BlockingQueue is different from Collection in that, for no specified reason, it never permits null elements. null elements would be very useful in blocking queues to signal that some alternate action is required. In fact the documentation suggests a poison object in the case of shutting down the queue. If the code operating on the queue is itself generic, then finding a special object other than null is extremely problematic. In non-generic cases the workaround may merely be evil (like using a particular new String()).


Alternatively adding a BlockingQueue.wakeup() method, in analogy to Selector.wakeup(), together with slight changes to blocking methods woul work. Workaround include checking additional state for each element read (assuming something will be sent through the queue at the same time), modifying the queued object if that is available and wrapping every element in a new nullable object (below). None of the workarounds are particularly nice.

    static class Nullable<T> {
        private final T obj;
        public Nullable() {
            this(null);
        }
        public Nullable(T obj) {
            this.obj = obj;
        }
        public T get() {
            return obj;
        }
    }