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

Using Guava's Maps class' utility methods to compute the difference of 2 maps you can do it in a single line, with a method signature which makes it more clear what you are trying to accomplish:

public static void main(final String[] args) {
    // Create some maps
    final Map map1 = new HashMap();
    map1.put(1, "Hello");
    map1.put(2, "There");
    final Map map2 = new HashMap();
    map2.put(2, "There");
    map2.put(3, "is");
    map2.put(4, "a");
    map2.put(5, "bird");

    // Add everything in map1 not in map2 to map2
    map2.putAll(Maps.difference(map1, map2).entriesOnlyOnLeft());
}