Dynamic Array resizing

Question

I have a class with some data members in it. Data for the objects comes frm a XML file. I have to parse out the xml file and create objects. Before reading the XML file i dunno as to how many objects i need to create. I wud like to know a way where I can dynamically increase the size of the object array as I read through the XMl file.
Thanks in advance,
Kris.

Answer 1

This is the source:
* @(#)ArrayList.java     1.36 01/12/03
* Copyright 2002 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
package java.util;
* Resizable-array implementation of the <tt>List</tt> interface.  Implements
* all optional list operations, and permits all elements, including
* <tt>null</tt>.  In addition to implementing the <tt>List</tt> interface,
* this class provides methods to manipulate the size of the array that is
* used internally to store the list.  (This class is roughly equivalent to
* <tt>Vector</tt>, except that it is unsynchronized.)<p>
* The <tt>size</tt>, <tt>isEmpty</tt>, <tt>get</tt>, <tt>set</tt>,
* <tt>iterator</tt>, and <tt>listIterator</tt> operations run in constant
* time.  The <tt>add</tt> operation runs in <i>amortized constant time</i>,
* that is, adding n elements requires O(n) time.  All of the other operations
* run in linear time (roughly speaking).  The constant factor is low compared
* to that for the <tt>LinkedList</tt> implementation.<p>
* Each <tt>ArrayList</tt> instance has a <i>capacity</i>.  The capacity is
* the size of the array used to store the elements in the list.  It is always
* at least as large as the list size.  As elements are added an ArrayList,
* its capacity grows automatically.  The details of the growth policy are not
* specified beyond the fact that adding an element has constant amortized
* time cost.<p>
* An application can increase the capacity of an <tt>ArrayList</tt> instance
* before adding a large number of elements using the <tt>ensureCapacity</tt>
* operation.  This may reduce the amount of incremental reallocation.<p>
* <strong>Note that this implementation is not synchronized.</strong> If
* multiple threads access an <tt>ArrayList</tt> instance concurrently, and at
* least one of the threads modifies the list structurally, it <i>must</i> be
* synchronized externally.  (A structural modification is any operation that
* adds or deletes one or more elements, or explicitly resizes the backing
* array; merely setting the value of an element is not a structural
* modification.)  This is typically accomplished by synchronizing on some
* object that naturally encapsulates the list.  If no such object exists, the
* list should be "wrapped" using the <tt>Collections.synchronizedList</tt>
* method.  This is best done at creation time, to prevent accidental
* unsynchronized access to the list:
* <pre>
*     List list = Collections.synchronizedList(new ArrayList(...));
* </pre><p>
* The iterators returned by this class's <tt>iterator</tt> and
* <tt>listIterator</tt> methods are <i>fail-fast</i>: if list is structurally
* modified at any time after the iterator is created, in any way except
* through the iterator's own remove or add methods, the iterator will throw a
* ConcurrentModificationException.  Thus, in the face of concurrent
* modification, the iterator fails quickly and cleanly, rather than risking
* arbitrary, non-deterministic behavior at an undetermined time in the
* future.
* <p>Note that the fail-fast behavior of an iterator cannot be guaranteed
* as it is, generally speaking, impossible to make any hard guarantees in the
* presence of unsynchronized concurrent modification.  Fail-fast iterators
* throw <tt>ConcurrentModificationException</tt> on a best-effort basis.
* Therefore, it would be wrong to write a program that depended on this
* exception for its correctness: <i>the fail-fast behavior of iterators
* should be used only to detect bugs.</i>
* @author  Josh Bloch
* @version 1.36, 12/03/01
* @see         Collection
* @see         List
* @see         LinkedList
* @see         Vector
* @see         Collections#synchronizedList(List)
* @since   1.2
public class ArrayList extends AbstractList
        implements List, RandomAccess, Cloneable, java.io.Serializable
    private static final long serialVersionUID = 8683452581122892189L;
     * The array buffer into which the elements of the ArrayList are stored.
     * The capacity of the ArrayList is the length of this array buffer.
    private transient Object elementData[];
     * The size of the ArrayList (the number of elements it contains).
