- By Java 8
// if choices are unique by name.
Map<String, Choice> result = choices.stream()
.collect(Collectors.toMap(Choice::getName, Function.identity()));
// owtherwise group by
Map<String, List<Choice>> result = choices.stream()
.collect(Collectors.groupingBy(Choice::getName));- By abacus-common
Map<String, Choice> result = Stream.of(choices).toMap(Choice::getName, Fn.identity());
// group by
Map<String, List<Choice>> result = Stream.of(choices).groupTo(Choice::getName);###Map<String,List<String>> to Pair<String,String>
- By Java 8
List<Pair<String, String>> result =
map.entrySet()
.stream()
.flatMap(
entry -> entry.getValue()
.stream()
.map(string -> new Pair<>(entry.getKey(), string)))
.collect(Collectors.toList());- By abacus-common
List<Pair<String, String>> result = Stream.of(map)
.flatMap(e -> Stream.of(e.getValue()).map(v -> Pair.of(e.getKey(), v))).toList();- By Java 8
targetLongList = sourceLongList.stream()
.filter(l -> l > 100)
.collect(Collectors.toList());- By abacus-common
targetLongList = Stream.of(sourceLongList).filter(l -> l > 100).toList();- By Java 8
???- By abacus-common
stream.parallel(threadNum);- By Java 8
String[] names = {"Sam", "Pamela", "Dave", "Pascal", "Erik"};
IntStream.range(0, names.length)
.filter(i -> names[i].length() <= i)
.mapToObj(i -> names[i])
.collect(Collectors.toList());- By abacus-common
String[] names = {"Sam", "Pamela", "Dave", "Pascal", "Erik"};
IntStream.range(0, names.length)
.filter(i -> names[i].length() <= i)
.mapToObj(i -> names[i])
.toList();
// Or: indexed for any type of collection/iterator.
Stream.of(collection).indexed()...;How can I throw CHECKED exceptions from inside Java 8 streams? (Not wrapping it into unchecked exceptions)
- By Java 8
Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
.map(className -> try {
Class.forName(className))
} catch (ClassNotFoundException ) {
throw new RuntimeException(e);
}
.collect(Collectors.toList());- By abacus-common
Stream.of("java.lang.Object", "java.lang.Integer", "java.lang.String")
.map(Fn.ff(className -> Class.forName(className)))
.toList();- By Java 8
Optional<Other> result =
things.stream()
.map(this::resolve)
.flatMap(o -> o.isPresent() ? Stream.of(o.get()) : Stream.empty())
.findFirst();- By abacus-common
Optional<Other> result =
things.stream()
.map(this::resolve)
.flatMap(Optional::stream)
.first();- By Java 8
// ??? No easy way to do it until Java 9
IntStream
.iterate(1, n -> n + 1)
.takeWhile(n -> n < 10)
.forEach(System.out::println);- By abacus-common
IntStream
.iterate(1, n -> n + 1)
.takeWhile(n -> n < 10)
.forEach(Fn.println);- By Java 8
List<Answer> answerList = new ArrayList<>();
answerList.add(new Answer(1, true));
answerList.add(new Answer(2, true));
answerList.add(new Answer(3, null));
Map<Integer, Boolean> answerMap = answerList.stream()
.collect(Collectors.toMap(Answer::getId, Answer::getAnswer)); // throw NullPointerException- By abacus-common
// Works well.
Map<Integer, Boolean> answerMap = answerList.stream().toMap(Answer::getId, Answer::getAnswer);- By Java 8
Stream stream = Stream.concat(
Stream.concat(
stream1.filter(x -> x!=0), stream2)
.filter(x -> x!=1),
Stream.of(element))
.filter(x -> x!=2);- By abacus-common
stream1.append(stream2).append(element);- By Java 8
Map<String, String> x;
Map<String, Integer> y =
x.entrySet().stream()
.collect(Collectors.toMap(
e -> e.getKey(),
e -> Integer.parseInt(e.getValue())
));- By abacus-common
Map<String, Integer> y = Stream.of(x).toMap(e -> e.getKey(), e -> Integer.parseInt(e.getValue()));- By Java 8
???- By abacus-common
Yes, with the most beautiful design: Pair, Triple, Tuple
- By Java 8
List<User> resultUserList = users.stream()
.filter(user -> user.getId() == 1)
.limit(2)
.collect(Collectors.toList());
if (resultUserList.size() != 1) {
throw new IllegalStateException();
}
User resultUser = resultUserList.get(0);
// Or by singletonCollector()
public static <T> Collector<T, ?, T> singletonCollector() {
return Collectors.collectingAndThen(
Collectors.toList(),
list -> {
if (list.size() != 1) {
throw new IllegalStateException();
}
return list.get(0);
}
);
}- By abacus-common
users.stream().filter(user -> user.getId() == 1).onlyOne();- By Java 8
// Answers are included in the above link.- By abacus-common
Kotlin vs Java 8 on Collection
- By Java 8
???- By abacus-common
String[] a = {"a", "b", "c"};
String[] b = {"1", "2", "3"};
Stream.zip(a, b, Pair::of)...;- By Java 8
Map<String, String> phoneBook =
people.stream()
.collect(Collectors.toMap(
Person::getName,
Person::getAddress,
(address1, address2) -> {
System.out.println("duplicate key found!");
return address1;
}
));- By abacus-common
Map<String, String> phoneBook = people.stream().toMap(Person::getName, Person::getAddress, Fn.ignoringMerger()); // Fn.replacingMerger()- By Java 8
Stream.of(objects).filter(c -> c instanceof Client)
.map(c -> ((Client) c).getID()).forEach(System.out::println);- By abacus-common
Stream.of(objects).select(Client.class).forEach(Fn.println);- By Java 8
stream.forEach(a -> safeFoo(a));
private void safeFoo(final A a) {
try {
a.foo();
} catch (Exception ex) {
throw new RuntimeException(ex);
}
}- By abacus-common
stream.forEach(a -> Try.run(() -> a.foo()));- By Java 8
???- By abacus-common
stream.reversed()...
