compareTo with generics for heapSort

Question

For class I had to either implement a BST or a heapSort. I did the BST but figured it would be good to know this too but now I'm stuck. This is my first time working with heaps(and really coding with generics/implementing Comparable so I apologize for all the errors) and im running into an issue implementing compareTo.

Essentially I want to be able to add generic objects to my heap Array and then compare them for the Heap sorting. I use compareTo to check a new entry when adding to the heap and for swapping in the reheap method.

My errors returned:

Heap.java:64: error: bad operand types for binary operator '<'
if (this  < other)
          ^
first type:  Heap<T>
second type: Heap<T>

where T is a type-variable:
T extends Comparable<T> declared in class Heap

Im not sure how to work around that though. I understand that my binary operator isnt for generics but I dont know how to work around it. Thanks for any input. Sorry about all the beginners mistakes you may find! Heres my code:

import java.util.*;

class Heap<T extends Comparable <T>>  implements Comparable<Heap<T>>{

private T[] heap;
private int lastIndex;
private static final int CAPACITY = 25;

public Heap(){

    this(CAPACITY);

}

public Heap(int capacity){

    heap = (T[])new Comparable[capacity+1];
    lastIndex = 0;
}

public void add(T newEntry){

    lastIndex++;
    if(lastIndex>=heap.length)
    doubleArray();

    int newIndex = lastIndex;
    int parentIndex = newIndex/2;

    while((parentIndex>0)&&(heap[parentIndex].compareTo(newEntry)>0))
    {
        heap[newIndex] = heap[parentIndex];
        newIndex = parentIndex;
        parentIndex = newIndex/2;
    }  
    heap[newIndex] = newEntry;
}

public void display()
{
    for(int i=1;i<heap.length;i++)
    {
        System.out.println(heap[i]);
    }

}

private void doubleArray()
{
T[] oldHeap = heap;
int oldSize = heap.length;

heap = (T[]) new Object[2*oldSize];

    for(int i =0; i < oldSize-1;i++)
    {
        heap[i] = oldHeap[i];
    }
}

public int compareTo(Heap<T> other)
{
    int sort = 0;
    if (this  < other)
    {
            sort = -1;
    }
    else if (this> other)
    {
            sort = 1;
    }
    else
    {
            sort = 0;
    }
    return sort;
}

private <T extends Comparable<T>> void reheap(T[] heap, int rootIndex, int lastIndex)
{
    boolean done=false;
    T orphan = heap[rootIndex];
    int leftChildIndex = 2 * rootIndex + 1;

    while(!done && (leftChildIndex<=lastIndex))
    {
    int largerChildIndex = leftChildIndex;
    int rightChildIndex = leftChildIndex + 1;

        if(rightChildIndex<=lastIndex &&     (heap[rightChildIndex].compareTo(heap[largerChildIndex])>0))
            largerChildIndex = rightChildIndex;
        if(orphan.compareTo(heap[largerChildIndex])<0)
        {
        //  System.out.println(orphan+ "--" + largerChildIndex);

            heap[rootIndex] = heap[largerChildIndex];
            rootIndex = largerChildIndex;
            leftChildIndex = 2 * rootIndex+1;
        }
        else
            done = true;
    }
heap[rootIndex] = orphan;
}

public <T extends Comparable<T>> void heapSort(int n)
{
    for(int rootIndex = n/2-1;rootIndex >=0;rootIndex--)
        reheap(heap,rootIndex,n-1);

    swap(heap,0,n-1);

    for(int lastIndex = n-2;lastIndex > 0;lastIndex--)
    {   
        reheap(heap,0,lastIndex);
        swap(heap,0,lastIndex);
    }
}

private <T extends Comparable<T>> void swap(T[] a,int first, int last)
{
T temp;

temp = a[first];
a[first] = a[last];
a[last] = temp;
}

}

Any help with any of this is very very appreciated

Answer 1

1. You don't want your heap to be Comparable; you want to compare its members. Therefore remove implements Comparable<Heap<T>> from your class declaration and remove the compareTo method.

2. Many of your methods (reheap, heapSort, swap) redundantly declare <T extends Comparable<T>> where you are already in the context of your class parameterized with T. Remove those declarations.

Answer 2

I think you need to implement the compareTo on you T object, not on the heap itself. You have to make sure T is comparable for it to be in the heap.