Adding a value to terms in a Map

Question

I'm trying to add specific values to a map in Java, where the key is quite complex, but the value is a simple Double.

Currently I'm using, where foos is an instance of java.util.TreeMap<Foo, Double>, and amount is a Double, code like:

for (java.util.Map.Entry<Foo, Double> entry : foos.entrySet()){     
    foos.put(entry.getKey(), entry.getValue() + amount);    
}   

but this looks quite dirty in that I have to reinsert the element, and I'm worried about invalidating the iterator.

Is there a better way of doing this? I'm using the latest version of Java.

Answer 1

Since you only want to modify the values, you can use Map#replaceAll:

foos.replaceAll((k, v) -> v + amount);

merge is useful if you potentially have new keys to insert, but that's not your case since you are iterating over the same set of keys.

Answer 2

One way is to use the Map#merge that, since Java version 8, has been available:

for (java.util.Map.Entry<Foo, Double> entry : foos.entrySet()){ 
    foos.merge(entry.getKey(), amount, Double::sum);    
}      

Note the use of the method reference symbol ::, not to be confused with the scope resolution operator of C++.

Here I'm using the "canned" function Double::sum but you can have lots of fun building your own which makes this approach particularly powerful. For more details see https://docs.oracle.com/javase/8/docs/api/java/util/Map.html#merge-K-V-java.util.function.BiFunction-