Java Program to Show Stack Implementation

This is a simple Java program to demonstrate the Stack data structure implementation. This code provides a simple implementation of a stack with basic operations (push, pop, and isEmpty). Understanding how a stack works is fundamental, and this code demonstrates the concept in a straightforward manner. Additionally, it introduces the concept of an inner class (Node), which is commonly used in data structure implementations. Internally, the stack is implemented as a linked list.

push Method:

• The push method is used to add a new item to the top of the stack.
• It creates a new node (newTop) and assigns the value N to its item field.
• The next field of the new node is set to the current top of the stack.
• Finally, the top reference is updated to the new node.

pop Method:

• The pop method removes the top item from the stack and returns its value.
• It first retrieves the value of the top item (topItem), updates the top reference to the next node in the stack, and then returns the retrieved value.
• Note: This method does not handle the case where an attempt is made to pop from an empty stack. It would be better to throw a specific exception in that case.

public class NumberStack {

private static class Node {
  // An object of type Node holds one of the
  // items on the stack;
  double item;
  Node next;
}

  // Pointer to the Node that is at the top of
  // of the stack. If top == null, then the
  // stack is empty.
  private Node top; 

  // Add N to the top of the stack.
public void push( double N ) {
  Node newTop = new Node();
  newTop.item = N;
  newTop.next = top;
  top = newTop;
}

// Remove the top item from the stack, and return it.
// Note that this routine will throw a NullPointerException
// if an attempty is made to pop an item from an empty
// stack. (It would be better style to define a new
// type of Exception to throw in this case.)
public double pop() {
  double topItem = top.item;
  top = top.next;
  return topItem;
}

// Returns true if the stack is empty. Returns false
// if there are one or more items on the stack.

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

}