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

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

/**
 *  The {@code ResizingArrayBag} 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 resizing array.
 *  See {@link LinkedBag} for a version that uses a singly linked list.
 *  The <em>add</em> operation takes constant amortized time; the
 *  <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="https://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 ResizingArrayBag<Item> implements Iterable<Item> {
    // initial capacity of underlying resizing array
    private static final int INIT_CAPACITY = 8;

    private Item[] a;         // array of items
    private int n;            // number of elements on bag

    /**
     * Initializes an empty bag.
     */
    public ResizingArrayBag() {
        a = (Item[]) new Object[INIT_CAPACITY];
        n = 0;
    }

    /**
     * Is this bag empty?
     * @return true if this bag is empty; false otherwise
     */
    public boolean isEmpty() {
        return n == 0;
    }

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

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

    /**
     * Adds the item to this bag.
     * @param item the item to add to this 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
    }


    /**
     * Returns an iterator that iterates over the items in the bag in arbitrary order.
     * @return an iterator that iterates over the items in the bag in arbitrary order
     */
    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++];
        }
    }

    /**
     * Unit tests the {@code ResizingArrayBag} data type.
     *
     * @param args the command-line arguments
     */
    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);
    }

}

Copyright © 2000–2019, Robert Sedgewick and Kevin Wayne.
Last updated: Sun Sep 6 14:02:50 EDT 2020.