Help with array program!!

Question

hi friends
i am a new comer to java and I have been studying Arrays recently. I came across this program on the internet ( thanks to Peter Williams)
package DataStructures;
import java.util.NoSuchElementException;
* An array implementation of a stack
* @author Peter Williams
public class StackArray implements Stack {
private int top; // for storing next item
public StackArray() {
stack = new Object[1];
top = 0;
public boolean isEmpty() {
return top == 0;
public void push(Object item) {
if (top == stack.length) {
// expand the stack
Object[] newStack = new Object[2*stack.length];
System.arraycopy(stack, 0, newStack, 0, stack.length);
stack = newStack;
stack[top++] = item;
public Object pop() {
if (top == 0) {
throw new NoSuchElementException();
} else {
return stack[--top];
interface Stack {
* Indicates the status of the stack.
* @return <code>true</code> if the stack is empty.
public boolean isEmpty();
* Pushes an item onto the stack.
* @param <code>item</code> the Object to be pushed.
public void push(Object item);
* Pops an item off the stack.
* @return the item most recently pushed onto the stack
* @exception NoSuchElementException if the stack is empty.
public Object pop();
class StackDemo {
public static void main(String[] args) {
     Stack s = new StackArray();
     // Stack s = new StackList();
     for (int i = 0; i < 8; i++) {
     s.push(new Integer(i));
     while (!s.isEmpty ()) {
     System.out.println(s.pop());
what baffles me is as below:
there is an 'if' construct in the push method, which talks about if(top == stack.length)
I fail to understand how top can at any time be equal to stack.length.
Could you help me understand this program?
a thousand apologies and thanks in advance

Answer 1

figurativelly speaking:
if you take an array and put it standing on your tesk, so that start of array points to floor, and end of array points to roof, then you can start filling that array as stack.
if you put books into stack, then you push (put) them on top of the stack, so the first book ever but into stack is adiacent to array element with index zero (0) and then second book in stack would be adiacent to array element that has index one (1) and so on.
if you have array of size 5, then fift book in stack would be adiacent to array element with index four (4)
after pushing that Object to stack, the top variable will get incremented (top++ in code) and be equal to size of array.
however, if you would like to push another Object to stack, then it would fall over the end of the array, so longer array is needed...