How to sort a HashSet?

Question

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 =

Solution 4

You can use a TreeSet instead.

Solution 5

Use java.util.TreeSet as the actual object. When you iterate over this collection, the values come back in a well-defined order.

If you use java.util.HashSet then the order depends on an internal hash function which is almost certainly not lexicographic (based on content).

View more solutions

Comments

[Alexis C.]

A HashSet is an unordered collection.

[fge]

You can't, since a Set does not have random access methods (ie, .get() an element at a given index), which is basically required for sort algorithms ;)

[demongolem]

You could convert it to a list first then sort if you need to sort

[user207421]

You can't since a HashSet doesn't have a defined order. Your question embodies a contradiction in terms.

[Tendai]

use a TreeSet and if you can't control the source see the conversion an usage here stackoverflow.com/a/52987487/5153955

[fatimasajjad]

The following answer can be very helpful for you stackoverflow.com/a/59964344/7397820

[Sotirios Delimanolis]

Why are you assuming they store String values?

[P45 Imminent]

I'm not though perhaps my use of lexographic is imprecise ;-)

[Sotirios Delimanolis]

It's very wrong. It doesn't store keys in lexographic (sp?) order. It either uses their natural ordering (which depends on Comparable interface which the keys implement) or uses the provided Comparator.

[P45 Imminent]

I've edited. Better do you think , or should I delete the answer?

[Sotirios Delimanolis]

I would just quote the javadoc.

[Luiggi Mendoza]

In case you move a HashSet to TreeSet, your class must implement Comparable interface or provide a custom Comparator. Otherwise, since you cannot sort a HashSet, just convert it into a List and sort it.

[Diana]

I guess,I can use TreeSet with the constructor- TreeSet(Comparator<? super E> comparator).

[Thomas Eizinger]

I don't see where HashSet or Set has a method toList.

[Roberto]

That looks like Scala to me, which unfortunately doesn't solve the problem in Java.

[Onic Team]

toList method , How is possible ?

[wisbucky]

Also, if you're using a collection of Strings, then List<String> sortedList = new ArrayList<String>(yourHashSet);

[Mounika]

By using TreeSet myTreeSet = new TreeSet(myHashSet); you can avoid adding all the elements to Treeset again.

[Jess]

Can you please, Add the logic to sort it in a specific order ?

[Jess]

Just putting the elements wont give the flexibility of sorting on any order with any element inside it. The above solution does.

[LazerBanana]

@Jess i dont know whats the specific order for you Jess, you can sort it as you desire using the comparator.

[Jess]

I meant, how to define the Ascending or Descending

[devconsole]

new TreeSet<>(hashSet) is more concise and probably more efficient.

[Jaco-Ben Vosloo]

Your statement "TreeSet will sort all the elements automatically every time you insert an element" is untrue. If you change the value used for comparing of an object in the list, and then add a new object it will be placed as if the object you changed has its original value. Thus only the new item gets placed in the list at the correct point assuming none of the elements changed their value used to sort the list.

[off99555]

@Jaco-BenVosloo Thanks. I've edited the answer to reflect your comment.

[ScanQR]

This will remove duplicates as OP asking to sort HashSet and it may contain duplicate values.

[Abdullah Khan]

Hashset may contain duplicate values!! Really!??

[ceklock]

I got this error: The method sort(List<T>) in the type Collections is not applicable for the arguments (List<capture#2-of ?>)