Does Java have a multiset data structure like the one in C++ STL?

Question

I need a data structure which works like the STL multiset but the TreeSet in Java doesn\'t allow duplicate elements. Is there any built-in data structure in Java which is eq

Answer 1

Algorithms 4th edition has a Bag implementation that's available on the book's web site. You can view the javadoc online as well.

The Bag class represents a bag (or multiset) of generic items. It supports insertion and iterating over the items in arbitrary order.

You'll probably just want to have a look at the source code if Bag is all you need, but there are a lot of other interesting things in algs4.jar, so it's worth a look.

Answer 2

There is no Multiset in the standard Java libraries. You should use the Google Guava framework which contains Multiset classes. See

Answer 3

Since 1.8, Map allows you to do this:

  map.computeIfAbsent(key, k -> new HashSet<V>()).add(v);

Each value is added into a Set, which is created on demand.

You may need to change remove() too.

Answer 4

Apache Commons Collections has Bag and SortedBag interfaces. It sounds like TreeBag might meet your needs, but there are plenty of implementations to choose from.

Answer 5

TreeSet<Integer> set = new TreeSet<>();
TreeSet<Integer> multiset = new TreeSet<>((i, j) -> i < j ? 1 : -1);

Make a set multiset just by making 1 != 1!

Answer 6

You could use a PriorityQueue The elements are sorted in their natural order, or if you provide one, by comparator, and duplicates are allowed.