public interface StackInterface<E>

public void push(E x);

// places x on a stack

public E pop();

// removes and returns the top item

// returns null if the stack is empty

public boolean empty();

// returns true if no elements are on the stack

public E peek();

// returns the top item, does not alter the stack

// returns null if the stack is empty

public int size();

// returns the number of items on the stack