LinkedStack

//----------------------------------------------------------------------
// LinkedStack.java         by Dale/Joyce/Weems                Chapter 2
//
// Implements StackInterface using a linked list to hold the elements.
//-----------------------------------------------------------------------

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

   public LinkedStack() {
      top = null;
   }

   public void push(T element) {
   // Places element at the top of this stack.
      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()) {
         top = top.getLink();
      }
      else
         throw new StackUnderflowException("Pop attempted on an empty stack.");
   }

   public T top() {
   // Throws StackUnderflowException if this stack is empty,
   // otherwise returns top element from this stack.
     if (!isEmpty())
        return top.getInfo();
     else
        throw new StackUnderflowException("Top attempted on an empty stack.");
   }

   public boolean isEmpty() {
   // Returns true if this stack is empty, otherwise returns false.
     if (top == null)
        return true;
     else
        return false;
   }
}

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