Ranch!

Let's see what's happening here.

In line 12 you create a new House with argument "x ". That will call the constructor in line 9.

The first thing that constructor does, by calling this();, is call the no-args constructor in line 8.

Since there is no explicit call to a superclass constructor (super(...);), first the no-args constructor of the superclass will be called. That's on line 2. That constructor prints: b

Then we continue on line 8. The constructor there prints h

Then we continue on line 10. There, hn x is printed.

So, in the end we have: b h hn x

The constructor of Building which takes a String argument (lines 3 - 5) is not called anywhere. Why do you expect it to be called?

[Campbell Ritchie]

Stuart James wrote:This() calls the default constructor on the sub class. . . .

Not quite. It calls the no-arguments constructor in the same class (House). Because you have defined a constructor, there is no default constructor. Since there is no explicit super(...) call in that no-arguments constructor, the JVM calls the no-arguments constructor in the superclass.

And I see Jesper has beaten me to it.

[Campbell Ritchie]

Jesper, you missed out the bit where the no-arguments constructor in Building calls the no-arguments constructor in Object.

[Campbell Ritchie]

. . . and welcome to the Ranch

[shuba gopal]

Thank you all...
Jesper thank you for the detailed explanation. Since the superclass constructors are invoked first, I thought the superclass constructor with a string argument will be invoked when Horse is instantiated with a string argument. Now it sounds silly.

Thanks for the welcome

[Jesper de Jong]

Campbell Ritchie wrote:Jesper, you missed out the bit where the no-arguments constructor in Building calls the no-arguments constructor in Object.

Yes, you're right Campbell, since all classes implicitly extend Object, the no-args constructor of Object is also called (although that's not visible in the code itself).