Implementing Comparator class in Java

Question

I am new to java. I have not understood one thing while implementing the following code:

public class Demo 
{
class DemoComparator implements Comparator<Board>
{
    @Override
    public int compare(Board A, Board B) { 
        return A.f()-B.f();
    }
PriorityQueue<Board>Q = new PriorityQueue<>(10, new DemoComparator());

}

Here, Board is a class which I haven't shown and f() is a function that returns an integer value. My question is if I write:

Q.add(element1);
Q.add(element2);
Q.add(element3);

where, element1,element2 and element3 are of type Board and have their own f values returned from their respective f() function.

Will element1,element2 and element3 be stored in the priority queue according to their f values considering any increasing or decreasing order? and when the line: return A.f()-B.f(); will be called?

why don't you consider Comparable<T> instead of Comparator<T>? — Andrew
– Andrew, Commented Apr 5, 2016 at 18:14

Robert Moskal · Accepted Answer · 2016-04-05 18:54:26Z

4

Yes, the elements will be ordered by the Comparator you pass the constructor. From the docs:

https://docs.oracle.com/javase/7/docs/api/java/util/PriorityQueue.html

The head of the queue is the item with the lowest value. The comparator will be called on the constructor or when you call add() from the collection interface or the offer() method from the queue interface. Just look at the source code:

http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/PriorityQueue.java#267

Or write a little test that proves it's happening.

edited Apr 5, 2016 at 18:54

answered Apr 5, 2016 at 18:13

Robert Moskal

22.7k8 gold badges69 silver badges91 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

user6149854 Over a year ago

will the comparator be called when I call add() from the second time and later? @Robert Moskal

Boo Radley · Accepted Answer · 2016-04-05 18:57:31Z

1

If the f values are themselves Comparable you may want to use

return A.f().compareTo(B.f());

This would ensure that your data is sorted by the natural ordering of f, otherwise you can simply define the Comparator functionality yourself as you did in the provided example.

answered Apr 5, 2016 at 18:57

Boo Radley

6821 gold badge13 silver badges25 bronze badges

Collectives™ on Stack Overflow

Implementing Comparator class in Java

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related