How to swap arrayMap values and keys in Java

Question

I\'m having a bit of trouble reversing a given map and storing its reversed keys and values into another map. I have a method prototype as follows:

public st         


        

Answer 1

You will need to loop over the entries in your map, and then, since the values are stored in a set, you will need to loop over that set. You will need to check your result map for each key and create a new set whenever a key does not yet exist.

public static Map> reverse (Map > graph) {
    Map> result = new HashMap>();
    for (Map.Entry> graphEntry: graph.entrySet()) {
        for (String graphValue: graphEntry.getValue()) {
            Set set = result.get(graphValue);
            if (set == null) {
                set = new HashSet();
                result.put(graphValue, set);
            }
            set.add(graphEntry.getKey());
        }
    }
    return result;
}