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Construct a new map using List of Maps using Java 8 streams

I converted below json data (in example) to List<Map<String, String>> and from that i want to construct a new Map using Java 8 streams and the output should look like below. could someone help me with this?
Key value
Service1: DEACTIVATED
Service2: ACTIVATED
Service3: DEACTIVATED
Ex:
[
{
name=Service1,
desiredState=DEACTIVATED
},
{
name=Service2,
desiredState=ACTIVATED
},
{
name=Service3,
desiredState=DEACTIVATED
}
]

From what I could comprehend, you aim to convert List<Map<String,String>> to Map<String,String>.
List<Map<String,String>> myMap = .... // map which you have already.
Map<String,String> resultMap = myMap.stream()
.flatMap(map -> map.entrySet().stream()) // Get a flatMap of the entryset. This will form a stream of Map.Entry
.collect(Collectors.toMap(e -> e.getKey(), e -> e.getValue(), (k1, k2) -> k2));
Here (k1,k2) -> k2 is a merge function in case if there are multiple entries for same keys while constructing the resultMap.

If I get it well, you need to merge your maps in a way such that the value of the name key is the key that maps to the value of the desiredState key. You could do it this way:
Map<String, String> result = listOfMaps.stream()
.collect(Collectors.toMap(
m -> m.get("name"),
m -> m.get("desiredState"),
(o, n) -> n));
This assumes that all maps from the list have name and desiredState entries. The (o, n) -> n merge function must be provided, in case there are collisions when creating the result map (i.e. entries with the same key). Here, between old and new values, I've chosen the new value.

Java Map with List value to list using streams?

I am trying to rewrite the method below using streams but I am not sure what the best approach is? If I use flatMap on the values of the entrySet(), I lose the reference to the current key.
private List<String> asList(final Map<String, List<String>> map) {
final List<String> result = new ArrayList<>();
for (final Entry<String, List<String>> entry : map.entrySet()) {
final List<String> values = entry.getValue();
values.forEach(value -> result.add(String.format("%s-%s", entry.getKey(), value)));
}
return result;
}
The best I managed to do is the following:
return map.keySet().stream()
.flatMap(key -> map.get(key).stream()
.map(value -> new AbstractMap.SimpleEntry<>(key, value)))
.map(e -> String.format("%s-%s", e.getKey(), e.getValue()))
.collect(Collectors.toList());
Is there a simpler way without resorting to creating new Entry objects?

A stream is a sequence of values (possibly unordered / parallel). map() is what you use when you want to map a single value in the sequence to some single other value. Say, map "alturkovic" to "ALTURKOVIC". flatMap() is what you use when you want to map a single value in the sequence to 0, 1, or many other values. Hence why a flatMap lambda needs to turn a value into a stream of values. flatMap can thus be used to take, say, a list of lists of string, and turn that into a stream of just strings.
Here, you want to map a single entry from your map (a single key/value pair) into a single element (a string describing it). 1 value to 1 value. That means flatMap is not appropriate. You're looking for just map.
Furthermore, you need both key and value to perform your mapping op, so, keySet() is also not appropriate. You're looking for entrySet(), which gives you a set of all k/v pairs, juts what we need.
That gets us to:
map.entrySet().stream()
.map(e -> String.format("%s-%s", e.getKey(), e.getValue()))
.collect(Collectors.toList());
Your original code makes no effort to treat a single value from a map (which is a List<String>) as separate values; you just call .toString() on the entire ordeal, and be done with it. This means the produced string looks like, say, [Hello, World] given a map value of List.of("Hello", "World"). If you don't want this, you still don't want flatmap, because streams are also homogenous - the values in a stream are all of the same kind, and thus a stream of 'key1 value1 value2 key2 valueA valueB' is not what you'd want:
map.entrySet().stream()
.map(e -> String.format("%s-%s", e.getKey(), myPrint(e.getValue())))
.collect(Collectors.toList());
public static String myPrint(List<String> in) {
// write your own algorithm here
}
Stream API just isn't the right tool to replace that myPrint method.
A third alternative is that you want to smear out the map; you want each string in a mapvalue's List<String> to first be matched with the key (so that's re-stating that key rather a lot), and then do something to that. NOW flatMap IS appropriate - you want a stream of k/v pairs first, and then do something to that, and each element is now of the same kind. You want to turn the map:
key1 = [value1, value2]
key2 = [value3, value4]
first into a stream:
key1:value1
key1:value2
key2:value3
key2:value4
and take it from there. This explodes a single k/v entry in your map into more than one, thus, flatmapping needed:
return map.entrySet().stream()
.flatMap(e -> e.getValue().stream()
.map(v -> String.format("%s-%s", e.getKey(), v))
.collect(Collectors.toList());
Going inside-out, it maps a single entry within a list that belongs to a single k/v pair into the string Key-SingleItemFromItsList.

