泛型和Java8流的未经检查的调用错误

Question

I've got an odd warning from my IDE, but the code still runs fine.

The code is

class TaskQueue<T extends Comparable> {
    private final PriorityQueue<QueueItem<T>> queue = 
            new PriorityQueue<>(Comparator.comparing(QueueItem::getDeadline));
}

The IDE gives me a warning and an error:

• Warning
  Unchecked call to 'PriorityQueue(Comparator<? super E>)' as a member of raw type 'java.util.PriorityQueue'

• Error referring to QueueItem::getDeadline
Non-static method cannot be referenced from a static context

I'm new to Java and, altough the code works, I'd like to know if I can write it in a better way or if there is a proper way.

Thank you

Edit

The code for QueueItem

class QueueItem<T extends Comparable>{
    private final T deadline;

    QueueItem(T deadline) {
        this.deadline = deadline;
    }


    T getDeadline() {
        return deadline;
    }
}

Answer 1

Shouldn't that QueueItem look a bit different(same goes for TaskQueue):

 QueueItem<T extends Comparable<T>>

notice the T extends Comparable<T>

Comparable is a generic type, but in your code you did not specify the type of that Comparable ; instead it is raw.

Answer 2

The method QueueItem::getDeadline return some kind of object that does not implement Comparable .

So the solution will be:

class Deadline implements Comparable<Deadline> {

    @Override
    public int compareTo(Deadline o) {
        return 0 /* Some comparing logic here */;
    }

}

And the Comparator.comparing expects something that returns Comparable object.