Min Heap using Generic

MinHeap.java

/*
 * referenced 
 * 1. http://www.techiedelight.com/min-heap-max-heap-implementation-in-java/
 * 2. https://gist.github.com/flexelem/70b120ac9bf2965f419f
 * */
class MinHeap<E extends Comparable<E>> {
	private ArrayList<E> heap;
	
	MinHeap() {
		heap = new ArrayList<>();
	}
	
	public void insert(E item) {
		heap.add(item);
		int i = heap.size() - 1;
		int parent = parent(i);
		
		while (parent != i && heap.get(i).compareTo(heap.get(parent)) < 0) {
			swap(i, parent);
			i = parent;
			parent = parent(i);
		}
	}
	
	public E remove() {
		E root;
		if (size() == 0) 
			return null;
		if (size() == 1) {
			root = heap.remove(0);
			return root;
		}
		
		root = heap.get(0);
		E lastItem = heap.remove(size() - 1);
		heap.set(0, lastItem);
		
		heapifyDown(0);
		
		return root;
	}
	
	private void heapifyDown(int i) {
		int left = left(i);
		int right = right(i);
		int smallest = i;
		if (left < size() && heap.get(left).compareTo(heap.get(i)) < 0) {
			smallest = left;
		}
		if (right < size() && heap.get(right).compareTo(heap.get(smallest)) < 0) {
			smallest = right;
		}
		if (smallest != i) {
			swap(i, smallest);
			heapifyDown(smallest);
		}
	}
	public void print() {
		for (int i = 0; i < heap.size(); i++) {
			/* print */
		}
	}
	
	public boolean validate() {
		for (int i = 1; i < heap.size(); i++) {
			if (heap.get(i).compareTo(heap.get(parent(i))) < 0) {
				print();
				return false;
			}
		}
		print();
		return true;
	}
	
	public int size() {
		return heap.size();
	}
	
	public void clear() {
		heap.clear();
	}
	
	private int parent(int i) {
		if (i == 0) { /* if i is already a root node */
			return 0;
		}
		return (i - 1) / 2;
	}
	
	private int left(int i) {
		return (2 * i + 1);
	}
	
	private int right(int i) {
		return (2 * i + 2);
	}
	
	private void swap(int i, int parent) {
		E tmp = heap.get(parent); 
		heap.set(parent, heap.get(i));
		heap.set(i, tmp);
	}
}