Adding my two cents to excellent answer by #rzwitserloot. Already flatmap and map is explained in his answer.
List<String> resultLists = myMap.entrySet().stream()
.flatMap(mapEntry -> printEntries(mapEntry.getKey(),mapEntry.getValue())).collect(Collectors.toList());
System.out.println(resultLists);
Splitting this to a separate method gives good readability IMO,
private static Stream<String> printEntries(String key, List<String> values) {
return values.stream().map(val -> String.format("%s-%s",key,val));
}

How to create a reverse map when original map contains collection as the value?

Let's say my original Map contains the following:
Map<String, Set<String>> original = Maps.newHashMap();
original.put("Scott", Sets.newHashSet("Apple", "Pear", "Banana");
original.put("Jack", Sets.newHashSet("Banana", "Apple", "Orange");
And I want to create a reversed Map containing the following:
"Apple": ["Scott", "Jack"]
"Pear": ["Scott"]
"Banana": ["Scott", "Jack"]
"Orange": ["Jack"]
I know it can be done in old fashion (pre-Java 8), but how do I achieve the same using Java Stream API?
Map<String, Set<String>> reversed = original.entrySet().stream().map(x -> ????).collect(??)
There's similar question posted here, but that only works for single valued Maps.

You can break the Map into key-value pairs (where each key and value is a single String) by using flatMap, and then collect them as you wish:
Map<String,Set<String>> rev =
original.entrySet ()
.stream ()
.flatMap (e -> e.getValue ()
.stream ()
.map (v -> new SimpleEntry<String,String>(v,e.getKey ())))
.collect(Collectors.groupingBy (Map.Entry::getKey,
Collectors.mapping (Map.Entry::getValue,
Collectors.toSet())));
System.out.println (rev);
Output:
{Apple=[Jack, Scott], Pear=[Scott], Orange=[Jack], Banana=[Jack, Scott]}

A more imperative but simpler solution would be using forEach :
Map<String, Set<String>> original,result; // initialised
original.forEach((key, value) -> value.forEach(v ->
result.computeIfAbsent(v, k -> new HashSet<>()).add(key)));

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

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?

Collectors.groupingBy is more suitable than Collectors.toMap for this task (though both can be used).
Map<String, List<Apple>> sortedApples =
appleMap.values()
.stream()
.collect(Collectors.groupingBy(Apple::getColour));
Or, to group them into Sets use:
Map<String, Set<Apple>> sortedApples =
appleMap.values()
.stream()
.collect(Collectors.groupingBy(Apple::getColour,
Collectors.mapping(Function.identity(),
Collectors.toSet())));
or (as Aomine commented):
Map<String, Set<Apple>> sortedApples =
appleMap.values()
.stream()
.collect(Collectors.groupingBy(Apple::getColour, Collectors.toSet()));

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>>

You can use Collectors.groupingBy and Collectors.toSet()
Map<String, Set<Apple>> sortedApples = appleMap.values() // Collection<Apple>
.stream() // Stream<Apple>
.collect(Collectors.groupingBy(Apple::getColour, // groupBy colour
Collectors.mapping(a -> a, Collectors.toSet()))); // collect to Set

You've asked how to do it with streams, yet here's another way:
Map<String, Set<Apple>> result = new LinkedHashMap<>();
appleMap.values().forEach(apple ->
result.computeIfAbsent(apple.getColor(), k -> new LinkedHashSet<>()).add(apple));
This uses Map.computeIfAbsent, which either returns the set mapped to that color or puts an empty LinkedHashSet into the map if there's nothing mapped to that color yet, then adds the apple to the set.
EDIT: I'm using LinkedHashMap and LinkedHashSet to preserve insertion order, but could have used HashMap and HashSet, respectively.

