How to putAll on Java hashMap contents of one to another, but not replace existing keys and values?

Question

I need to copy all keys and values from one A HashMap onto another one B, but not to replace existing keys and values.

Whats the best way to do that?

I was t

Answer 1

Just iterate and add:

for(Map.Entry e : a.entrySet())
  if(!b.containsKey(e.getKey())
    b.put(e.getKey(), e.getValue());

Edit to add:

If you can make changes to a, you can also do:

a.putAll(b)

and a will have exactly what you need. (all the entries in b and all the entries in a that aren't in b)