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Grouping of inner Maps with Java Streams

i have following structure:
Map<String,Map<String,Map<String,Integer>>>
Now i want to disregard the First-level-Maps and group (and sum up) the 3rd-Level-Maps according to the key of the 2nd-Level-Maps.
To Clarify some example-Entries:
Entry 1: ["1"["A"[[a,1];[b,2]];"B"[[a,3];[c,1]]]]
Entry 2: ["2"["A"[[b,2];[c,1]];"B"[[a,5];[b,0]]]]
Desired output:
Entry 1: ["A"[[a,1];[b,4];[c,1]]]
Entry 4: ["B"[[a,8];[b,0];[c,1]]]
So to do this I first group my Entry-stream according to my 2nd-Level-Keys ("A","B") and, if nothing else done, end up with a structure like the following:
Map<String,List<Entry<String,Map<String,Integer>>>>
And here is where I am stuck. How do i go about getting my Map<String,Integer>from my List of Entries (for each outer Map, secifically)?
The simple code which I assume is guaranteed to be needed:
initialMap.values().stream()
.flatMap(m -> m.entrySet().stream())
.collect(Collectors.groupingBy(Map.Entry::getKey));
Summary:
How do I transform a Map<String,Map<String,Map<String,Integer>>> to a Map<String<Map<String,Integer>>, disregarding the outermost Map, grouping my innermost Maps according to my 2nd-Layer-Keys and summing my Integer-values by key-values of the Innermost Map.
Additionally the outermost Maps each have a Key-Value-Pair for each 2nd-Level-Map, so each will have the same 2nd-Level-Keys. In the 3rd-Level-Keysets can be Keys not found in other 3rd-Level-Maps

Map<String, Map<String, Integer>> result =
initialMap
.values()
.stream()
.flatMap(m -> m.entrySet().stream())
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.groupingBy(e -> mapToFirstEntry(e.getValue()).getKey(),
Collectors.summingInt(e -> mapToFirstEntry(e.getValue()).getValue()))));
but it assumes that a third-level Map<String, Integer> contains one entry and there is a method to get that entry:
public static Map.Entry<String, Integer> mapToFirstEntry(Map<String, Integer> map) {
return map.entrySet().iterator().next();
}

If you have the liberty of using Java9, I would suggest you to use the flatMapping collector to solve this problem. This approach is much more readable and generates less visual clutter to me. Here's how it looks.
Map<String, Map<String, Integer>> summaryMap = map.values().stream()
.flatMap(m -> m.entrySet().stream())
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.flatMapping(e -> e.getValue().entrySet().stream(),
Collectors.groupingBy(Map.Entry::getKey,
Collectors.summingInt(Map.Entry::getValue)))));
This program produces the following output:
{A={a=1, b=4, c=1}, B={a=8, b=0, c=1}}

A thing to keep in mind here: streams conceptually represent a single element coming down through a "pipe" of sorts. It's always single element when the stream runs, no matter if source has one, multiple or infinite number of elements backed up in total.
What you're trying to do here is represent several nested loops, along the lines of:
Map<String, Map<String, Integer>> result = new HashMap<>();
for (Map<String, Map<String, Integer>> firstMap : inputMap.values()) {
for (Entry<String, Map<String, Integer>> firstEntry : firstMap.entrySet()) {
String upperCaseKey = firstEntry.getKey();
Map<String, Ingeter> resultEntry = result.computeIfAbsent(
upperCaseKey,
_k -> new HashMap<>());
for (Entry<String, Integer> secondEntry : firstEntry.getValue().entrySet()) {
resultEntry.merge(secondEntry.getKey(), secondEntry.getValue(), Integer::sum);
}
}
}
Among the better ways to do it with streams would be via Collector composition:
inputMap.values().stream()
.flatMap(map -> map.entrySet().stream())
.flatMap(firstEntry -> firstEntry.getValue()
.entrySet().stream()
.map(secondEntry -> new SimpleImmutableEntry(
firstEntry.getKey(),
secondEntry)
)
)
.collect(Collectors.groupingBy(
Entry::getKey,
Collectors.groupingBy(
compositeEntry -> compositeEntry.getValue().getKey(),
Collectors.summingInt(compositeEntry -> compositeEntry.getValue().getValue())
)
));
That should do the trick in general, but note how I had to first build up a composite entry, to keep element count as 1, and then nested two grouping collectors. This is why I'm of the opinion that tasks like yours aren't good fit for the API. It's also very likely to require a little help from you to compiler, as it may struggle to infer all the types correctly.
Also note, that this is not the only way to do it: the Stream API is very flexible, and you're likely to see many more other ways to do the same.

