Recursive Polymorphic Stack

/* A recursive, polymorphic implementation of a stack */
public class Stack<T> {
    /** Return true if the collection is empty */
    public boolean isEmpty() { return true; }

    /** Return the top element on the stack */
    public T peek() { return null; }

    /** Return the stack, sans top element */
    public Stack<T> pop() { return this; }

    /** Return a new stack with a given item at the top */
    public Stack<T> push(final T elem) {
        return new Stack<T>() {
            public T peek() { return elem; }
            public Stack<T> pop() { return Stack.this; }
            public boolean isEmpty() { return false; }
        };}
}

/* Sample usages */

/* Add entries to a stack */
Stack<String> animals = new Stack<T>()
    .push("Dog")
    .push("Rat")
    .push("Cat");

/* A generic print method for a stack */
static <T> void print(Stack<T> stack) {
    while (!stack.isEmpty()) {
        System.out.println(stack.peek());
        stack = stack.pop();
    }
}