stack.java

public class Stack { 
    private Object[] elements; 
    private int size = 0; 

    public Stack(int initialCapacity) { 
        this.elements = new Object[initialCapacity]; 
    }

    public void push(Object e) { 
        ensureCapacity(); 
        elements[size++] = e; 
    }    

    public Object pop() { 
        if (size == 0) 
            throw new EmptyStackException(); 
        return elements[--size]; 
    }    

    /**    
     * Ensure space for at least one more element, roughly 
     * doubling the capacity each time the array needs to grow. 
     */     
    private void ensureCapacity() { 
        if (elements.length == size) { 
            Object[] oldElements = elements; 
            elements = new Object[2 * elements.length + 1]; 
            System.arraycopy(oldElements, 0, elements, 0, size); 
        }
    }
}