LinkedBag.java

/*************************************************************************
 *  Compilation:  javac LinkedBag.java
 *  Execution:    java LinkedBag < input.txt
 *
 *  A generic bag or multiset, implemented using a singly-linked list.
 *
 *  % more tobe.txt 
 *  to be or not to - be - - that - - - is
 *
 *  % java Bag < tobe.txt
 *  size of bag = 14
 *  is
 *  -
 *  -
 *  -
 *  that
 *  -
 *  -
 *  be
 *  -
 *  to
 *  not
 *  or
 *  be
 *  to
 *
 *************************************************************************/

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

/**
 *  The <tt>LinkedBag</tt> class represents a bag (or multiset) of 
 *  generic items. It supports insertion and iterating over the 
 *  items in arbitrary order.
 *  <p>
 *  This implementation uses a singly-linked list with a non-static nested class Node.
 *  See {@link Bag} for a version that uses a static nested class.
 *  The <em>add</em>, <em>isEmpty</em>, and <em>size</em> operations
 *  take constant time. Iteration takes time proportional to the number of items.
 *  <p>
 *  For additional documentation, see <a href="http://algs4.cs.princeton.edu/13stacks">Section 1.3</a> of
 *  <i>Algorithms, 4th Edition</i> by Robert Sedgewick and Kevin Wayne.
 *
 *  @author Robert Sedgewick
 *  @author Kevin Wayne
 */
public class LinkedBag<Item> implements Iterable<Item> {
    private int N;         // number of elements in bag
    private Node first;    // beginning of bag

    // helper linked list class
    private class Node {
        private Item item;
        private Node next;
    }

    /**
     * Initializes an empty bag.
     */
    public LinkedBag() {
        first = null;
        N = 0;
    }

    /**
     * Is this bag empty?
     * @return true if this bag is empty; false otherwise
     */
    public boolean isEmpty() {
        return first == null;
    }

    /**
     * Returns the number of items in this bag.
     * @return the number of items in this bag
     */
    public int size() {
        return N;
    }

    /**
     * Adds the item to this bag.
     * @param item the item to add to this bag
     */
    public void add(Item item) {
        Node oldfirst = first;
        first = new Node();
        first.item = item;
        first.next = oldfirst;
        N++;
    }


    /**
     * Returns an iterator that iterates over the items in the bag.
     */
    public Iterator<Item> iterator()  {
        return new ListIterator();  
    }

    // an iterator, doesn't implement remove() since it's optional
    private class ListIterator implements Iterator<Item> {
        private Node current = first;

        public boolean hasNext()  { return current != null;                     }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            Item item = current.item;
            current = current.next; 
            return item;
        }
    }

    /**
     * Unit tests the <tt>LinkedBag</tt> data type.
     */
    public static void main(String[] args) {
        LinkedBag<String> bag = new LinkedBag<String>();
        while (!StdIn.isEmpty()) {
            String item = StdIn.readString();
            bag.add(item);
        }

        StdOut.println("size of bag = " + bag.size());
        for (String s : bag) {
            StdOut.println(s);
        }
    }


}