I usually write my own class or methods for this.

For example, if you need a MuliSet I use a Map<?, Integer> to represent how many times a key is in the set, for a MultiMap you can use a Map<?, List<?>> to store all the values.

As for me, I usually just write my own class extending java.util.TreeMap

Counterquestion: why on earth you would need multimap?

You can use this

    static class MultiSet <T> extends HashMap/*Change to TreeMap if you want */<T,Integer>
    {
	public void add(T key)
	{
	    Integer q = super.get(key);
	    if(q == null)
		super.put(key, 1);
	    else
		super.put(key, q+1);
	}
	@Override
	public Integer remove(Object key) {
	    Integer q = super.get(key);
	    if(q != null)
	    {
		if(q == 1)
		    super.remove(key);
		else
		    super.put((T)key, q-1);
	    }
	    else
		throw new NullPointerException("The specified key does not exist");

	    return q;
	}
    }