Inverse Map where getValue returns a List

I would like to transform a Map<String, List<Object>> so it becomes Map<String, String>. If it were just Map<String, Object> it is easy in Java8;
stream().collect(k -> k.getValue().getMyKey(), Entry::getKey);
But this will not work because getValue returns a List Map<List<Object>, String> in my example. Assume Object contains a getter to be used for the key and that Object does not contain the key in the first map.
Any thoughts?

Stream over the list of objects and extract the key you need then map --> flatten --> toMap
source.entrySet()
.stream()
.flatMap(e -> e.getValue()
.stream()
.map(x -> new SimpleEntry<>(x.getMyKey(), e.getKey())))
.collect(toMap(SimpleEntry::getKey, SimpleEntry::getValue));
use a merge function if there is expected to be duplicate getMyKey() values:
source.entrySet()
.stream()
.flatMap(e -> e.getValue()
.stream()
.map(x -> new SimpleEntry<>(x.getMyKey(), e.getKey())))
.collect(toMap(SimpleEntry::getKey, SimpleEntry::getValue, (l, r) -> l));
Note: the above uses the source map keys as the values of the resulting map as that's what you seem to be illustrating in your post, if however you want the key of the source map to remain as the key of the resulting map then change new SimpleEntry<>(x.getMyKey(), e.getKey()) to new SimpleEntry<>(e.getKey(),x.getMyKey()).

If preference could be to choose any amongst the multiple values mapped as a key, you can simply use:
Map<String, List<YourObject>> existing = new HashMap<>();
Map<String, String> output = new HashMap<>();
existing.forEach((k, v) -> v.forEach(v1 -> output.put(v1.getMyKey(), k)));
Essentially this would put the 'first' such myKey along with its corresponding value which was the key of the existing map.

Java-Stream, toMap with duplicate keys

So there might be one abc for several payments, now I have:
//find abc id for each payment id
Map<Long, Integer> abcIdToPmtId = paymentController.findPaymentsByIds(pmtIds)
.stream()
.collect(Collectors.toMap(Payment::getAbcId, Payment::getPaymentId));
But then I reallize this could have duplicate keys, so I want it to return a
Map<Long, List<Integer>> abcIdToPmtIds
which an entry will contain one abc and his several payments.
I know I might can use groupingBy but then I think I can only get Map<Long, List<Payments>> .

Use the other groupingBy overload.
paymentController.findPaymentsByIds(pmtIds)
.stream()
.collect(
groupingBy(Payment::getAbcId, mapping(Payment::getPaymentId, toList());

Problem statement: Converting SimpleImmutableEntry<String, List<String>> -> Map<String, List<String>>.
For Instance you have a SimpleImmutableEntry of this form [A,[1]], [B,[2]], [A, [3]] and you want your map to looks like this: A -> [1,3] , B -> [2].
This can be done with Collectors.toMap but Collectors.toMap works only with unique keys unless you provide a merge function to resolve the collision as said in java docs.
https://docs.oracle.com/javase/8/docs/api/java/util/stream/Collectors.html#toMap-java.util.function.Function-java.util.function.Function-java.util.function.BinaryOperator-
So the example code looks like this:
.map(returnSimpleImmutableEntries)
.collect(Collectors.toMap(SimpleImmutableEntry::getKey,
SimpleImmutableEntry::getValue,
(oldList, newList) -> { oldList.addAll(newList); return oldList; } ));
returnSimpleImmutableEntries method returns you entries of the form [A,[1]], [B,[2]], [A, [3]] on which you can use your collectors.

