How to sort a HashSet?

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

Solution 1:

A HashSet does not guarantee any order of its elements. If you need this guarantee, consider using a TreeSet to hold your elements.

However if you just need your elements sorted for this one occurrence, then just temporarily create a List and sort that:

Set<?> yourHashSet = new HashSet<>();

...

List<?> sortedList = new ArrayList<>(yourHashSet);
Collections.sort(sortedList);

Solution 2:

Add all your objects to the TreeSet, you will get a sorted Set. Below is a raw example.

HashSet myHashSet = new HashSet();
myHashSet.add(1);
myHashSet.add(23);
myHashSet.add(45);
myHashSet.add(12);

TreeSet myTreeSet = new TreeSet();
myTreeSet.addAll(myHashSet);
System.out.println(myTreeSet); // Prints [1, 12, 23, 45]

Update

You can also use TreeSet's constructor that takes a HashSet as a parameter.

HashSet myHashSet = new HashSet();
myHashSet.add(1);
myHashSet.add(23);
myHashSet.add(45);
myHashSet.add(12);

TreeSet myTreeSet = new TreeSet(myHashSet);
System.out.println(myTreeSet); // Prints [1, 12, 23, 45]

Thanks @mounika for the update.

Solution 3:

Java 8 way to sort it would be:

fooHashSet.stream()
  .sorted(Comparator.comparing(Foo::getSize)) //comparator - how you want to sort it
  .collect(Collectors.toList()); //collector - what you want to collect it to

*Foo::getSize it's an example how to sort the HashSet of YourItem's naturally by size.

*Collectors.toList() is going to collect the result of sorting into a List the you will need to capture it with List<Foo> sortedListOfFoo =