Map<String, List<String>> map1 = new HashMap<>();
Map<String, List<String>> map2 = new HashMap<>();
map1.put("a", Lists.newArrayList("1","123"));
map1.put("b", Lists.newArrayList("2","223"));
map2.put("c", Lists.newArrayList("11","1123"));
map2.put("a", Lists.newArrayList("22","2223"));
Map<String, List<List<String>>> collect = Stream.of(map1, map2)
.flatMap(m -> m.entrySet().stream())
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.mapping(Map.Entry::getValue,
Collectors.toList())
));
System.out.println(collect);
print: {a=[[1, 123], [22, 2223, 1, 123]], b=[[2, 223]], c=[[11, 1123]]}
how can print:
{a=[22, 2223, 1, 123], b=[2, 223], c=[11, 1123]}
You may use a simple for loop and do it like this.
List<Map<String, List<String>>> source = Arrays.asList(map1, map2);
for (Map<String, List<String>> m : source) {
for (Map.Entry<String, List<String>> e : m.entrySet()) {
target.computeIfAbsent(e.getKey(), unused -> new ArrayList<>())
.addAll(e.getValue());
}
}
A stream based counterpart would be something like this.
Map<String, List<String>> res = source.stream()
.flatMap(m -> m.entrySet().stream())
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.flatMapping(e -> e.getValue().stream(), Collectors.toList())));
The Collectors.flatMapping is available from java9 onwards. If you are using java8, I would recommend you to write your own custom collector for flatMapping and use it here. This will give you an easier migration strategy for Java9 too. You may find an explanation here.
I wouldn't create List<List<String>>s in the first place
Map<String, List<String>> map1 = new HashMap<>();
Map<String, List<String>> map2 = new HashMap<>();
map1.put("a", Lists.newArrayList("1","123"));
map1.put("b", Lists.newArrayList("2","223"));
map2.put("c", Lists.newArrayList("11","1123"));
map2.put("a", Lists.newArrayList("22","2223"));
Map<String, List<String>> collect = Stream.of(map1, map2)
.flatMap(m -> m.entrySet().stream())
.collect(Collectors.groupingBy(Map.Entry::getKey,
Collectors.flatMapping(e -> e.getValue().stream(), Collectors.toList())));
I want to translate a List of objects into a Map using Java 8's streams and lambdas.
This is how I would write it in Java 7 and below.
private Map<String, Choice> nameMap(List<Choice> choices) {
final Map<String, Choice> hashMap = new HashMap<>();
for (final Choice choice : choices) {
hashMap.put(choice.getName(), choice);
}
return hashMap;
}
I can accomplish this easily using Java 8 and Guava but I would like to know how to do this without Guava.
In Guava:
private Map<String, Choice> nameMap(List<Choice> choices) {
return Maps.uniqueIndex(choices, new Function<Choice, String>() {
#Override
public String apply(final Choice input) {
return input.getName();
}
});
}
And Guava with Java 8 lambdas.
private Map<String, Choice> nameMap(List<Choice> choices) {
return Maps.uniqueIndex(choices, Choice::getName);
}
Based on Collectors documentation it's as simple as:
Map<String, Choice> result =
choices.stream().collect(Collectors.toMap(Choice::getName,
Function.identity()));
If your key is NOT guaranteed to be unique for all elements in the list, you should convert it to a Map<String, List<Choice>> instead of a Map<String, Choice>
Map<String, List<Choice>> result =
choices.stream().collect(Collectors.groupingBy(Choice::getName));
Use getName() as the key and Choice itself as the value of the map:
Map<String, Choice> result =
choices.stream().collect(Collectors.toMap(Choice::getName, c -> c));
Most of the answers listed, miss a case when the list has duplicate items. In that case there answer will throw IllegalStateException. Refer the below code to handle list duplicates as well:
public Map<String, Choice> convertListToMap(List<Choice> choices) {
return choices.stream()
.collect(Collectors.toMap(Choice::getName, choice -> choice,
(oldValue, newValue) -> newValue));
}
Here's another one in case you don't want to use Collectors.toMap()
Map<String, Choice> result =
choices.stream().collect(HashMap<String, Choice>::new,
(m, c) -> m.put(c.getName(), c),
(m, u) -> {});
One more option in simple way
Map<String,Choice> map = new HashMap<>();
choices.forEach(e->map.put(e.getName(),e));
For example, if you want convert object fields to map:
Example object:
class Item{
private String code;
private String name;
public Item(String code, String name) {
this.code = code;
this.name = name;
}
//getters and setters
}
And operation convert List To Map:
List<Item> list = new ArrayList<>();
list.add(new Item("code1", "name1"));
list.add(new Item("code2", "name2"));
Map<String,String> map = list.stream()
.collect(Collectors.toMap(Item::getCode, Item::getName));
If you don't mind using 3rd party libraries, AOL's cyclops-react lib (disclosure I am a contributor) has extensions for all JDK Collection types, including List and Map.
