how to merge more than one hashmaps also sum the values of same key in java

Question

ı am trying to merge more than one hashmaps also sum the values of same key, ı want to explain my problem with toy example as follows

    HashMap

        

Answer 1

If the key exists, add to it's value. If not insert.

Here is a simple example which merges one map into another:

Foo oldVal = map.get(key);
if oldVal == null
{
   map2.put(key, newVal);
}
else
{
   map2.put(key, newVal + oldVal);
}

Obviously you have to loop over the first map so you can process all of it's entries but that's trivial.