With Collectors.toMap:
Map<Long, Integer> abcIdToPmtId = paymentController.findPaymentsByIds(pmtIds)
.stream()
.collect(Collectors.toMap(
Payment::getAbcId,
p -> new ArrayList<>(Arrays.asList(p.getPaymentId())),
(o, n) -> { o.addAll(n); return o; }));
Though it's more clear and readable to use Collectors.groupingBy along with Collectors.mapping.
You don't need streams to do it though:
Map<Long, Integer> abcIdToPmtId = new HashMap<>();
paymentController.findPaymentsByIds(pmtIds).forEach(p ->
abcIdToPmtId.computeIfAbsent(
p.getAbcId(),
k -> new ArrayList<>())
.add(p.getPaymentId()));

Flatten a Map<Integer, List<String>> to Map<String, Integer> with stream and lambda

I would like to flatten a Map which associates an Integer key to a list of String, without losing the key mapping.
I am curious as though it is possible and useful to do so with stream and lambda.
We start with something like this:
Map<Integer, List<String>> mapFrom = new HashMap<>();
Let's assume that mapFrom is populated somewhere, and looks like:
1: a,b,c
2: d,e,f
etc.
Let's also assume that the values in the lists are unique.
Now, I want to "unfold" it to get a second map like:
a: 1
b: 1
c: 1
d: 2
e: 2
f: 2
etc.
I could do it like this (or very similarly, using foreach):
Map<String, Integer> mapTo = new HashMap<>();
for (Map.Entry<Integer, List<String>> entry: mapFrom.entrySet()) {
for (String s: entry.getValue()) {
mapTo.put(s, entry.getKey());
}
}
Now let's assume that I want to use lambda instead of nested for loops. I would probably do something like this:
Map<String, Integer> mapTo = mapFrom.entrySet().stream().map(e -> {
e.getValue().stream().?
// Here I can iterate on each List,
// but my best try would only give me a flat map for each key,
// that I wouldn't know how to flatten.
}).collect(Collectors.toMap(/*A String value*/,/*An Integer key*/))
I also gave a try to flatMap, but I don't think that it is the right way to go, because although it helps me get rid of the dimensionality issue, I lose the key in the process.
In a nutshell, my two questions are :
Is it possible to use streams and lambda to achieve this?
Is is useful (performance, readability) to do so?

You need to use flatMap to flatten the values into a new stream, but since you still need the original keys for collecting into a Map, you have to map to a temporary object holding key and value, e.g.
Map<String, Integer> mapTo = mapFrom.entrySet().stream()
.flatMap(e->e.getValue().stream()
.map(v->new AbstractMap.SimpleImmutableEntry<>(e.getKey(), v)))
.collect(Collectors.toMap(Map.Entry::getValue, Map.Entry::getKey));
The Map.Entry is a stand-in for the nonexistent tuple type, any other type capable of holding two objects of different type is sufficient.
An alternative not requiring these temporary objects, is a custom collector:
Map<String, Integer> mapTo = mapFrom.entrySet().stream().collect(
HashMap::new, (m,e)->e.getValue().forEach(v->m.put(v, e.getKey())), Map::putAll);
This differs from toMap in overwriting duplicate keys silently, whereas toMap without a merger function will throw an exception, if there is a duplicate key. Basically, this custom collector is a parallel capable variant of
Map<String, Integer> mapTo = new HashMap<>();
mapFrom.forEach((k, l) -> l.forEach(v -> mapTo.put(v, k)));
But note that this task wouldn’t benefit from parallel processing, even with a very large input map. Only if there were additional computational intense task within the stream pipeline that could benefit from SMP, there was a chance of getting a benefit from parallel streams. So perhaps, the concise, sequential Collection API solution is preferable.

