Continuing this tour of Guava we get to the Multiset. I probably don't use this as much as Multimaps or Bimaps, but it certainly does have it's uses.

So what's a Multiset then?

Well as you might be able to guess it's a set that can hold multiple instances of the same object.

Isn't that just a List?

In Java there are two basic differences between Lists and Sets. Lists can hold duplicates of the same object, and Lists are always ordered. Sets can't hold duplicates, and there's no guarantee of order by the Set interface. (Some implementations - LinkedHashSet, SortedSet etc. - do of course provide a guaranteed order!)

So a Multiset occupies a sort of grey area between a List and a Set. Duplicates allowed, but no guaranteed order.

This collection is also sometimes called a Bag, in fact this is what Apache Commons Collections calls it's Mutlisets.

So what would I use one for?

The great thing about Multisets is they keep track of the counts of each particular object in the set. So you can use them for counting stuff.

Have you ever written code like the following:

Map<MyClass,Integer> objectCounts = new HashMap<MyClass,Integer>();
 
public void incrementCount(MyClass obj) {
    Integer count = objectCounts.get(obj);
    if (count == null) {
        objectCounts.put(obj,0);
    } else {
        objectCounts.put(obj,count++);
    }
}
 
public int getCount(MyClass obj) {
    Integer count = objectCounts.get(obj);
    if (count == null) {
        return 0;
    } else {
        return count;
    }
}

Bit unwieldy? Lets see how we might use a Multiset instead: 

Multiset<MyClass> myMultiset = HashMultiset.create();
 
MyClass myObject = new MyClass();
 
myMultiset.add(myObject);
myMultiset.add(myObject);  // add it a second time.
 
System.out.println(myMultiset.count(myObject)); // 2
 
myMultiset.remove(myObject);
System.out.println(myMultiset.count(myObject)); // 1

As you can see that's much simpler! It's even possible to add/remove more than one object at at time 

Multiset<MyClass> myMultiset = HashMultiset.create();
 
MyClass myObject = new MyClass();
myMultiset.add(myObject,5); // Add 5 copies of myObject
 
System.out.println(myMultiset.count(myObject)); // 5
 
myMultiset.remove(myObject,2); // remove 2 copies
 
System.out.println(myMultiset.count(myObject)); // 3

Pretty useful eh? As usual there's several implementations available depending on your requirements, and I recommend taking a look at the API: http://docs.guava-libraries.googlecode.com/git-history/v9.0/javadoc/com/google/common/collect/Multiset.html