     * @serial
    private int size;
     * Constructs an empty list with the specified initial capacity.
     * @param   initialCapacity   the initial capacity of the list.
     * @exception IllegalArgumentException if the specified initial capacity
     *            is negative
    public ArrayList(int initialCapacity) {
     super();
        if (initialCapacity < 0)
            throw new IllegalArgumentException("Illegal Capacity: "+
                                               initialCapacity);
     this.elementData = new Object[initialCapacity];
     * Constructs an empty list with an initial capacity of ten.
    public ArrayList() {
     this(10);
     * Constructs a list containing the elements of the specified
     * collection, in the order they are returned by the collection's
     * iterator.  The <tt>ArrayList</tt> instance has an initial capacity of
     * 110% the size of the specified collection.
     * @param c the collection whose elements are to be placed into this list.
     * @throws NullPointerException if the specified collection is null.
    public ArrayList(Collection c) {
        size = c.size();
        // Allow 10% room for growth
        elementData = new Object[
                      (int)Math.min((size*110L)/100,Integer.MAX_VALUE)];
        c.toArray(elementData);
     * Trims the capacity of this <tt>ArrayList</tt> instance to be the
     * list's current size.  An application can use this operation to minimize
     * the storage of an <tt>ArrayList</tt> instance.
    public void trimToSize() {
     modCount++;
     int oldCapacity = elementData.length;
     if (size < oldCapacity) {
         Object oldData[] = elementData;
         elementData = new Object[size];
         System.arraycopy(oldData, 0, elementData, 0, size);
     * Increases the capacity of this <tt>ArrayList</tt> instance, if
     * necessary, to ensure  that it can hold at least the number of elements
     * specified by the minimum capacity argument.
     * @param   minCapacity   the desired minimum capacity.
    public void ensureCapacity(int minCapacity) {
     modCount++;
     int oldCapacity = elementData.length;
     if (minCapacity > oldCapacity) {
         Object oldData[] = elementData;
         int newCapacity = (oldCapacity * 3)/2 + 1;
             if (newCapacity < minCapacity)
          newCapacity = minCapacity;
         elementData = new Object[newCapacity];
         System.arraycopy(oldData, 0, elementData, 0, size);
     * Returns the number of elements in this list.
     * @return  the number of elements in this list.
    public int size() {
     return size;
     * Tests if this list has no elements.
     * @return  <tt>true</tt> if this list has no elements;
     *          <tt>false</tt> otherwise.
    public boolean isEmpty() {
     return size == 0;
     * Returns <tt>true</tt> if this list contains the specified element.
     * @param elem element whose presence in this List is to be tested.
     * @return  <code>true</code> if the specified element is present;
     *          <code>false</code> otherwise.
    public boolean contains(Object elem) {
     return indexOf(elem) >= 0;
     * Searches for the first occurence of the given argument, testing
     * for equality using the <tt>equals</tt> method.
     * @param   elem   an object.
     * @return  the index of the first occurrence of the argument in this
     *          list; returns <tt>-1</tt> if the object is not found.
     * @see     Object#equals(Object)
    public int indexOf(Object elem) {
     if (elem == null) {
         for (int i = 0; i < size; i++)
          if (elementData==null)
          return i;
     } else {
     for (int i = 0; i < size; i++)
          if (elem.equals(elementData[i]))
          return i;
     return -1;
* Returns the index of the last occurrence of the specified object in
* this list.
* @param elem the desired element.
* @return the index of the last occurrence of the specified object in
* this list; returns -1 if the object is not found.
public int lastIndexOf(Object elem) {
     if (elem == null) {
     for (int i = size-1; i >= 0; i--)
          if (elementData[i]==null)
          return i;
     } else {
     for (int i = size-1; i >= 0; i--)
          if (elem.equals(elementData[i]))
          return i;
     return -1;
* Returns a shallow copy of this <tt>ArrayList</tt> instance. (The
* elements themselves are not copied.)