You should use flatMap as follows:
entrySet.stream()
.flatMap(e -> e.getValue().stream()
.map(s -> new SimpleImmutableEntry(e.getKey(), s)));
SimpleImmutableEntry is a nested class in AbstractMap.

Hope this would do it in simplest way. :))
mapFrom.forEach((key, values) -> values.forEach(value -> mapTo.put(value, key)));

This should work. Please notice that you lost some keys from List.
Map<Integer, List<String>> mapFrom = new HashMap<>();
Map<String, Integer> mapTo = mapFrom.entrySet().stream()
.flatMap(integerListEntry -> integerListEntry.getValue()
.stream()
.map(listItem -> new AbstractMap.SimpleEntry<>(listItem, integerListEntry.getKey())))
.collect(Collectors.toMap(AbstractMap.SimpleEntry::getKey, AbstractMap.SimpleEntry::getValue));

Same as the previous answers with Java 9:
Map<String, Integer> mapTo = mapFrom.entrySet()
.stream()
.flatMap(entry -> entry.getValue()
.stream()
.map(s -> Map.entry(s, entry.getKey())))
.collect(toMap(Entry::getKey, Entry::getValue));

How to convert Map to List in Java 8

How to convert a Map<String, Double> to List<Pair<String, Double>> in Java 8?
I wrote this implementation, but it is not efficient
Map<String, Double> implicitDataSum = new ConcurrentHashMap<>();
//....
List<Pair<String, Double>> mostRelevantTitles = new ArrayList<>();
implicitDataSum.entrySet()
.stream()
.sorted(Comparator.comparing(e -> -e.getValue()))
.forEachOrdered(e -> mostRelevantTitles.add(new Pair<>(e.getKey(), e.getValue())));
return mostRelevantTitles;
I know that it should works using .collect(Collectors.someMethod()). But I don't understand how to do that.

Well, you want to collect Pair elements into a List. That means that you need to map your Stream<Map.Entry<String, Double>> into a Stream<Pair<String, Double>>.
This is done with the map operation:
Returns a stream consisting of the results of applying the given function to the elements of this stream.
In this case, the function will be a function converting a Map.Entry<String, Double> into a Pair<String, Double>.
Finally, you want to collect that into a List, so we can use the built-in toList() collector.
List<Pair<String, Double>> mostRelevantTitles =
implicitDataSum.entrySet()
.stream()
.sorted(Comparator.comparing(e -> -e.getValue()))
.map(e -> new Pair<>(e.getKey(), e.getValue()))
.collect(Collectors.toList());
Note that you could replace the comparator Comparator.comparing(e -> -e.getValue()) by Map.Entry.comparingByValue(Comparator.reverseOrder()).

Note that if you want efficient implementation, you should consider this:
List<Pair<String, Double>> mostRelevantTitles =
implicitDataSum.entrySet()
.stream()
.map(e -> new Pair<>(e.getKey(), e.getValue()))
.collect(Collectors.toList());
mostRelevantTitles.sort(Comparators.comparing(Pair::getSecond, Comparator.reverseOrder()));
I assume that your Pair class have getSecond getter.
Using the sorted() stream pipeline step you create intermediate buffer, store everything to that buffer, convert it into array, sort that array, then store the result into the ArrayList. My approach, though less functional, stores data directly into the target ArrayList, then sorts it in-place without any additional copying. So my solution would take less time and intermediate memory.

public List<TeamResult> process(final Map<String, Team> aggregatedMap) {
return aggregatedMap.entrySet()
.stream()
.map(e -> new TeamResult(e.getKey(),e.getValue()))
.collect(Collectors.toList());
}

Sort the Map based on values in reverse order and collect the keys in list and also limit only first 2 results in the list
List<String> list = map.keySet().stream()
.sorted((k1, k2)->map.get(k2)- map.get(k1))
.limit(2)
.collect(Collectors.toList())