public class GenericStack <Item>  {
  
  private static final int defaultSize = 1000;
    
  private Item [] data;
  private int numElements = 0;
  
  GenericStack () {
    data = (Item []) new Object[defaultSize];
  }
  
  GenericStack (int initialSize) {
    data = (Item []) new Object[initialSize];
  }
  
  public boolean isEmpty() {
    return (numElements == 0);
  }
  
  public void push(Item c) {
    if (numElements == data.length) {
      Item [] tmpData = (Item []) new Object [Math.max(2*data.length, 1)];
      for (int i = 0; i<numElements; i++)
        tmpData[i]=data[i];
      data = tmpData;
    }
      
    data[numElements++]=c;
  }
    
  public Item pop () {
    Item ret = data[--numElements];
    data[numElements] = null; // to prevent loitering -- i.e., to allow garbage collector to reclaim the space used by this String
    return ret;
  }
}