ListX<Choices> choices;
Map<String, Choice> map = choices.toMap(c-> c.getName(),c->c);
You can create a Stream of the indices using an IntStream and then convert them to a Map :
Map<Integer,Item> map =
IntStream.range(0,items.size())
.boxed()
.collect(Collectors.toMap (i -> i, i -> items.get(i)));
I was trying to do this and found that, using the answers above, when using Functions.identity() for the key to the Map, then I had issues with using a local method like this::localMethodName to actually work because of typing issues.
Functions.identity() actually does something to the typing in this case so the method would only work by returning Object and accepting a param of Object
To solve this, I ended up ditching Functions.identity() and using s->s instead.
So my code, in my case to list all directories inside a directory, and for each one use the name of the directory as the key to the map and then call a method with the directory name and return a collection of items, looks like:
Map<String, Collection<ItemType>> items = Arrays.stream(itemFilesDir.listFiles(File::isDirectory))
.map(File::getName)
.collect(Collectors.toMap(s->s, this::retrieveBrandItems));
I will write how to convert list to map using generics and inversion of control. Just universal method!
Maybe we have list of Integers or list of objects. So the question is the following: what should be key of the map?
create interface
public interface KeyFinder<K, E> {
K getKey(E e);
}
now using inversion of control:
static <K, E> Map<K, E> listToMap(List<E> list, KeyFinder<K, E> finder) {
return list.stream().collect(Collectors.toMap(e -> finder.getKey(e) , e -> e));
}
For example, if we have objects of book , this class is to choose key for the map
public class BookKeyFinder implements KeyFinder<Long, Book> {
#Override
public Long getKey(Book e) {
return e.getPrice()
}
}
I use this syntax
Map<Integer, List<Choice>> choiceMap =
choices.stream().collect(Collectors.groupingBy(choice -> choice.getName()));
It's possible to use streams to do this. To remove the need to explicitly use Collectors, it's possible to import toMap statically (as recommended by Effective Java, third edition).
import static java.util.stream.Collectors.toMap;
private static Map<String, Choice> nameMap(List<Choice> choices) {
return choices.stream().collect(toMap(Choice::getName, it -> it));
}
Another possibility only present in comments yet:
Map<String, Choice> result =
choices.stream().collect(Collectors.toMap(c -> c.getName(), c -> c)));
Useful if you want to use a parameter of a sub-object as Key:
Map<String, Choice> result =
choices.stream().collect(Collectors.toMap(c -> c.getUser().getName(), c -> c)));
Map<String, Set<String>> collect = Arrays.asList(Locale.getAvailableLocales()).stream().collect(Collectors
.toMap(l -> l.getDisplayCountry(), l -> Collections.singleton(l.getDisplayLanguage())));
This can be done in 2 ways. Let person be the class we are going to use to demonstrate it.
public class Person {
private String name;
private int age;
public String getAge() {
return age;
}
}
Let persons be the list of Persons to be converted to the map
1.Using Simple foreach and a Lambda Expression on the List
Map<Integer,List<Person>> mapPersons = new HashMap<>();
persons.forEach(p->mapPersons.put(p.getAge(),p));
2.Using Collectors on Stream defined on the given List.
Map<Integer,List<Person>> mapPersons =
persons.stream().collect(Collectors.groupingBy(Person::getAge));
Here is solution by StreamEx
StreamEx.of(choices).toMap(Choice::getName, c -> c);
Map<String,Choice> map=list.stream().collect(Collectors.toMap(Choice::getName, s->s));
Even serves this purpose for me,
Map<String,Choice> map= list1.stream().collect(()-> new HashMap<String,Choice>(),
(r,s) -> r.put(s.getString(),s),(r,s) -> r.putAll(s));
If every new value for the same key name has to be overridden:
public Map < String, Choice > convertListToMap(List < Choice > choices) {
return choices.stream()
.collect(Collectors.toMap(Choice::getName,
Function.identity(),
(oldValue, newValue) - > newValue));
}
If all choices have to be grouped in a list for a name:
public Map < String, Choice > convertListToMap(List < Choice > choices) {
return choices.stream().collect(Collectors.groupingBy(Choice::getName));
}
List<V> choices; // your list
Map<K,V> result = choices.stream().collect(Collectors.toMap(choice::getKey(),choice));
//assuming class "V" has a method to get the key, this method must handle case of duplicates too and provide a unique key.
