Is this a cyclic reference?

Question

Please consider this an attempt to understand the api and not questioning the judgement of the architect.

Referring to the constructor of the javafx.event.Event, I need to pass an EventType of Event to instantiate an Event. Am I reading this right ? If so, wouldn't this be a cyclic reference ?

As far as I see on the API docs, Event depends on EventType, but EventType does not depend on Event, so there is no cyclic reference. — Guillaume, Jan 31 '13 at 04:06

score 3 · Answer 1 · answered Jan 31 '13 at 04:06

3

No its not, because generic parameters are resolved at compile time. At runtime there's nothing being passed into the Event constructor but an EventType.

answered Jan 31 '13 at 04:06

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score 2 · Answer 2 · answered Jan 31 '13 at 04:09

Not in the sense that you mean.

An EventType instance can refer to other EventType instances, but not to Event instances. Take a look at the constructor and methods of EventType, paying close attention to the method / constructor parameter and return types.

score 1 · Answer 3 · edited May 23 '17 at 10:24

This is not cyclical, but it really, really takes a lot of thinking to understand.

From the javadoc you provided:

Event(EventType<? extends Event> eventType) 
Construct a new Event with the specified event type.

The EventType<? extends Event> should be read as "An EventType that works with a certain type of Event".

Here is another SO answer that does a better job of explaining the <? extends Class>: https://stackoverflow.com/a/3009779/463196

For example, let's say we have a FooEvent that extends Event. We would then have an EventType that works with FooEvent.

Also, if you look at the JavaDoc for EventType, it says:

This class represents a specific event type associated with an Event.

Is this a cyclic reference?

3 Answers3