Java Stream collect - how to deduce type?

I have been given a stream of words, Stream<String> words, and a class Pair<String,Integer> which realizes a simple tuple for (someString, someInt) with getter and setter methods for both elements called getFirst,setFirst,getSecond,setSecond.
I am now supposed to box each word of the stream into a Pair (word, 1), and then use a Collector to somehow make the whole thing tell me how often each word is in the text. Now I've looked up a Collector that should let me do what I want to, and passed it as .collect(...) to the stream.
But the whole thing is looking so complex, and the type inference and deduction and wildcards that are floating around in that topic aren't making it any easier, so that I got now no clue, just what it is I've created.
I've tried deducing it from the API, and tried all the things I could come up with, but none of it seems to match:
words
.map(x -> new Pair<String,Integer>(x,1))
.collect(Collectors.groupingBy(
x -> x.getFirst(),
Collectors.reducing(
(a,b) -> new Pair<String,Integer>(a.getFirst(), a.getSecond() + b.getSecond())
)
));

Try using Collectors.toMap:
Collection<Pair<String, Integer>> values = words.collect(Collectors.toMap(
Function.identity(),
s -> new Pair<>(s, 1),
(a, b) -> {a.setSecond(a.getSecond() + b.getSecond()); return a;}
)).values();
It creates a map from your stream, using provided:
keyMapper - a mapping function to produce keys
valueMapper - a mapping function to produce values
mergeFunction - a merge function, used to resolve collisions between values associated with the same key
So it groups your Pairs by string value to a map, and then you just call .values() to get a collection of Pairs

The easiest (though not necessarily most efficient) solution would be to group to a map and then convert the entries to pairs:
List<Pair<String, Integer>> pairs = words
.collect(Collectors.groupingBy(x -> x, Collectors.summingInt(x -> 1)))
.entrySet()
.stream()
.map(e -> new Pair(e.getKey(), e.getValue()))
.collect(Collectors.toList());

I agree that entering the world of collectors can be a bit frightening at the beginning, particularly if you need to deal with generic type parameters.
There are many ways to solve your problem, both with and without streams.
With streams:
Map<String, Pair<String, Integer>> map = words.stream()
.collect(Collectors.toMap(
word -> word,
word -> new Pair<>(word, 1),
(o, n) -> {
o.setSecond(o.getSecond() + n.getSecond());
return o;
}));
Collection<Pair<String, Integer>> result = map.values();
Collectors.toMap works by transforming each element of the stream into the keys (this is the 1st argument word -> word, which means we leave the word as is, so that it will be the key of the map), and by transforming each element of the stream into the values (this is the 2nd argument word -> new Pair<>(word, 1), which means that we've found the word for the first time, so we're creating a new Pair instance for that word with a count of 1).
The 3rd argument is a merge function that is to be used to merge values when the 1st argument returns a key that already belongs to the map. As maps can't have more than one entry for the same key, we need a way to merge the value that is already in the map for that key, with the new value produced by the 2nd argument. In this case, o stands for the old value and n for the new value. The way I merge values is by summing the counts for the word and setting the new count in the Pair instance that corresponds to the old value. There's no need to create a new instance of Pair with the word and the new count, as it's safe to accumulate the count by mutating the old instance of Pair.
Without streams:
Map<String, Pair<String, Integer>> map = new HashMap<>();
words.forEach(word -> map.merge(
word,
new Pair<>(word, 1),
(o, n) -> {
o.setSecond(o.getSecond() + n.getSecond());
return o;
}));
Collection<Pair<String, Integer>> result = map.values();
This uses Map.merge and has similar semantics as the previous code.