Convert Map<String, Object> to Map<String, Set<Object>> with filter and streams

M4V3N :

I would like to convert my map which looks like this:

{
  key="someKey1", value=Apple(id="1", color="green"),
  key="someKey2", value=Apple(id="2", color="red"),
  key="someKey3", value=Apple(id="3", color="green"),
  key="someKey4", value=Apple(id="4", color="red"),
}

to another map which puts all apples of the same color into the same list:

{
  key="red", value=list={apple1, apple3},
  key="green", value=list={apple2, apple4},  
}

I tried the following:

Map<String, Set<Apple>> sortedApples = appleMap.entrySet()
    .stream()
    .collect(Collectors.toMap(l -> l.getColour, ???));

Am I on the right track? Should I use filters for this task? Is there an easier way?

Ousmane D. :

if you want to proceed with toMap you can get the result as follows:

map.values()  // get the apples
   .stream() // Stream<Apple>
   .collect(toMap(Apple::getColour, // group by colour
             v ->  new HashSet<>(singleton(v)), // have values as set of apples
          (l, r) -> {l.addAll(r); return l;})); // merge colliding apples by colour

• stream over the map values instead of entrySet because we're not concerned with the map keys.
• Apple::getColour is the keyMapper function used to extract the "thing" we wish to group by, in this case, the Apples colour.
• v -> new HashSet<>(singleton(v)) is the valueMapper function used for the resulting map values
• (l, r) -> {l.addAll(r); return l;} is the merge function used to combine two HashSet's when there is a key collision on the Apple's colour.
• finally, the resulting map is a Map<String, Set<Apple>>

but this is better with groupingBy and toSet as downstream:

map.values().stream().collect(groupingBy(Apple::getColour, toSet()));

• stream over the map values instead of entrySet because we're not concerned with the map keys.

• groups the Apple's by the provided classification function i.e. Apple::getColour and then collect the values in a Set hence the toSet downstream collector.

• finally, the resulting map is a Map<String, Set<Apple>>

short, readable and the idiomatic approach.

You could also do it without a stream:

Map<String, Set<Apple>> res = new HashMap<>();
map.values().forEach(a -> res.computeIfAbsent(a.getColour(), e -> new HashSet<>()).add(a));

• iterate over the map values instead of entrySet because we're not concerned with the map keys.
• if the specified key a.getColour() is not already associated with a value, attempts to compute its value using the given mapping function e -> new HashSet<>() and enters it into the map. we then add the Apple to the resulting set.
• if the specified key a.getColour() is already associated with a value computeIfAbsent returns the existing value associated with it and then we call add(a) on the HashSet to enter the Apple into the set.
• finally, the resulting map is a Map<String, Set<Apple>>