How to get inverse of a comparator in java

Question

In a method I receive a generic object E extends Comparable as an argument. Now i want to create two priority queues.One which uses the comparato

Answer 1

The single argument of Collections.reverseOrder is a Comparator and not a Collection. For your code simply use reverseOrder without an argument. You have to use a non-zero inital size, too. The following code will work.

queue2=new PriorityQueue<E>(1, Collections.reverseOrder());

Answer 2

Your object E extends java.lang.Comparable, but it is not a java.util.Comparator.

Create your first queue w/o a Comparator and you'll get the ordering in your compareTo function, then create a java.util.Comparator that does the comparison in reverse (just call a.compareTo(b) and then negate the result) and create your second queue with that comparator.

Answer 3

Below Program depicts how to do it.

I have StringLengthComparator which compares based on string length. Using Collections.reverseOrder I have created queue which is reverse ordered and another queue which is ordered correctly.

import java.util.Collections;
import java.util.Comparator;
import java.util.PriorityQueue;

public class TestReverseorder {
public static void main(String[] args) {
    Comparator<String> comparator = new TestReverseorder().new StringLengthComparator();
    PriorityQueue<String> reverse = new PriorityQueue<String>(10,
            Collections.reverseOrder(comparator));
    PriorityQueue<String> queue = new PriorityQueue<String>(10,comparator);
    queue.add("1");
    queue.add("12");
    queue.add("123");

    reverse.add("1");
    reverse.add("12");
    reverse.add("123");

    while (!queue.isEmpty()) {
        System.out.println(queue.poll());
    }

    while (!reverse.isEmpty()) {
        System.out.println(reverse.poll());
    }


}

public class StringLengthComparator implements Comparator<String> {
    @Override
    public int compare(String x, String y) {
        // Assume neither string is null. Real code should
        // probably be more robust
        if (x.length() < y.length()) {
            return -1;
        }
        if (x.length() > y.length()) {
            return 1;
        }
        return 0;
    }
}
}

It will print output

Normal Order:
1
12
123
Reverse Order:
123
12
1

Answer 4

Look at Collections.reverseOrder.