Thread: Help needed with Comparator objects and Collections

I'm having issues with using Comparator objects and Collections and would be grateful to receive assistance on the following:

Write code for a Comparator object that orders List<Integer> objects in order of their length. Then write code for a Comparator object that orders List<Integer> objects in the order of the sum of their integers (the sum is all the integers in the list added together). Use these Comparator objects and the two-argument method max from Java’s class Collections to find the longest list and the list which adds up to the most from the set of lists from a HashSet<List<Integer>> object, for example as created in question 2.

The following is the code that I already have (to summarise it's consists of a HashSet<List<Integer>> object that will store values of type List<Integer>, I'm at the point now where I need to re-order the HashSet and then run both Comparators):

import java.util.*;
 
class Ex5b implements Comparator<HashSet<List<Integer>>>
{
    public static void main(String[] args) 
    {
        Scanner in = new Scanner(System.in);
        String line, ask;
        boolean run = true;
        HashSet<List<Integer>> hs = new HashSet<List<Integer>>();
 
        while(run)
        {
            System.out.println("Enter some integers (all on one line):");
            line = in.nextLine();
 
            String[] words = line.split(" ");
 
            List<Integer> l = new LinkedList<Integer>();
 
            for(int i=0; i<words.length; i++)
            {
                l.add(new Integer(words[i]));
            }
 
            hs.add(l);
 
            System.out.println("Would you like to enter more integers?: (y/n)");
            ask = in.nextLine();
            if(ask.equalsIgnoreCase("N"))
            {
                run = false;
            }   
 
        }
 
        System.out.println("Contents of list: " + hs);
 
        //PLACE WHERE I AM STUCKFOR REST OF CODE
    }
 
    // ATTEMPT TO CREATE A COMPARATOR THAT WOULD SORT HASHSET IN ORDER OF LENGTH OF EACH LIST 
    public HashSet<List<Integer>> compareLength(HashSet<List<Integer>> hs) 
    {
        HashSet<List<Integer>> hsLength = new HashSet<List<Integer>>();
 
        // INCOMPLETE - I PRESUME THAT THIS SHOULD BE DONE CONSTRUCTIVELY AND NOT DESTRUTIVELY - ALSO MEANS RETURNING A HASHSET WHICH I AM HAVING PROBLEMS WITH (NON-STATIC METHOD BEING REFERENCED FROM A STATIC CONTEXT)
 
    }
}

INTERFACE FOR COMPARATOR(S)

import java.util.*;
 
interface Comparator<T>
{
     public T compareLength(T o1);
}
 
// I PRESUME I WILL REQUIRE ANOTHER METHOD FOR ORDERING BY SIZE HERE

I'd be grateful if anyone could assist me as I've been working on this on/off over the last couple of days and are completely lost...

What method of what class are you using that requires a Comparator object?

Oh would I require another class that contain a method which would run the comparators?

I'm pretty new to using Interfaces if the case...

Thanks for replying.

First I'd recommend you read up on what an interface is for and how to use them.

What Is an Interface? (The Java™ Tutorials > Learning the Java Language > Object-Oriented Programming Concepts)
Lesson: Interfaces and Inheritance (The Java™ Tutorials > Learning the Java Language)