Remove from HashMap if there is no key in the list

Question

Is there any elegant way to remove an item from a hash map where the key is not listed in the specified list of items? I would really appreciate if anyone would give a piece of code. If it weren’t for me, I probably would have done something like this:

public HashMap<Integer, NameAndID> getTasksWithWordInFormula(Session session, 
        HashMap<Integer, NameAndID> taskMap, int sectionID, int topicID, int wordID) {
    @SuppressWarnings("unchecked")
    List<Integer> goodList = session.createCriteria(Frbw.class)
            .add(Restrictions.in("id.formulaId", taskMap.keySet()))
            .add(Restrictions.eq("sectionId", sectionID))
            .add(Restrictions.eq("topicId", topicID))
            .add(Restrictions.eq("wordId", wordID))
            .setProjection(Projections.projectionList()
                 .add(Projections.property("id.formulaId")))
            .setCacheable(true).setCacheRegion("query.DBParadox").list();
    ArrayList<Integer> toRemove = new ArrayList<Integer>();
    for (Integer formulaID : taskMap.keySet()) 
        if (!goodList.contains(formulaID))
            toRemove.add(formulaID);
    for (Integer formulaID : toRemove) 
        taskMap.remove(formulaID);
    return taskMap;
}

Answer 1

You can use Set#retainAll:

taskMap.keySet().retainAll(goodList);

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From Map#keySet:

Returns a representation of the Setkeys contained in this map. The set is supported by the map, so changes in the map are reflected in the set and vice versa.

(my emphasis)