Its a fair question. Certainly the stream construction you suggest will do the job; you’re looking for a more compact way to do it. Note that of course you could write your own static method that encapsulates the stream operation and you’d be done.
Map.replace/replaceAll are in-place, mutative methods, and so make sense as instance methods on Map. What you want is a “construct a new map”, which is more of a static factory. Two reasons it was not prioritized: (a) as a static method, anyone can write it, and put it anywhere, so omitting it from the JDK doesn’t really prevent anyone from doing this, and (b) as a static method, there’s no value in having specialized implementations in the various JDK Map implementations. On Apr 22, 2014, at 6:10 AM, Frank van Heeswijk <fvheesw...@outlook.com> wrote: > > > > > 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 > >
