Java8 HashMap X Y to HashMap X Z using Stream Map-Reduce Collector

Question

I know how to  transform  a simple Java List from Y -  Z  i e    List lt String gt  x  List lt Integer gt  y   x stream            map s - gt  Integer parseInt s            collect Collectors toList       Now I d like to do basically the same with a Map  i e    INPUT       key1  - gt   41          41  and  42     key2  - gt   42         are Strings    OUTPUT       key1  - gt  41          41 and 42    key2  - gt  42          are Integers     The solution should not be limited to String -  Integer  Just like in the List example above  I d like to call any method  or constructor

User · Answer

Here are some variations on Sotirios Delimanolis' answer, which was pretty good to begin with (+1). Consider the following:

static <X, Y, Z> Map<X, Z> transform(Map<? extends X, ? extends Y> input,
                                     Function<Y, Z> function) {
    return input.keySet().stream()
        .collect(Collectors.toMap(Function.identity(),
                                  key -> function.apply(input.get(key))));
}

A couple points here. First is the use of wildcards in the generics; this makes the function somewhat more flexible. A wildcard would be necessary if, for example, you wanted the output map to have a key that's a superclass of the input map's key:

Map<String, String> input = new HashMap<String, String>();
input.put("string1", "42");
input.put("string2", "41");
Map<CharSequence, Integer> output = transform(input, Integer::parseInt);

(There is also an example for the map's values, but it's really contrived, and I admit that having the bounded wildcard for Y only helps in edge cases.)

A second point is that instead of running the stream over the input map's entrySet, I ran it over the keySet. This makes the code a little cleaner, I think, at the cost of having to fetch values out of the map instead of from the map entry. Incidentally, I initially had key -> key as the first argument to toMap() and this failed with a type inference error for some reason. Changing it to (X key) -> key worked, as did Function.identity().

Still another variation is as follows:

static <X, Y, Z> Map<X, Z> transform1(Map<? extends X, ? extends Y> input,
                                      Function<Y, Z> function) {
    Map<X, Z> result = new HashMap<>();
    input.forEach((k, v) -> result.put(k, function.apply(v)));
    return result;
}

This uses Map.forEach() instead of streams. This is even simpler, I think, because it dispenses with the collectors, which are somewhat clumsy to use with maps. The reason is that Map.forEach() gives the key and value as separate parameters, whereas the stream has only one value -- and you have to choose whether to use the key or the map entry as that value. On the minus side, this lacks the rich, streamy goodness of the other approaches. :-)

User · Answer

Map lt String  String gt  x  Map lt String  Integer gt  y       x entrySet   stream            collect Collectors toMap              e - gt  e getKey                e - gt  Integer parseInt e getValue                  It s not quite as nice as the list code  You can t construct new Map Entrys in a map   call so the work is mixed into the collect   call

User · Answer

A generic solution like so  public static  lt X  Y  Z gt  Map lt X  Z gt  transform Map lt X  Y gt  input          Function lt Y  Z gt  function        return input              entrySet                stream                collect                      Collectors toMap  entry  - gt  entry getKey                                 entry  - gt  function apply entry getValue           Example  Map lt String  String gt  input   new HashMap lt String  String gt     input put  string1    42    input put  string2    41    Map lt String  Integer gt  output   transform input               val  - gt  Integer parseInt val

User · Answer

If you don t mind using 3rd party libraries  my cyclops-react lib has extensions for all JDK Collection types  including Map  We can just transform the map directly using the  map  operator  by default map acts on the values in the map       MapX lt String Integer gt  y   MapX fromMap HashMaps of  hello   1                                     map Integer  parseInt     bimap can be used to transform the keys and values at the same time    MapX lt String Integer gt  y   MapX fromMap HashMaps of  hello   1                                    bimap this  newKey Integer  parseInt

User · Answer

An alternative that always exists for learning purpose is to build your custom collector through Collector of   though toMap   JDK collector here is succinct   1 here     Map lt String Integer gt  newMap   givenMap                  entrySet                    stream   collect Collector of                    - gt  new HashMap lt String Integer gt                             mutableMap entryItem - gt  mutableMap put entryItem getKey   Integer parseInt entryItem getValue                              map1 map2 - gt   map1 putAll map2   return map1

User · Answer

Does it absolutely have to be 100  functional and fluent  If not  how about this  which is about as short as it gets   Map lt String  Integer gt  output   new HashMap lt  gt     input forEach  k  v  - gt  output put k  Integer valueOf v       if you can live with the shame and guilt of combining streams with side-effects

User · Answer

My StreamEx library which enhances standard stream API provides an EntryStream class which suits better for transforming maps   Map lt String  Integer gt  output   EntryStream of input  mapValues Integer  valueOf  toMap

User · Answer

Guava s function Maps transformValues is what you are looking for  and it works nicely with lambda expressions   Maps transformValues originalMap  val - gt

User · Answer

The declarative and simpler solution would be     yourMutableMap replaceAll  key  val  -  return value of bi your function   Nb  be aware your modifying your map state  So this may not be what you want   Cheers to   http   www deadcoderising com 2017-02-14-java-8-declarative-ways-of-modifying-a-map-using-compute-merge-and-replace

[java] Java8: HashMap<X, Y> to HashMap<X, Z> using Stream / Map-Reduce / Collector

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