Test memory leak in java code

MemoryLeakTest.java

import java.util.WeakHashMap;

import junit.framework.Assert;

import org.junit.Test;

public class MemoryLeakTest {

	@Test
	public void test_stack_memory_leak() {
		WeakHashMap<Integer, String> weakMap = new WeakHashMap<Integer, String>();

		Stack<Integer> stack = new Stack<Integer>();
		Integer i = new Integer(5);
		stack.push(i);
		weakMap.put(i, i.toString());

		i = new Integer(6);
		stack.push(i);
		weakMap.put(i, i.toString());
		i = null;

		stack.pop();
		stack.pop();

		testMap(weakMap);

	}

	private void testMap(WeakHashMap<Integer, String> weakMap) {
		System.gc();
		try {
			Thread.sleep(100);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

		Assert.assertTrue(weakMap.isEmpty());
	}

	public class Stack<Item> {

		private class Node {
			private Item item;
			private Node next;
		}

		private Node front;

		public void push(Item item) {
			Node oldFront = front;
			front = new Node();
			front.item = item;
			front.next = oldFront;
		}

		public Item pop() {
			Item item = null;

			if (front != null) {
				item = front.item;
				front = front.next;
			}
			
			return item;

		}
	}

}