How to select a random key from a HashMap in Java?

Question

I\'m working with a large ArrayList>, and I would repeatedly need to select a random key from a random HashMap (and do some stuff with it).

Answer 1

You need access to the underlying entry table.

// defined staticly
Field table = HashMap.class.getDeclaredField("table");
table.setAccessible(true);
Random rand = new Random();

public Entry randomEntry(HashMap map) {
    Entry[] entries = (Entry[]) table.get(map);
    int start = rand.nextInt(entries.length);
    for(int i=0;i

This still has to traverse the entries to find one which is there so the worst case is O(n) but the typical behaviour is O(1).