How to sort a HashSet?

Question

For lists, we use the Collections.sort(List) method. What if we want to sort a HashSet?

Answer 1

If you want want the end Collection to be in the form of Set and if you want to define your own natural order rather than that of TreeSet then -

1. Convert the HashSet into List
2. Custom sort the List using Comparator
3. Convert back the List into LinkedHashSet to maintain order
4. Display the LinkedHashSet

Sample program -

package demo31;

import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Set;

public class App26 {
    public static void main(String[] args) {
        Set set = new HashSet<>();
        addElements(set);
        List list = new LinkedList<>();
        list = convertToList(set);
        Collections.sort(list, new Comparator() {
            @Override
            public int compare(String s1, String s2) {
                int flag = s2.length() - s1.length();
                if(flag != 0) {
                    return flag;
                } else {
                    return -s1.compareTo(s2);
                }
            }
        });
        Set set2 = new LinkedHashSet<>();
        set2 = convertToSet(list);
        displayElements(set2);
    }
    public static void addElements(Set set) {
        set.add("Hippopotamus");
        set.add("Rhinocerous");
        set.add("Zebra");
        set.add("Tiger");
        set.add("Giraffe");
        set.add("Cheetah");
        set.add("Wolf");
        set.add("Fox");
        set.add("Dog");
        set.add("Cat");
    }
    public static List convertToList(Set set) {
        List list = new LinkedList<>();
        for(String element: set) {
            list.add(element);
        }
        return list;
    }
    public static Set convertToSet(List list) {
        Set set = new LinkedHashSet<>();
        for(String element: list) {
            set.add(element);
        }
        return set;
    }
    public static void displayElements(Set set) {
        System.out.println(set);
    }
}

Output -

[Hippopotamus, Rhinocerous, Giraffe, Cheetah, Zebra, Tiger, Wolf, Fox, Dog, Cat]

Here the collection has been sorted as -

First - Descending order of String length
Second - Descending order of String alphabetical hierarchy