java: passing argument to copy constructor

Question

Inside of class ATester
{
   private A<Integer> p1,p2;

    p1 = new B<Integer>();
    p2 = new B<Integer>( p1);

}

public class B<E extends Comparable<? super E>> implements A<E>
{
     public B()   // default constructor
     {
        // skip
     }

     public B(B other)  // copy constructor
     {
        // skip
     }

}

I want to define a copy constructor, which takes another B as argument but when I pass p1 into

p2 = new B<Integer>( p1);

when compile, it gives me error message

"no suitable constructor found for B< A < Integer > >"

What should I change or add?

@MarkoTopolnik.. It won't help. His reference type of p1, is A<Integer> — Rohit Jain
– Rohit Jain, Commented Nov 24, 2012 at 21:17

Yogendra Singh · Accepted Answer · 2012-11-24 21:21:16Z

2

You need to cast your p1 to B<Integer> before calling the copy constructor.

    p2 = new B<Integer>( (B<Integer>)p1);

Or you can define another constructor accepting the Interface type e.g.

    public B(A<E> other)  // copy constructor
    {
         //type cast here and use it
    }

answered Nov 24, 2012 at 21:21

Yogendra Singh

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Comments

Jatin · Accepted Answer · 2012-11-24 21:18:56Z

1

Change it to

Or call as p2 = new B<Integer>( (B<Integer>)p1);

Because what you are trying to do is send A<Integer> to B in the constructor. Ultimately it is

B b = element of type A<Integer>

Which is wrong due to contra-variance of argument type. Either change the argument type in you B constructor as per design or do the above mentioned

answered Nov 24, 2012 at 21:18

Jatin

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4 Comments

hibc Over a year ago

THank you very much! WOuld you please explain little more about (B<Integer>)p1? what does it mean?

Jatin Over a year ago

Ok for example: Animal a = new Dog() is valid. because Dog is also an animal. What you are trying to do is: Dog a = new Animal(). Now dog cant be all animals. Hence it is wrong

Jatin Over a year ago

A super type can be used instead of subtype, but not other way around. Above hence, I am type casting p1 from type A to type B<Integer>

Jatin Over a year ago

And if I were you, I would change constructor of B to :public B(B<E> other) to avoid compile time warning

HRgiger · Accepted Answer · 2012-11-24 21:24:51Z

0

Your B already implements A, So change constructor arg from B to A:

public class B<E extends Comparable<? super E>> implements A<E>
{
     public B()   // default constructor
     {
        // skip
     }

     public B(A other)  // copy constructor
     {
        // skip
     }



}

Then you can use both A and B as a valid cons parameter;

    A<Integer> p1, p2;
    B<Integer> c = new B<Integer>();

    p1 = new B<Integer>(c);
    p2 = new B<Integer>( p1);

answered Nov 24, 2012 at 21:24

HRgiger

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1 Comment

Jatin Over a year ago

This actually depends on design. I personally wouldn;t do it

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