* @return a clone of this <tt>ArrayList</tt> instance.
public Object clone() {
     try {
     ArrayList v = (ArrayList)super.clone();
     v.elementData = new Object[size];
     System.arraycopy(elementData, 0, v.elementData, 0, size);
     v.modCount = 0;
     return v;
     } catch (CloneNotSupportedException e) {
     // this shouldn't happen, since we are Cloneable
     throw new InternalError();
* Returns an array containing all of the elements in this list
* in the correct order.
* @return an array containing all of the elements in this list
*      in the correct order.
public Object[] toArray() {
     Object[] result = new Object[size];
     System.arraycopy(elementData, 0, result, 0, size);
     return result;
* Returns an array containing all of the elements in this list in the
* correct order; the runtime type of the returned array is that of the
* specified array. If the list fits in the specified array, it is
* returned therein. Otherwise, a new array is allocated with the runtime
* type of the specified array and the size of this list.<p>
* If the list fits in the specified array with room to spare (i.e., the
* array has more elements than the list), the element in the array
* immediately following the end of the collection is set to
* <tt>null</tt>. This is useful in determining the length of the list
* <i>only</i> if the caller knows that the list does not contain any
* <tt>null</tt> elements.
* @param a the array into which the elements of the list are to
*          be stored, if it is big enough; otherwise, a new array of the
*           same runtime type is allocated for this purpose.
* @return an array containing the elements of the list.
* @throws ArrayStoreException if the runtime type of a is not a supertype
* of the runtime type of every element in this list.
public Object[] toArray(Object a[]) {
if (a.length < size)
a = (Object[])java.lang.reflect.Array.newInstance(
a.getClass().getComponentType(), size);
     System.arraycopy(elementData, 0, a, 0, size);
if (a.length > size)
a[size] = null;
return a;
// Positional Access Operations
* Returns the element at the specified position in this list.
* @param index index of element to return.
* @return the element at the specified position in this list.
* @throws IndexOutOfBoundsException if index is out of range <tt>(index
*           < 0 || index >= size())</tt>.
public Object get(int index) {
     RangeCheck(index);
     return elementData[index];
* Replaces the element at the specified position in this list with
* the specified element.
* @param index index of element to replace.
* @param element element to be stored at the specified position.
* @return the element previously at the specified position.
* @throws IndexOutOfBoundsException if index out of range
*          <tt>(index < 0 || index >= size())</tt>.
public Object set(int index, Object element) {
     RangeCheck(index);
     Object oldValue = elementData[index];
     elementData[index] = element;
     return oldValue;
* Appends the specified element to the end of this list.
* @param o element to be appended to this list.
* @return <tt>true</tt> (as per the general contract of Collection.add).
public boolean add(Object o) {
     ensureCapacity(size + 1); // Increments modCount!!
     elementData[size++] = o;
     return true;
* Inserts the specified element at the specified position in this
* list. Shifts the element currently at that position (if any) and
* any subsequent elements to the right (adds one to their indices).
* @param index index at which the specified element is to be inserted.
* @param element element to be inserted.
* @throws IndexOutOfBoundsException if index is out of range
*          <tt>(index < 0 || index > size())</tt>.
public void add(int index, Object element) {
     if (index > size || index < 0)
     throw new IndexOutOfBoundsException(
          "Index: "+index+", Size: "+size);
     ensureCapacity(size+1); // Increments modCount!!
     System.arraycopy(elementData, index, elementData, index + 1,
               size - index);
     elementData[index] = element;
     size++;
* Removes the element at the specified position in this list.
* Shifts any subsequent elements to the left (subtracts one from their
* indices).
* @param index the index of the element to removed.
* @return the element that was removed from the list.
