When interviewing, I often like to ask a candidate to discuss inheritance vs composition using a Stack as an example.

Example 1.

public class EvilStack<T> extends Vector<T> {
    public T pop() {
        // ...
    }
    public void push(T item) {
        // ...
    }
}

Example 2.

public class Stack<T> {
    private Vector<T> items = new Vector<T>();
    public T pop() {
        // ...
    }
    public void push(T item) {
        // ...
    }
}

Extending Vector as in example 1 weakens the encapsulation of the class. Suddenly, methods to get and insert elements at specific positions are available to clients of the stack. We move from trying to create a well behaved LIFO stack to creating a socially irresponsible monster: an EvilStack.

So I was surprised when looking at the Java source to see that java.util.Stack actually extends Vector! Naughty and things aren’t any better in Java 7 and 8.