//----------------------------------------------------------------------
// LinkedStack.java         by Dale/Joyce/Weems                Chapter 3
//
// Implements UnboundedStackInterface using a linked list 
// to hold the stack elements.
// ajg version: format, if-then-else
//-----------------------------------------------------------------------

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; }
}
// Local Variables:
// compile-command: "cd ../..; javac ch03/stacks/LinkedStack.java"
// End: