Here’s an implementation of a multiset in Java 8. It uses a Map<T, Integer> to count how many times an object was inserted (multiplicity). It is not thread-safe, but there’s synchronizedMultiset(...) and it always knows the current size in O(1). With the method asMap() you can get a view of the multiset as a Map, that will allow modification of the underlying multiset.
The code is on GitHub: github.com/claudemartin/multiset
This started as a simple implementation and is not rather complex. The iterator allows to remove elements during iteration. I tried to write my own Spliterator, but it wasn’t faster than the default implementaion. Many methods are optimized and I added some set operations for multisets (union, intersect, minus). With merge(...) you can merge two multisets with any operation.