import java.util.Iterator;
import java.util.NoSuchElementException;

public class Bag<I> implements Iterable<I> {
	private static final int DEFAULT_CAPACITY = 10;
	private int size;
	private I[] theItems;

	public Bag() {
		clear();
	}

	private void clear() {
		size = 0;
		ensureCapacity(DEFAULT_CAPACITY);
	}

	private void ensureCapacity(int newCapacity) {
		if (newCapacity < size) {
			return;
		} else {
			I[] oldIs = theItems;
			theItems = (I[]) new Object[newCapacity];
			for (int i = 0; i < size(); i++) {
				theItems[i] = oldIs[i];
			}
		}
	}

	public void add(I item) {
		if (theItems.length == size()) {
			ensureCapacity(2 * size() + 1);
		}
		theItems[size()] = item;
		size++;
	}

	public boolean isEmpty() {
		return size() == 0;
	}

	public int size() {
		return size;
	}

	@Override
	public Iterator<I> iterator() {
		return new BagIterator();
	}

	private class BagIterator implements Iterator<I> {
		private int current = 0;

		@Override
		public boolean hasNext() {
			return current < size();
		}

		@Override
		public I next() {
			if (!hasNext()) {
				throw new NoSuchElementException();
			}
			return theItems[current++];
		}

	}

	public static void main(String[] args) {
		Bag<Integer> bag = new Bag<Integer>();
		bag.add(1);
		bag.add(3);
		bag.add(6);
		bag.add(4);
		Iterator<Integer> it = bag.iterator();
		while (it.hasNext()) {
			System.out.println(it.next());
		}
	}
}
 

output：

1
3
6
4