An implementation of a minheap

MinHeap.java

import java.util.ArrayList;
import java.util.Random;

public class MinHeap<T extends Comparable<T>> {
  private ArrayList<T> values;

  public MinHeap() {
    values = new ArrayList<T>();
  }

  public void push(T value) {
    int index = values.size();
    values.add(value);

    while (index != 0) {
      int parentIndex = (index - 1) / 2;
      if (value.compareTo(values.get(parentIndex)) >= 0) {
        break;
      }
      values.set(index, values.get(parentIndex));
      values.set(parentIndex, value);
      index = parentIndex;
    }
  }

  public T peek() {
    return values.get(0);
  }

  public T pop() {
    T value = peek();

    int lastIndex = values.size() - 1;
    T insertedValue = values.get(lastIndex);
    values.set(0, insertedValue);
    values.remove(lastIndex);

    int index = 0;
    int leftChildIndex = 1;
    int rightChildIndex = 2;
    while (leftChildIndex < lastIndex) {
      if (rightChildIndex >= lastIndex) {
        if (insertedValue.compareTo(values.get(leftChildIndex)) > 0) {
          values.set(index, values.get(leftChildIndex));
          values.set(leftChildIndex, insertedValue);
        }
        break;
      }
      if (insertedValue.compareTo(values.get(leftChildIndex)) <= 0 &&
          insertedValue.compareTo(values.get(rightChildIndex)) <= 0) {
        break;
      }
      if (values.get(leftChildIndex).compareTo(values.get(rightChildIndex)) <= 0) {
        values.set(index, values.get(leftChildIndex));
        values.set(leftChildIndex, insertedValue);
        index = leftChildIndex;
      } else {
        values.set(index, values.get(rightChildIndex));
        values.set(rightChildIndex, insertedValue);
        index = rightChildIndex;
      }
      leftChildIndex = (index * 2) + 1;
      rightChildIndex = leftChildIndex + 1;
    }

    return value;
  }

  public boolean isEmpty() {
    return values.isEmpty();
  }

  public String toString() {
    return values.toString();
  }

  public static void main(String[] args) {
    Random random = new Random();

    MinHeap<Integer> heap = new MinHeap<>();

    for (int i = 0; i < 15; i++) {
      int value = random.nextInt(100);
      System.out.println("push " + value);
      heap.push(value);
      System.out.println("=> " + heap);
    }

    while (!heap.isEmpty()) {
      System.out.println("pop " + heap.pop());
      System.out.println("=> " + heap);
    }
  }
}