Map.replaceAll when changing value types

[Frank van Heeswijk]

Hello,

First of all, this is the first time I am sending a mail to any java mailing list, so please correct me if I did something wrong.

I am a regular visitor and answered on stackoverflow ( http://stackoverflow.com/users/2057294/skiwi ) and noticed that there are quite a large amount of questions now already asking for a way to easily replace values in maps when their type fundamentally changes.
I however also have a few concerns while doing this, which will be listed below.

Take first as example the following code, it may not be the best example, but it should show the intention:

Map<String, Integer> map = new HashMap<>();
map.put("test", 5);
Map<String, String> mapped = map.entrySet().stream()
        .collect(Collectors.toMap(Map.Entry::getKey, entry -> String.valueOf(entry.getValue())));

The intention here was to map the integer values to their string counterparts in a single map, functionally this is equivalent to the following:

Map<String, Integer> map = new HashMap<>();
map.put("test", 5);
map.replaceAll((k, v) -> v + 1);

One would expect the first case to also be a one-liner, possibly somewhat like:

Map<String, String> mapped = map.replaceAllSpecial((k, v) -> String.valueOf(v));

Based on this I have the following concerns:
1. How would the method exactly need to look like? Should it also have the option to take a Function<V_IN, V_OUT> valueMapper as argument?
2. How could this be optimized? Surely we do not *really* want to take all map entries out of one map and insert the modified value in a new map, I was thinking that an in-place modification would be the best, but then you have the issue where the type changes.

Lastly, I hope that we can agree on that for such a trivial usecase, the usage of streams is possibly too complicated and furthermore point (2) is very important.

As some more reference, here are some of the stackoverflow questions that ask for it:
- http://stackoverflow.com/questions/23213891/how-to-map-values-in-a-map-in-java-8
- http://stackoverflow.com/questions/22840170/in-java-8-functional-style-how-can-i-map-the-values-over-a-java-util-map

Regards,

Frank van Heeswijk