public class BoxDemo2 {

    public static void main(String[] args) {

        // ONLY place Integer objects into this box!
        Box integerBox = new Box();

        // Imagine this is one part of a large application
        // modified by one programmer. 
        integerBox.add("10"); // note how the type is now String

        // ... and this is another, perhaps written
        // by a different programmer
        Integer someInteger = (Integer)integerBox.get();
        System.out.println(someInteger);
    }
}