Bag

A bag is a collection where removing items is not supported—its purpose is to provide clients with the ability to collect items and then to iterate through the collected items.

API

public interface Bag<Item> extends Iterable<Item>
{
    public void add(Item item);
    public boolean isEmpty();
    public int size();
}

Implementation

Resized Array

/*************************************************************************
 *  Compilation:  javac ResizingArrayBag.java
 *  Execution:    java ResizingArrayBag
 *
 *  Bag implementation with a resizing array.
 *
 *************************************************************************/

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

public class ResizingArrayBag<Item> implements Iterable<Item> {
    private Item[] a;         // array of items
    private int N = 0;        // number of elements on stack

    // create an empty bag
    public ResizingArrayBag() {
        a = (Item[]) new Object[2];
    }

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


    // resize the underlying array holding the elements
    private void resize(int capacity) {
        assert capacity >= N;
        Item[] temp = (Item[]) new Object[capacity];
        for (int i = 0; i < N; i++)
            temp[i] = a[i];
        a = temp;
    }

    // add a new item to the bag
    public void add(Item item) {
        if (N == a.length) resize(2*a.length);    // double size of array if necessary
        a[N++] = item;                            // add item
    }

    public Iterator<Item> iterator()  { return new ArrayIterator();  }

    // an iterator, doesn't implement remove() since it's optional
    private class ArrayIterator implements Iterator<Item> {
        private int i = 0;
        public boolean hasNext()  { return i < N;                               }
        public void remove()      { throw new UnsupportedOperationException();  }

        public Item next() {
            if (!hasNext()) throw new NoSuchElementException();
            return a[i++];
        }
    }



   /***********************************************************************
    * Test routine.
    **********************************************************************/
    public static void main(String[] args) {
        ResizingArrayBag<String> bag = new ResizingArrayBag<String>();
        bag.add("Hello");
        bag.add("World");
        bag.add("how");
        bag.add("are");
        bag.add("you");

        for (String s : bag)
            StdOut.println(s);
    }

}

Linked List

The implementation is the same as Stack.java except for changing the name of push() to add() and removing pop().

/*************************************************************************
 *  Compilation:  javac Bag.java
 *  Execution:    java Bag < input.txt
 *
 *  A generic bag or multiset, implemented using a 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>Bag</tt> class represents a bag (or multiset) of
 *  generic items. It supports insertion and iterating over the
 *  items in arbitrary order.
 *  <p>
 *  The <em>add</em>, <em>isEmpty</em>, and <em>size</em>  operation
 *  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.
 */
public class Bag<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;
    }

   /**
     * Create an empty stack.
     */
    public Bag() {
        first = null;
        N = 0;
        assert check();
    }

   /**
     * Is the BAG empty?
     */
    public boolean isEmpty() {
        return first == null;
    }

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

   /**
     * Add the item to the bag.
     */
    public void add(Item item) {
        Node oldfirst = first;
        first = new Node();
        first.item = item;
        first.next = oldfirst;
        N++;
        assert check();
    }

    // check internal invariants
    private boolean check() {
        if (N == 0) {
            if (first != null) return false;
        }
        else if (N == 1) {
            if (first == null)      return false;
            if (first.next != null) return false;
        }
        else {
            if (first.next == null) return false;
        }

        // check internal consistency of instance variable N
        int numberOfNodes = 0;
        for (Node x = first; x != null; x = x.next) {
            numberOfNodes++;
        }
        if (numberOfNodes != N) return false;

        return true;
    }


   /**
     * Return 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;
        }
    }

   /**
     * A test client.
     */
    public static void main(String[] args) {
        Bag<String> bag = new Bag<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);
        }
    }
}