IntStream.of(1, 5, 3).reverseSorted()...- By Java 8
// For Array/List only.
IntStream.range(1, arrayList.size())
.mapToObj(i -> new Pair(arrayList.get(i-1), arrayList.get(i)))
.forEach(System.out::println);
// There is no general solution Stream created from iterator/collection..- By abacus-common
// For any stream created from iterator/Collection/Array/List...
stream().slidingMap(Pair::of).forEach(Fn.println());- By Java 8
T last = stream.reduce((a, b) -> b).orElse(null);- By abacus-common
T last = stream.last().orElse(null);- By Java 8
???- By abacus-common
stream.split(batchSize)...- By Java 8
Map<String, Integer> m1 = ImmutableMap.of("a", 2, "b", 3);
Map<String, Integer> m2 = ImmutableMap.of("a", 3, "c", 4);
Map<String, Integer> mx = Stream.of(m1, m2)
.map(Map::entrySet) // converts each map into an entry set
.flatMap(Collection::stream) // converts each set into an entry stream, then
// "concatenates" it in place of the original set
.collect(
Collectors.toMap( // collects into a map
Map.Entry::getKey, // where each entry is based
Map.Entry::getValue, // on the entries in the stream
Integer::max // such that if a value already exist for
// a given key, the max of the old
// and new value is taken
)
);- By abacus-common
Stream.of(m1).append(m2.entrySet()).toMap(Fn.key(), Fn.value(), Integer::max);- By Java 8
List<String> list = ...;
return IntStream.range(0, list.size())
.filter(n -> n % 3 == 0)
.mapToObj(list::get)
.collect(Collectors.toList());- By abacus-common
Stream.of(list).step(3).toList();- By Java 8
Integer[] numbers = new Integer[] { 1, 2, 1, 3, 4, 4 };
Set<Integer> allItems = new HashSet<>();
Set<Integer> duplicates = Arrays.stream(numbers)
.filter(n -> !allItems.add(n)) //Set.add() returns false if the item was already in the set.
.collect(Collectors.toSet());- By abacus-common
Stream.of(numbers).groupByToEntry(Fn.identity(), Fn.counting()).filterByValue(occur -> occur > 1).keys().toList();
// Or:
Multiset.of(numbers).entryStream().filterByValue(occur -> occur > 1).keys().toList();- By Java 8
Collection<DataSet> convert(List<MultiDataPoint> multiDataPoints) {
Map<String, DataSet> result = new HashMap<>();
multiDataPoints.forEach(pt ->
pt.keyToData.forEach((key, value) ->
result.computeIfAbsent(
key, k -> new DataSet(k, new ArrayList<>()))
.dataPoints.add(new DataPoint(pt.timestamp, value))));
return result.values();
}
// Or:
Collection<DataSet> convert(List<MultiDataPoint> multiDataPoints) {
return multiDataPoints.stream()
.flatMap(mdp -> mdp.keyToData.entrySet().stream().map(e ->
new Object() {
String key = e.getKey();
DataPoint dataPoint = new DataPoint(mdp.timestamp, e.getValue());
}))
.collect(
collectingAndThen(
groupingBy(t -> t.key, mapping(t -> t.dataPoint, toList())),
m -> m.entrySet().stream().map(e -> new DataSet(e.getKey(), e.getValue())).collect(toList())));
}- By abacus-common
Stream.of(multiDataPoints)
.flatMap(mdp -> Stream.of(mdp.keyToData).map(e -> Pair.of(e.getKey(), new DataPoint(mdp.timestamp, e.getValue()))))
.groupBy(Entry::getKey, Entry::getValue).map(e -> new DataSet(e.getKey(), e.getValue())).toList();- By Java 8
String result = "Hello world."
.codePoints()
//.parallel() // uncomment this line for large strings
.map(c -> c == ' ' ? ' ': '*')
.collect(StringBuilder::new,
StringBuilder::appendCodePoint, StringBuilder::append)
.toString();- By abacus-common
CharStream.of(result).map(c -> c == ' ' ? ' ': '*').println();- By Java 8
???- By abacus-common
Stream.of(resultSet)...
// Or:
Stream.of(entityClass, resultSet)...
//Or:
sqlExecutor.stream(sql, parameters);- By Java 8
int[] one = new int[]{1, 2, 3};
int[] two = new int[]{3, 4};
List<IntIntPair> list = new ArrayList<>();
IntStream.of(one).forEach(i ->
IntStream.of(two).mapToObj(j -> PrimitiveTuples.pair(i, j)).forEach(list::add));
System.out.println(list);- By abacus-common
Stream.of(1, 2, 3).cartesianProduct(Arrays.asList(3, 4)).forEach(Fn.println());