send object of an abstract class to constructor of a concrete class Java

Question

I have an abstract class LinearStructure. Class LinkedList and CircularList implements the abstract functions declared in LinearStructure. I also have Queue, Stack and PriorityQueue.

My constructor for Queue looks like this:

public class Queue<T>
{
  private LinearStructure<T> dataStructure;
  public Queue(LinearStructure<T> c)
  {
        dataStructure =  c;
  }
  .....
}

And in my copy constructor for stack I want to do this:

public Stack(Stack<T> other)
{
      Queue<T> temp = new Queue<T>(new LinearStructure<T>());
      this.elements = new Queue<T>(new LinearStructure<T>());
      T val;
      ......
}

But I can't because LinearStructure is abtract. So in my main I want to do something like this:

LinkedList<Integer> ll = new LinkedList<Integer>();
CircularList<Integer> cl = new CircularList<Integer>();
Stack<Integer> s = new Stack<Integer>(ll);
Queue<Integer> q = new Queue<Integer>(cl);

So in other words Stack and Queue can receive either an object of LinkedList or CircularList.

Answer 1

If you wish to make sure that the LinearStructure<T> in the copy is of the same type as in the original, add this method to LinearStructure<T> :

LinearStructure<T> makeEmpty();

Each subclass should override this method to return an empty collection of its own subclass. Now you can code your copy constructor as follows:

public Stack(Stack<T> other) {
    Queue<T> temp = new Queue<T>(other.makeEmpty());
    this.elements = new Queue<T>(other.makeEmpty());
    T val;
    ......
}

Now the type of LinearStructure<T> in the copy and in the original would match.

You could go further and implement a copy function instead, like this:

LinearStructure<T> makeCopy(LinearStructure<? extends T> other);

Doing so would let you combine copying with creation of subclass, which may be important, because each subclass could optimize its creation separately.