* @throws IndexOutOfBoundsException if index out of range <tt>(index
*           < 0 || index >= size())</tt>.
public Object remove(int index) {
     RangeCheck(index);
     modCount++;
     Object oldValue = elementData[index];
     int numMoved = size - index - 1;
     if (numMoved > 0)
     System.arraycopy(elementData, index+1, elementData, index,
               numMoved);
     elementData[--size] = null; // Let gc do its work
     return oldValue;
* Removes all of the elements from this list. The list will
* be empty after this call returns.
public void clear() {
     modCount++;
     // Let gc do its work
     for (int i = 0; i < size; i++)
     elementData[i] = null;
     size = 0;
* Appends all of the elements in the specified Collection to the end of
* this list, in the order that they are returned by the
* specified Collection's Iterator. The behavior of this operation is
* undefined if the specified Collection is modified while the operation
* is in progress. (This implies that the behavior of this call is
* undefined if the specified Collection is this list, and this
* list is nonempty.)
* @param c the elements to be inserted into this list.
* @return <tt>true</tt> if this list changed as a result of the call.
* @throws IndexOutOfBoundsException if index out of range <tt>(index
*          < 0 || index > size())</tt>.
* @throws NullPointerException if the specified collection is null.
public boolean addAll(Collection c) {
     modCount++;
     int numNew = c.size();
     ensureCapacity(size + numNew);
     Iterator e = c.iterator();
     for (int i=0; i<numNew; i++)
     elementData[size++] = e.next();
     return numNew != 0;
* Inserts all of the elements in the specified Collection into this
* list, starting at the specified position. Shifts the element
* currently at that position (if any) and any subsequent elements to
* the right (increases their indices). The new elements will appear
* in the list in the order that they are returned by the
* specified Collection's iterator.
* @param index index at which to insert first element
*          from the specified collection.
* @param c elements to be inserted into this list.
* @return <tt>true</tt> if this list changed as a result of the call.
* @throws IndexOutOfBoundsException if index out of range <tt>(index
*          < 0 || index > size())</tt>.
* @throws NullPointerException if the specified Collection is null.
public boolean addAll(int index, Collection c) {
     if (index > size || index < 0)
     throw new IndexOutOfBoundsException(
          "Index: "+index+", Size: "+size);
     int numNew = c.size();
     ensureCapacity(size + numNew); // Increments modCount!!
     int numMoved = size - index;
     if (numMoved > 0)
     System.arraycopy(elementData, index, elementData, index + numNew,
               numMoved);
     Iterator e = c.iterator();
     for (int i=0; i<numNew; i++)
     elementData[index++] = e.next();
     size += numNew;
     return numNew != 0;
* Removes from this List all of the elements whose index is between
* fromIndex, inclusive and toIndex, exclusive. Shifts any succeeding
* elements to the left (reduces their index).
* This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements.
* (If <tt>toIndex==fromIndex</tt>, this operation has no effect.)
* @param fromIndex index of first element to be removed.
* @param toIndex index after last element to be removed.
protected void removeRange(int fromIndex, int toIndex) {
     modCount++;
     int numMoved = size - toIndex;
System.arraycopy(elementData, toIndex, elementData, fromIndex,
numMoved);
     // Let gc do its work
     int newSize = size - (toIndex-fromIndex);
     while (size != newSize)
     elementData[--size] = null;
* Check if the given index is in range. If not, throw an appropriate
* runtime exception.
private void RangeCheck(int index) {
     if (index >= size || index < 0)
     throw new IndexOutOfBoundsException(
          "Index: "+index+", Size: "+size);
* Save the state of the <tt>ArrayList</tt> instance to a stream (that
* is, serialize it).
* @serialData The length of the array backing the <tt>ArrayList</tt>
* instance is emitted (int), followed by all of its elements
* (each an <tt>Object</tt>) in the proper order.
private synchronized void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException{
     // Write out element count, and any hidden stuff
     s.defaultWriteObject();
// Write out array length
s.writeInt(elementData.length);
     // Write out all elements in the proper order.
     for (int i=0; i<size; i++)
s.writeObject(elementData[i]);
* Reconstitute the <tt>ArrayList</tt> instance from a stream (that is,
* deserialize it).
private synchronized void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
     // Read in size, and any hidden stuff
     s.defaultReadObject();
// Read in array length and allocate array
int arrayLength = s.readInt();
elementData = new Object[arrayLength];
     // Read in all elements in the proper order.
     for (int i=0; i<size; i++)
elementData[i] = s.readObject();