public void makeHeap(int[] arr)
{
for(int i = 0; i <= arr.length; i++)//for each element in the array, we we check its children and sift it down
{
siftDown(arr, i);
}
}
//insert a new root in the correct position. Byu comparing the top element and swapping it with its largest child.
public void siftDown(int[]heap, int i)
{
int c = i * 2; //grab the children of the current index we are at.
if(c == 0) // 0*2 is 0 so we make it 1 so it will register the first nodes parents
{
c+=1;
}
if(c >= heap.length || c+1 >= heap.length) // so we dont go off the end of the array
{
return;
}
if(heap[c] < heap[c + 1]) //Is child 1 less than child 2? 
{
c += 1; //if it is we want c to be the greater child to eventually move upwards
}
if(heap[i] < heap[c])//If the parent we have just gotten is smaller than the child defined last loop, then we swap the two. We then call sift down again to compare child and parent.
{
heap = swap(heap,i,c);//swap the values
siftDown(heap, c);//call again to compare the new root with its children.
}
}

 public int[] swap(int[]heap, int i, int c)
{
//capture the two values in variables
int ele1 = heap[i];
int ele2 = heap[c];
heap[i] = ele2;//change heap i to heap c
heap[c] = ele1;//change heap c to heap i
return heap;
}

    int c = i * 2; //grab the children of the current index we are at.
if(c == 0) // 0*2 is 0 so we make it 1 so it will register the first nodes parents
{
c+=1;
}

Given parentIdx in 0-based array,
leftChildIdx = parentIdx * 2 + 1
rightChildIdx = parentIdx * 2 + 2