Putting a new value into a Map if not present, or adding it if it is

Question

I have a java.util.Map for a key-type class Foo. Let\'s call the instance of the map map.

I want to add {

Answer 1

this is what the merge function on maps is for.

map.merge(foo, f, (f1, f2) -> f1 + f2)

this can be reduced even further to

map.merge(foo, f, Double::sum)

it is basically the equivalent of

if(map.contains(foo)){
    double x = map.get(foo);
    map.put(foo, x + f)
} else {
    map.put(foo, f)      
}