Question

i just found while studying the JDK 1.6 ArrayBlockingQueue – The constructor makes a call to one of the public overridable methods ! I thought this is a bad practice specially for an API.

 public ArrayBlockingQueue(int capacity, boolean fair,
                          Collection<? extends E> c) {
    this(capacity, fair);
    if (capacity < c.size())
        throw new IllegalArgumentException();

    for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
        add(it.next()); // -> surprise:  add is public 
}

I was actually trying to extend the ArrayBlockingQueue , and add some some state and overrided add() , and I promptly got the java.lang.NullPointerException when I invoked super(capacity,fair,col) constructor. Am I missing some design concept here ?

Answer 1

This isn’t so out of the ordinary; essentially, you can think of this as just a one-line syntax for the lines

ArrayBlockingQueue<E> queue = new ArrayBlockingQueue<E>(capacity, fair);
queue.addAll(c);

which is overridable. Calling overridable methods isn’t usually a good idea, but it’s perfectly reasonable here.

But…that said, extending any collection from java.util or java.util.concurrent is kind of evil. You should decorate them, rather than extending them directly.