Question

I am new to java language and have been reading over the API documentations. I was wondering if someone could help me with what some of the symbols mean? For example, in PriorityQueue there is:

Constructor Summary
...
PriorityQueue(Collection<? extends E> c)
...

Method Summary
...
<T> T[]
...

My problem is around ‘?’, ‘E’, ‘c’ and ‘T’. I think I have worked out a few, like ‘T’ I think is Type. If someone could help with understanding I would be much greatful. A link to website that describes would be great! Thank you!

Answer 1

You’re right that T is a type parameter. In this case it can be replaced by any type, since it does not have constraints.

This constructor has a type constraint:

PriorityQueue(Collection<? extends E> c)

and should be read as: create a new PriorityQueue instance using a Collection taking as type parameter any type that extends the type E (including E itself), where E is the type parameter of the PriorityQueue. Example:

List<String> list = new ArrayList<String>();
PriorityQueue<String> pq = new PriorityQueue<String>(list);

In this case E (the type parameter of pq) is String and the type parameter of list matches the predicate ? extends E, since it is also String.

This would also work:

List<String> list = new ArrayList<String>();
PriorityQueue<Object> pq = new PriorityQueue<Object>(list);

since String is a subclass of Object, but this would fail at compilation:

List<String> list = new ArrayList<String>();
PriorityQueue<Integer> pq = new PriorityQueue<Integer>(list);

I suggest you read more about Java generics here.