As an alternative to guava one can use kotlin-stdlib
private Map<String, Choice> nameMap(List<Choice> choices) {
return CollectionsKt.associateBy(choices, Choice::getName);
}
List<Integer> listA = new ArrayList<>();
listA.add(1);
listA.add(5);
listA.add(3);
listA.add(4);
System.out.println(listA.stream().collect(Collectors.toMap(x ->x, x->x)));
String array[] = {"ASDFASDFASDF","AA", "BBB", "CCCC", "DD", "EEDDDAD"};
List<String> list = Arrays.asList(array);
Map<Integer, String> map = list.stream()
.collect(Collectors.toMap(s -> s.length(), s -> s, (x, y) -> {
System.out.println("Dublicate key" + x);
return x;
},()-> new TreeMap<>((s1,s2)->s2.compareTo(s1))));
System.out.println(map);
Dublicate key AA
{12=ASDFASDFASDF, 7=EEDDDAD, 4=CCCC, 3=BBB, 2=AA}
I am trying to generate count based on keys, using the below code, this code is based on the word count example. Strangely if the mapValues function returns on a String then the groupBy works as mentioned in the commented line, but when I send a keypair of String as key and GenericRecord as value.
final Serde<String> stringSerde = Serdes.String();
final Serde<Long> longSerde = Serdes.Long();
final Map<String, String> serdeConfig = Collections.singletonMap("schema.registry.url","http://localhost:8081");
stringSerde.configure(serdeConfig, true); // `true` for record keys
final Serde<GenericRecord> valueGenericAvroSerde = new GenericAvroSerde();
valueGenericAvroSerde.configure(serdeConfig, false); // `false` for record values
StreamsBuilder builder = new StreamsBuilder();
KStream<String, GenericRecord> textLines =
builder.stream("ora-query-in",Consumed.with(stringSerde, valueGenericAvroSerde));
final KTable<String, Long> wordCounts = textLines
.mapValues(new ValueMapperWithKey<String, GenericRecord, KeyValue<String, GenericRecord>>() {
#Override
public KeyValue<String, GenericRecord> apply(String arg0, GenericRecord arg1) {
return new KeyValue<String, GenericRecord>(arg1.get("KEY_FIELD").toString(),arg1);
}
})
// .groupBy((key, value) -> value) //THIS WORKS if value is STRING
// .groupBy((key, value) -> key) //DOES NOT WORK EITHER
.groupByKey() //THIS does nothing
.count();
wordCounts.toStream().to("test.topic.out",Produced.with(stringSerde, longSerde));
Am I missing something in configuration
streamsConfiguration.put(AbstractKafkaAvroSerDeConfig.SCHEMA_REGISTRY_URL_CONFIG, "http://localhost:8081");
streamsConfiguration.put(StreamsConfig.DEFAULT_KEY_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
streamsConfiguration.put(StreamsConfig.DEFAULT_VALUE_SERDE_CLASS_CONFIG, Serdes.String().getClass().getName());
You haven't write what exactly is wrong but it seems it is an issue with Serialization
You can use:
KStream::groupBy(final KeyValueMapper<? super K, ? super V, KR> selector, final Grouped<KR, V> grouped).
someStream.groupByKey((key, value) -> value, Grouped.with(newKeySerdes, valueSerdes)
KGroupedStream::count(final Materialized<K, Long, KeyValueStore<Bytes, byte[]>> materialized)
someGroupedStream.count(Materialized.with(newKeySerdes, valueSerdes)
Can be same reason like:
Kafka Streams 2.1.1 class cast while flushing timed aggregation to store
KafkaStreams: Getting Window Final Results
Currently to do something simple with Collections of CompletionStage requires jumping through several ugly hoops:
public static CompletionStage<String> translate(String foo) {
// just example code to reproduce
return CompletableFuture.completedFuture("translated " + foo);
}
public static CompletionStage<List<String>> translateAllAsync(List<String> input) {
List<CompletableFuture<String>> tFutures = input.stream()
.map(s -> translate(s)
.toCompletableFuture())
.collect(Collectors.toList()); // cannot use toArray because of generics Arrays creation :-(
return CompletableFuture.allOf(tFutures.toArray(new CompletableFuture<?>[0])) // not using size() on purpose, see comments
.thenApply(nil -> tFutures.stream()
.map(f -> f.join())
.map(s -> s.toUpperCase())
.collect(Collectors.toList()));
}
What I want to write is:
public CompletionStage<List<String>> translateAllAsync(List<String> input) {
// allOf takes a collection< futures<X>>,
// and returns a future<collection<x>> for thenApply()
return XXXUtil.allOf(input.stream()
.map(s -> translate(s))
.collect(Collectors.toList()))
.thenApply(translations -> translations.stream()
.map(s -> s.toUpperCase())
.collect(Collectors.toList()));
}
The whole ceremony about toCompletableFuture and converting to an Array and join is boilerplate distracting from the actual code semantics.
