Selecting random key and value sets from a Map in Java

Question

I want to get random keys and their respective values from a Map. The idea is that a random generator would pick a key and display that value. The tricky part is that both k

Answer 1

If you don't mind the wasted space, one approach would be to separately keep a List of all keys that are in the Map. For best performance, you'll want a List that has good random-access performance (like an ArrayList). Then, just get a random number between 0 (inclusive) and list.size() (exclusive), pull out the key at that index, and look that key up.

Random rand = something
int randIndex = rand.nextInt(list.size());
K key = list.get(randIndex);
V value = map.get(key);

This approach also means that adding a key-value pair is a good deal cheaper than removing one. To add the key-value pair, you would test to see if the key is already in the map (if your values can be null, you'll have to separately call map.containsKey; if not, you can just add the key-value pair and see if the "old value" it returns is null). If the key is already in the map, the list is unchanged, but if not, you add the key to the list (an O(1) operation for most lists). Removing a key-value pair, though, involves an O(N) operation to remove the key from the list.

If space is a big concern, but performance is less so, you could also get an Iterator over the map's entry set (Map.entrySet()), and skip randIndex entries before returning the one you want. But that would be an O(N) operation, which kinda defeats the whole point of a map.

Finally, you can just get the entry set's toArray() and randomly index into that. That's simpler, though less efficient.