Chapter 23 Exercise 9, Introduction to Java Programming, Tenth Edition Y. Daniel LiangY.

23.9 (Generic Heap using Comparator ) Revise Heap in Listing 23.9, using a generic parameter and a Comparator for comparing objects. Define a new constructor with a Comparator as its argument as follows:
Heap(Comparator<? super E> comparator)

import java.util.Comparator;

public class Exercise09 {
 
 static class IntegerComparator implements Comparator<Integer> {
  @Override
  public int compare(Integer o1, Integer o2) {
   return o1.intValue() - o2.intValue();
  }  
 }

 /** Heap sort method */
 public static <E> void heapSort(E[] list, Comparator<? super E> comparator) {
  // Create a Heap of integers
  Heap<E> heap = new Heap<E>(comparator);

  // Add elements to the heap
  for (int i = 0; i < list.length; i++)
   heap.add(list[i]);

  // Remove elements from the heap
  for (int i = list.length - 1; i >= 0; i--)
   list[i] = heap.remove();
 }

 /** A test method */
 public static void main(String[] args) {
  Integer[] list = { -44, -5, -3, 3, 3, 1, -4, 0, 1, 2, 4, 5, 53 };
  heapSort(list, new IntegerComparator());
  for (int i = 0; i < list.length; i++)
   System.out.print(list[i] + " ");
 }
 
 
 static class Heap  <E>  {
  private Comparator<? super E> comparator;
  
  private java.util.ArrayList<E> list = new java.util.ArrayList<E>();

  /** Create a default heap */
  public Heap(Comparator<? super E> comparator) {
   this.comparator = comparator;
  }

  /** Create a heap from an array of objects */
  public Heap(E[] objects) {
   for (int i = 0; i < objects.length; i++)
    add(objects[i]);
  }

  /** Add a new object into the heap */
  public void add(E newObject) {
   list.add(newObject); // Append to the heap
   int currentIndex = list.size() - 1; // The index of the last node

   while (currentIndex > 0) {
    int parentIndex = (currentIndex - 1) / 2;
    // Swap if the current object is greater than its parent
    if (comparator.compare(list.get(currentIndex), (list.get(parentIndex))) > 0) {
     E temp = list.get(currentIndex);
     list.set(currentIndex, list.get(parentIndex));
     list.set(parentIndex, temp);
    } else
     break; // the tree is a heap now

    currentIndex = parentIndex;
   }
  }

  /** Remove the root from the heap */
  public E remove() {
   if (list.size() == 0)
    return null;

   E removedObject = list.get(0);
   list.set(0, list.get(list.size() - 1));
   list.remove(list.size() - 1);

   int currentIndex = 0;
   while (currentIndex < list.size()) {
    int leftChildIndex = 2 * currentIndex + 1;
    int rightChildIndex = 2 * currentIndex + 2;

    // Find the maximum between two children
    if (leftChildIndex >= list.size())
     break; // The tree is a heap
    int maxIndex = leftChildIndex;
    if (rightChildIndex < list.size()) {
     if (comparator.compare(list.get(maxIndex), (list.get(rightChildIndex))) < 0) {
      maxIndex = rightChildIndex;
     }
    }

    // Swap if the current node is less than the maximum
    if (comparator.compare(list.get(currentIndex), (list.get(maxIndex))) < 0) {
     E temp = list.get(maxIndex);
     list.set(maxIndex, list.get(currentIndex));
     list.set(currentIndex, temp);
     currentIndex = maxIndex;
    } else
     break; // The tree is a heap
   }

   return removedObject;
  }

  /** Get the number of nodes in the tree */
  public int getSize() {
   return list.size();
  }
 }
 
 
}