What is the meaning of the <?> token in Java?

Question

What is the meaning of the token in this code copied from www.JavaPractices.com? When I replace it with the more conventional looking

Answer 1

The joker ? can hold any type. If you want to use the same type for all methods/members you can make the whole class generic. By writing StopAlarmTask<T> you define the type T in the whole class.

private final class StopAlarmTask<T> implements Runnable 
{
    StopAlarmTask(ScheduledFuture<T> aSchedFuture)
    {
        fSchedFuture = aSchedFuture;
    }

    public void run() 
    { /* */ }

    private ScheduledFuture<T> fSchedFuture;
}

Answer 2

Assuming this.executorService is a subtype of ScheduledExecutorService (available since Java 1.5), the return type of scheduleWithFixedDelay() is ScheduledFuture<?>. You can't change the return type from ScheduledFuture<?> to ScheduledFuture<T>, ScheduledFuture<Integer> or anything else for that matter. You could, however, change it to just ScheduledFuture since <?> is a wildcard generic type parameter that approximates a raw type for backward compatibility.

See What is a raw type and why shouldn't we use it? for a good discussion on raw types and generics.