/**
 * A container for a single item. No copy is made of the contained item, so the
 * client retains an alias to the inserted item.
 * 
 * @convention value is null ==> isEmpty
 * @correspondence isEmpty ==> contains is empty <br />
 *                 !isEmpty ==> value in contains
 */
public class PlasticBox<T> implements Box<T> {

	/**
	 * The contained item, if one exists. If the PlasicBox is empty, there is no
	 * guarantee on value. In particular, it might or might not be null.
	 */
	private T value;

	/**
	 * True if and only if the PlasticBox is empty. When isEmpty is true, it
	 * might not be safe to dereference value, as that field might be null.
	 */
	private boolean isEmpty;

	/**
	 * Initializes a PlasticBox to be empty.
	 * 
	 * @ensures isEmpty
	 */
	PlasticBox() {
		isEmpty = true;
	}

	/**
	 * @see Box#contains(java.lang.Object)
	 */
	public boolean contains(T target) {
		if (!isEmpty) {
			if (target.equals(value)) {
				return true;
			}
		}
		return false;
	}

	/**
	 * @see Box#insert(java.lang.Object)
	 */
	public void insert(T item) {
		if (isEmpty) {
			isEmpty = false;
			value = item;
		}
	}

	/**
	 * @see Box#removeAny()
	 */
	public T removeAny() {
		assert (!isEmpty);
		isEmpty = true;
		return value;
	}

	/**
	 * @see Box#size()
	 */
	public int size() {
		if (isEmpty) {
			return 0;
		}
		return 1;
	}

}