String s = (String)o;

Lionel.

You can check if an object is of a certain type using 'instanceof'.

if (o instanceof String) {
   String s = (String) o;
}

however it would be better if you designed the Stack with generics.

/**
* A stack
* @param <T> the generic type
*/
public interface Stack<T> {
   /**
    * Push an object onto the stack
    * @param t the object
    */
   void push(T t);

   /**
    * @return pop an object from the stack
    */
   T pop();
}

A simple implementation used a linked list:

import java.util.LinkedList;

/**
* Implementation of Stack
*/
public class StackImpl<T> implements Stack<T> {
   private LinkedList<T> list = new LinkedList<T>();

   /**
    * {@inheritDoc}
    */
   public T pop() {
       return list.removeLast();
   }

   /**
    * {@inheritDoc}
    */
   public void push(T t) {
       list.addLast(t);
   }
}

Example usage:

public class StackTest {
   public static void main(String[] args) {
       Stack<String> stack = new StackImpl<String>();
       stack.push("hi");
       stack.push("joe");
       System.out.println(stack.pop());
       System.out.println(stack.pop());
   }
}

Cheers,
Martin.