public class ArrayStack implements StackInterface

{

private Object[] stack; // array of stack entries

private int topIndex; // index of top entry

private static final int DEFAULT_MAX_SIZE = 50;

public ArrayStack()

{

stack = new Object[DEFAULT_MAX_SIZE];

topIndex = -1;

} // end default constructor

public ArrayStack(int maxSize)

{

stack = new Object[maxSize];

topIndex = -1;

} // end constructor

public void push(Object newEntry)

{

if (isFull())

throw new IllegalStateException("Attempt to push element into a full stack.");

stack[++topIndex] = newEntry;

} // end push

public Object peek()

{

Object top = null;

if (!isEmpty())

top = stack[topIndex];

return top;

} // end peek

public Object pop()

{

Object top = null;

if (!isEmpty())

{

top = stack[topIndex];

stack[topIndex] = null;

topIndex--;

} // end if

return top;

} // end pop

public boolean isEmpty()

{

return topIndex < 0;

} // end isEmpty

private boolean isFull(){

return topIndex == stack.length - 1;

}

/**

* clear

*/

public void clear() {

topIndex = -1;

}

}