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

It looks like you are willing to create a temporary Map, so I'd do it like this:

Map tmp = new HashMap(patch);
tmp.keySet().removeAll(target.keySet());
target.putAll(tmp);

Here, patch is the map that you are adding to the target map.

Thanks to Louis Wasserman, here's a version that takes advantage of the new methods in Java 8:

patch.forEach(target::putIfAbsent);