package basics.collection;
import java.util.List;
import java.util.ArrayList;
import java.util.EmptyStackException;
final public class IntegerStack {
  final private List<Integer> l;
  public IntegerStack() { l = new ArrayList<Integer>(); }
  public boolean isEmpty() { return l.isEmpty(); }
  public void push(Integer x) {
    if (x == null)
      throw new IllegalArgumentException();
    l.add(x);
  }
  public Integer pop() {
    if (l.isEmpty())
      throw new EmptyStackException();
    return l.remove(l.size()-1);
  }
}