bag.java

Java程序设计(美) David D. Riley著 机械工业出版社 书籍配套 代码

/** Bag Class
 * Author: David D. Riley
 * Date: June, 2004
 *
 *  class invariant
 *      A Bag object ...
 *          * is a container of zero or more other objects
 *          * is capable of storing anything that conforms to Object 
 */
public class Bag<ItemType> extends SimpleList<ItemType> {

	/**	post:   this is an empty bag (i.e. size() == 0) */
	public Bag()  {
		super();
	} 
	
	/**	post:   result == number of items in this Bag
	 *	note:   this method is inherited 
     */
//	public int size()   

	/** post:   this ==  this@pre with some z inserted
     *          and  size() == size()@pre + 1
     *  note:   this method is inherited
     */
//	public void add(ItemType z)   

	/** pre:    count() > 0  <br>
	 *	post:   this ==  this@pre with some item removed
     *          and  size() == size()@pre - 1
     *  note:   this method is inherited 
     */
//	public void remove()   

	/**	pre:    size() > 0
     *  post:   result == one of the items from this Bag
     */
	public ItemType item()   {
		int skipCount;
		super.reset();
		// the following code randomizes which item is returned.
		skipCount = (int) (Math.random() * size());
		for ( ; skipCount>0; skipCount--)   {
			super.next();
		}
		return super.next();
	}

	public String toString()  {
		String str = "";
		super.reset();
		while (super.hasNext())   {
			str = str + " " + super.next().toString();
		}
		return str;
	} 


// The methods below are overridden to disable them.
	/**	post:   this method is disabled */
	public void reset() {
	} 

	/**	post:   this method is disabled */
	public ItemType next()  {
        return null;
	}  
}