Java: Basic problems setting up a class

Question

I am having some problems with the basic setup of my class.

The background: I am writing a class which implements a MinHeap based on a binary tree. I have a class BinaryTree, BinaryTreeNode, MinTreeHeap, DefaultComparator and an interface PriorityQueue.
MinTreeHeap is the class I am writing. It implements PriorityQueue and does some other things.
DefaultComparator implements functionality to compare two nodes.

A given test program calls MinTreeHeap with the following:

MinTreeHeap<Value> heap = new MinTreeHeap<>( new DefaultComparator<Value>() );

My problem is the class declaration of MinTreeHeap. I have the following so far.

import pa01.bintree.BinaryTree;
import pa01.bintree.BinaryTreeNode;
import java.util.Comparator;
import java.util.HashMap;
import java.util.NoSuchElementException;

public class MinTreeHeap<T extends DefaultComparator<T>> extends BinaryTree 
    implements PriorityQueue<T> {

  /** The actual heap. */
  private final BinaryTree<T> heap;

  /**
   * Default Constructor
   */
  public MinTreeHeap() {
    super();
    last = null;
  }

  public MinTreeHeap( T data ){
    super(data);
    last = heap.getRoot();
  }

I am getting errors in line public class MinTree... error:

Bound mismatch: The type T is not a valid substitute for the bounded parameter
<T extends Comparable<T>> of the type DefaultComparator<T>

and I have no idea how I'm getting the functionality of my DefaultComparator into the class.

If you need further information to clarify just ask.

Thanks in advance.

Answer 1

You are confusing the type of the data your MinTreeHeap ought to store with the type of the comparator:

public class MinTreeHeap<T extends DefaultComparator<T>> extends BinaryTree
//                       ↑                           ↑
//                       |                            the type to compare
//                       A type extending the Comparator

Saying that T shall be both, a Comparator and the type of the items the Comparator should compare, makes no sense.

And, the DefaultComparator expects a type extending Comparable which your T type variable does not fulfill.

What you most probably meant is something like this:

public class MinTreeHeap<E, C extends Comparator<E>> extends BinaryTree 
implements PriorityQueue<E> {

where E is the type of the items to store and C the type of the Comparator (without forcing it to be a DefaultComparator.

It is also possible to go without a type variable for the type of the Comparator:

public class MinTreeHeap<E> extends BinaryTree implements PriorityQueue<E> {

public MinTreeHeap(Comparator<? super E> comp) {
  // store comp in a field of type Comparator<? super E>
  // or pass it over to other implementation code
  …
}
…