Coursera_Algorithm class practice: Generic stack using variable size array implementation

gistfile1.txt

public class MyVarSizeArrayStack<Item>
{
  private Item[] s;
  private int index;
  
  public MyVarSizeArrayStack() {
    s = (Item[]) new Object[1];
    index = 0;
  }
  
  public void resize(int capacity) {
    Item[] copy = (Item[]) new Object[capacity];
    for(int i = 0; i < index; i++) {
      copy[i] = s[i];
    }
    s = copy;
  }
  
  public boolean isEmpty() {
    return index == 0;
  }
  
  public void push(Item item) {
    if( index == s.length ) resize( 2 * s.length);
    s[index++] = item;
  }
  
  public Item pop() {
    if( index == 0 ) return null;
    if( index > 0 && index < s.length / 4) resize ( s.length / 2); 
     return s[--index];    
  }
  
    public static void main(String[] args) {
    MyArrayStack<Integer> s1 =  new MyArrayStack<Integer>(5);
    
    s1.push(1);
    s1.push(2);
    s1.push(3);
    
    System.out.println(s1.pop());
    System.out.println(s1.pop());
    System.out.println(s1.pop());
    System.out.println(s1.pop());
    System.out.println(s1.isEmpty());

}
  
}