An object of type IntStack is a stack of real numbers, with the standard stack operations push(int N), pop(), and isEmpty(). A makeEmpty() operation is also provided to remove all items from the stack.

Internally, the stack is implemented as a linked list.

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public class NumberStack {

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

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

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

public double pop() {
// 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.)
double topItem = top.item;
top =;
return topItem;

public boolean isEmpty() {
// Returns true if the stack is empty. Returns false
// if there are one or more items on the stack.
return top == null;

} // end class NumberStack