Possibly having a version of allOf() returning a Future<Collection<Future<X>>> instead of Future<Collection<X>> may also be useful in some cases.
I could try implementing XXXUtil myself, but I wonder if there already is a mature 3rdparty library for this and similar issues (Such as Spotify's CompletableFutures). If so, I'd like to see the equivalent code for such a library as an answer.
Or maybe the original code posted above can somehow be written more compactly in a different way?
JUnit test code:
#Test
public void testTranslate() throws Exception {
List<String> list = translateAllAsync(Arrays.asList("foo", "bar")).toCompletableFuture().get();
Collections.sort(list);
assertEquals(list,
Arrays.asList("TRANSLATED BAR", "TRANSLATED FOO"));
}
I just looked into the source code of CompletableFuture.allOf, to find that it basically creates a binary tree of nodes handling two stages at a time. We can easily implement a similar logic without using toCompletableFuture() explicitly and handling the result list generation in one go:
public static <T> CompletionStage<List<T>> allOf(
Stream<? extends CompletionStage<? extends T>> source) {
return allOf(source.collect(Collectors.toList()));
}
public static <T> CompletionStage<List<T>> allOf(
List<? extends CompletionStage<? extends T>> source) {
int size = source.size();
if(size == 0) return CompletableFuture.completedFuture(Collections.emptyList());
List<T> result = new ArrayList<>(Collections.nCopies(size, null));
return allOf(source, result, 0, size-1).thenApply(x -> result);
}
private static <T> CompletionStage<Void> allOf(
List<? extends CompletionStage<? extends T>> source,
List<T> result, int from, int to) {
if(from < to) {
int mid = (from+to)>>>1;
return allOf(source, result, from, mid)
.thenCombine(allOf(source, result, mid+1, to), (x,y)->x);
}
return source.get(from).thenAccept(t -> result.set(from, t));
}
That’s it.
You can use this solution to implement the logic of your question’s code as
public static CompletionStage<List<String>> translateAllAsync(List<String> input) {
return allOf(input.stream().map(s -> translate(s)))
.thenApply(list -> list.stream()
.map(s -> s.toUpperCase())
.collect(Collectors.toList()));
}
though it would be more natural to use
public static CompletionStage<List<String>> translateAllAsync(List<String> input) {
return allOf(input.stream().map(s -> translate(s).thenApply(String::toUpperCase)));
}
Note that this solution maintains the order, so there is no need for sorting the result in the test case:
#Test
public void testTranslate() throws Exception {
List<String> list = translateAllAsync(Arrays.asList("foo", "bar")).toCompletableFuture().get();
assertEquals(list, Arrays.asList("TRANSLATED FOO", "TRANSLATED BAR"));
}
I am trying to process an ArrayList with content of Long type as in the given example below using Java 8's LongStream but I get the below error.
import java.util.*;
import java.util.stream.*;
public class HelloWorld{
public static void main(String []args){
List<Long> data=new LinkedList();
for(Long j=0L;j<300L;j++){
data.add(j);
}
int BATCH = 10;
LongStream.range(0, (data.size()+BATCH-1)/BATCH)
.mapToLong(i -> data.subList(i*BATCH, Math.min(data.size(), (i+1)*BATCH)))
.forEach(batch -> process(batch));
}
static void process(List<Long> list){
System.out.println(list);
}
}
But I get the below exception. I have tried with mapToLong insted of map but mapToLong is not recognzied
$javac HelloWorld.java
HelloWorld.java:13: error: incompatible types: possible lossy
conversion from long to int
.map(i -> data.subList(i*BATCH, Math.min(data.size(),
(i+1)*BATCH)))
^
HelloWorld.java:14: error: incompatible types: long cannot be
converted to List<Long>
.forEach(batch -> process(batch));
^
2 errors
map in LongStream is supposed to map an element of the LongStream to a long, not to a List.
Use mapToObj:
LongStream.range(0, (data.size()+BATCH-1)/BATCH)
.mapToObj(i -> data.subList((int)i*BATCH, (int)Math.min(data.size(), (i+1)*BATCH)))
.forEach(batch -> process(batch));
Or:
IntStream.range(0, (data.size()+BATCH-1)/BATCH)
.mapToObj(i -> data.subList(i*BATCH, Math.min(data.size(), (i+1)*BATCH)))
.forEach(batch -> process(batch));