package ch03.stacks;

public interface UnboundedStackInterface<T> extends StackInterface<T> {

  void push(T element);
}

================================================================

package support;
public class LLNode<T> {
  private LLNode<T> link = null;
  private T info;
  
  public LLNode(T info) { this.info = info; }
 
  public void setInfo(T info) { this.info = info; }

  public T getInfo() { return info; }
 
  public void setLink(LLNode<T> link) { this.link = link; }

  public LLNode<T> getLink() { return link; }
}

================================================================

package ch03.stacks;
import support.LLNode;

public class LinkedStack<T> implements UnboundedStackInterface<T> {
    protected LLNode<T> top;   // reference to the top of this stack

    public LinkedStack() { top = null; }

    public void push(T element) {
	LLNode<T> newNode = new LLNode<T>(element);
	newNode.setLink(top);
	top = newNode;
    }     

    public void pop() {
	// Throws StackUnderflowException if this stack is empty,
	// otherwise removes top element from this stack.
	if (isEmpty())
	    throw new StackUnderflowException("Helpful msg.");
	top = top.getLink();
    }

    public T top() {
	// Throws StackUnderflowException if this stack is empty,
	// otherwise returns top element from this stack.
	if (isEmpty())
	    throw new StackUnderflowException("Helpful msg.");
	return top.getInfo();
    }

    public boolean isEmpty() { return top == null; }
}