Constructor

[Vidya Singh]

Ques 3 :
Consider the following class definition:
1. public class Test extends Base {
2. public Test(int j) {
3. }
4. public Test(int j, int k) {
5. super(j, k);
6. }
7. }

Which of the following forms of constructor must exist explicitly in the definition of the Base class?


(1) Base() { }
(2) Base(int j) { }
(3) Base(int j, int k) { }
(4) Base(int j, int k, int l) { }


Answer : 1,3
Explanation :
1 and 3 are correct. In the constructor at lines 2 and 3, there is no explicit call to either this() or super(), which means that the compiler will generate a call to the zero argument superclass constructor, as in 1. The explicit call to super() at line 5 requires that the Base class must have a 7.constructor as in 3. This has two consequences. First, 3 must be one of the required constructors and therefore one of the answers.Second, the Base class must have at least that constructor defined explicitly, so the default constructor is not generated, but must be added explicitly. Therefore the constructor of 1 is also required and must be a correct answer.At no point in the Test class is there a call to either a superclass constructor with one or three arguments, so 2 and 4 need not explicitly exist.
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I do not understand why constructor Base() { } is necessary, and default constructor is mentioned explicitly

[sachin verma]

I do not understand why constructor Base() { } is necessary, and default constructor is mentioned explicitly

3rd is necessary and you know it.
But if you will have 3rd in your base class no default constructor would be generated in the base class.
And you can't say something like this

new Test(3);

because the first statement inserted would be a call to super()
like this

public Test(int j) {
[B]super()[/B]
 }

that is why 1st is necessary too

[Vidya Singh]

Thanks for explaining. So, that means if you explicitly defines any constructor, then default constructor does not come into picture, and one has explicitly define a default constructor.

[Vishal Matere]

Originally posted by Vidya Singh:
Thanks for explaining. So, that means if you explicitly defines any constructor, then default constructor does not come into picture, and one has explicitly define a default constructor.

You have to write default constructor only if you are going to use default constructor. (either explicitly or implicitly)

[sachin verma]

Yes if you define even a sigle constructor in the class.No default constructor would be generated in that class.That is always no-arg. It is well explained in K&B book

[Jesper de Jong]

Vidya, when you copy a question from a book or mock exam, we require that you quote your sources. So, please tell us where you copied it from.