How to sort a HashSet?

java

prompt-engineering

best-practices

performance

byAlex Kataev·Feb 21, 2025

// Conjure up a HashSet in true Hogwarts style:
HashSet<Integer> hashSet = new HashSet<>(Arrays.asList(4, 2, 3, 1));

// Transform HashSet into a List and call upon the almighty Collections.sort():
List<Integer> sortedList = new ArrayList<>(hashSet);
Collections.sort(sortedList); // sortedList is now [1, 2, 3, 4]

// Or let TreeSet perform some magic of its own:
TreeSet<Integer> sortedSet = new TreeSet<>(hashSet); // sortedSet is now [1, 2, 3, 4]

When the default order is too mainstream, customize it using a Comparator with Collections.sort() or TreeSet.

Unmasking the HashSet sorting mystery

When dealing with the unruly HashSet, which throws order out the window, we need to call upon the power of List or TreeSet to restore order. TreeSet sorts your elements and enforces uniqueness just like your trusty HashSet.

Custom order with a smidge of Comparator

Got a specific ordering in mind that differs from natural ordering? Declare a Comparator to your rescue. This fellow can help sort elements based on any custom conditions – whether you want to sort strings by length or witches by their spell potency.

Sorting performance – A Fighter's guide

While the journey from HashSet to List or TreeSet is rather simple, it's worth noting the performance tickets each method invoices you. If you'd rather save those for a rainy day, TreeSet comes as a knight in shining armor, adding all the elements of the HashSet into its realm with the addAll method or via its constructors swiftly, with no extra sorting needed.

Sorting the Java 8 way

Java 8 offers its powerful streams to help you sort a HashSet. The sorted method paired with Comparator.comparing lets you establish custom sorting criteria, in a neat and handy approach.

Set<String> strings = new HashSet<>(Arrays.asList("one", "two", "three"));
List<String> sortedStrings = strings.stream()
    .sorted(Comparator.comparing(String::length))
    .collect(Collectors.toList()); // "one" and "two" fighting for first place, "three" enjoying its spot at the end

TreeSet – The sentinel of uniqueness

When converting our unruly HashSet into a more manageable List, tread carefully lest you let duplicates creep in. If maintaining uniqueness is your game, then TreeSet is the name.

Service announcement for custom comparators users

If using custom Comparators, don't forget to honor the equals contract for the force to be with the TreeSet, which strictly adheres to comparison rules.

Fun with sorting – Some practical tips

Watch out – It's duplicate season!

Beware of inadvertently multiplying like rabbits (introducing duplicates) while transforming a HashSet to List due to unforeseen alterations affecting element equality.