I am trying to achieve the following
public class MyObject {
Map<String, String> myMap;
}
public class MyOtherObject {
List<MyObject> myObjects;
}
I want to be able to do the following
for (MyObject myObject: myObjects) {
Map<String,String> newMap = new Hashmap<String,String>();
for (Map.Entry<String, String> entry : myObject.getMyMap.entrySet() {
newMap.put(entry.key + "a" , entry.value)
}
}
How do I avoid this nested loop?
Regarding your current solution, you'll need to move the newMap
declaration outside the loop otherwise you're creating a new map at each iteration of the loop and that would not contain the result you'd expect.
You also have a typo on your map instantiation, change Hashmap to
HashMap.
As for avoiding nested loops, you can create a stream instance of the entrySet then simply perform a reduction operation on the stream using collect.
Map<String, String> copy = myObject.getMyMap.entrySet()
.stream()
.collect(Collectors.toMap(p -> p.getKey() + "a", Map.Entry::getValue));
if you want to copy all of the mappings from the specified map to another map then as svasa has suggested within the comments, you can do:
newMap.putAll(myObject.getMyMap.entrySet().stream().collect(Collectors.toMap(p -> p.getKey() + "a", Map.Entry::getValue)));
Use putAll:
Amap.putAll(Bmap);
The complexity will always be O(n x m), with n being the size of the list and m the maximum size of the inner maps.
With java 8, you can use:
Map<String, String> result = myObjects.stream()
.flatMap(myObject -> myObject.getMyMap().entrySet().stream())
.collect(Collectors.toMap(e -> e.getKey() + "a", Entry::getValue));
This assumes you want only one map with all the entries of all the inner maps, with their key modified.
I've used flatMap to flatten all the streams with the entries of the inner maps into one stream. Then, I've used Collectors.toMap to collect all these entries into a new map.
This will fail if there are repeated keys. In such case, you can use the overloaded version of Collectors.toMap, which accepts a function to merge values when there's a collision between keys:
Map<String, String> result = myObjects.stream()
.flatMap(myObject -> myObject.getMyMap().entrySet().stream())
.collect(Collectors.toMap(e -> e.getKey() + "a", Entry::getValue, String::concat));
This simply concatenates the values that are mapped more than once to the same key, but you can use any other merge function.
Related
Map<Integer,String> maper = new HashMap<>();
maper.put(1, "Naveen");
Map<Integer,Map<Integer,String>> map1 = new HashMap<>();
map1.put(1, maper);
Map<Integer,Map<Integer,Map<Integer,String>>> mapOne = new HashMap<>();
mapOne.put(1, map1);
How to get the String value from mapOne using streams in Java 8?
The simple answer to your question would just be:
mapOne.get(1).get(1).get(1)
where each get gets the inner map, or in the case of the last get, gets the final value.
Since you mentioned streams, I think you probably meant to ask how to flatten the nested map so that you can get the strings in the innermost map as a collection. In that case, you should use flatMap. N calls to flatMap is required for a map nested N levels deep.
mapOne.values().stream()
.flatMap(x -> x.values().stream())
.flatMap(x -> x.values().stream())
.collect(Collectors.toList()) // or toSet()
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.
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));
Anybody knows how to merge with Java 8 two maps of this type?
Map<String, List<String>> map1--->["a",{1,2,3}]
Map<String, List<String>> map2--->["a",{4,5,6}]
And obtain as result of the merge
Map<String, List<String>> map3--->["a",{1,2,3,4,5,6}]
I´m looking for a non verbose way if exist. I know how to do it in the old fashion way.
Regards.
The general idea is the same as in this post. You create a new map from the first map, iterate over the second map and merge each key with the first map thanks to merge(key, value, remappingFunction). In case of conflict, the remapping function is applied: in this case, it takes the two lists and merges them; if there is no conflict, the entry with the given key and value is put.
Map<String, List<String>> mx = new HashMap<>(map1);
map2.forEach((k, v) -> mx.merge(k, v, (l1, l2) -> {
List<String> l = new ArrayList<>(l1);
l.addAll(l2);
return l;
}));
You could try this, which gradually flattens the structure until you have a stream of tuples of the maps keys versus the lists values:
Map<K,List<V>> result = Stream.of(map1,map2) // Stream<Map<K,List<V>>>
.flatMap(m -> m.entrySet().stream()) // Stream<Map.Entry<K,List<V>>>
.flatMap(e -> e.getValue().stream() // Inner Stream<V>...
.map(v -> new AbstractMap.SimpleImmutableEntry<>(e.getKey(), v)))
// ...flatmapped into an outer Stream<Map.Entry<K,V>>>
.collect(Collectors.groupingBy(e -> e.getKey(), Collectors.mapping(e -> e.getValue(), Collectors.toList())));
Another option would avoid the internal streaming of the lists by using Collectors.reducing() as a second parameter of groupingBy, I guess. However, I would consider the accepted answer first
You have to use Set instead of List and can do it like this:
Map<String, Set<String>> map1--->["a",{1,2,3}]
Map<String, Set<String>> map2--->["a",{4,5,6}]
map1.forEach((k, v) -> v.addAll(map2.get(k) == null : new HashSet<> ? map2.get(k)));
I have the following Object and a Map:
MyObject
String name;
Long priority;
foo bar;
Map<String, List<MyObject>> anotherHashMap;
I want to convert the Map in another Map. The Key of the result map is the key of the input map. The value of the result map ist the Property "name" of My object, ordered by priority.
The ordering and extracting the name is not the problem, but I could not put it into the result map. I do it the old Java 7 way, but it would be nice it is possible to use the streaming API.
Map<String, List<String>> result = new HashMap<>();
for (String identifier : anotherHashMap.keySet()) {
List<String> generatedList = anotherHashMap.get(identifier).stream()...;
teaserPerPage.put(identifier, generatedList);
}
Has anyone an idea? I tried this, but got stuck:
anotherHashMap.entrySet().stream().collect(Collectors.asMap(..., ...));
Map<String, List<String>> result = anotherHashMap
.entrySet().stream() // Stream over entry set
.collect(Collectors.toMap( // Collect final result map
Map.Entry::getKey, // Key mapping is the same
e -> e.getValue().stream() // Stream over list
.sorted(Comparator.comparingLong(MyObject::getPriority)) // Sort by priority
.map(MyObject::getName) // Apply mapping to MyObject
.collect(Collectors.toList())) // Collect mapping into list
);
Essentially, you stream over each entry set and collect it into a new map. To compute the value in the new map, you stream over the List<MyOjbect> from the old map, sort, and apply a mapping and collection function to it. In this case I used MyObject::getName as the mapping and collected the resulting names into a list.
For generating another map, we can have something like following:
HashMap<String, List<String>> result = anotherHashMap.entrySet().stream().collect(Collectors.toMap(elem -> elem.getKey(), elem -> elem.getValue() // can further process it);
Above I am recreating the map again, but you can process the key or the value according to your needs.
Map<String, List<String>> result = anotherHashMap.entrySet().stream().collect(Collectors.toMap(
Map.Entry::getKey,
e -> e.getValue().stream()
.sorted(comparing(MyObject::getPriority))
.map(MyObject::getName)
.collect(Collectors.toList())));
Similar to answer of Mike Kobit, but sorting is applied in the correct place (i.e. value is sorted, not map entries) and more concise static method Comparator.comparing is used to get Comparator for sorting.