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Streams

Stream of Primitive

DoubleStream

Return an OptionalDouble:

  • max()
  • min()
  • average()
OptionalDouble optionalDouble;
//not get()!!!
double value = optionalDouble.getAsDouble()

Do not return an OptionalDouble

  • count(): return a long
  • sum(): return a double DoubleStream

DoubleSummaryStatistics

DoubleSummaryStatistics summaryStatistics = DoubleStream.iterate(1D, n -> n + n / 2)
    .limit(10L)
    .summaryStatistics();
double average = summaryStatistics.getAverage();

DoubleSummaryStatistics

LongStream

There ain't mapToLong() method in the LongStream.
LongStream

DoubleToLongFunction

long applyAsLong(double value);

DoubleToLongFunction

IntStream

IntStream rangeClosed = IntStream.rangeClosed(0, 9);
//note that IntStream.sum(0 returns an int, not a long
int sum = rangeClosed.sum();

IntStream

IntSummaryStatistics

IntSummaryStatistics intSummaryStatistics = IntStream.range(1, 100)
    .filter(n -> n % 2 == 0)
    .summaryStatistics();
double average = intSummaryStatistics.getAverage();
long sum = intSummaryStatistics.getSum();

Optional

Optional examples

Optional for primitives

  • OptionalInt > getAsInt()
  • OptionalLong -> getAsLong()
  • OptionalDouble -> getAsDouble()

Optional Exception

If a value is present, returns the value, otherwise throws NoSuchElementException.

opt.orElseThrow();

Optional.ofNullable

    String message = null;
    //this does NOT throw any exception
    var optOfNullable = Optional.ofNullable(message);
    //this does throw NPE
    var opt = Optional.of(message);

Infinite Stream

Stream.generate(() -> "1");
Stream.iterate(1, x -> x++);

Intermediate Operations

sorted

sorted

//default natural order
stream
    .sorted()
    .forEach(System.out::println);

//order set by comparator
stream
    .sorted((c1,c2)->c1-c2)
    .forEach(System.out::println);

sorted of elements not Comparable

record Name(String name){}
List<Name> list = Arrays.asList(new Name("John"), new Name("Mark"), new Name("Philip"));
//java.lang.ClassCastException: class ...Name cannot be cast to class java.lang.Comparable
list.stream()
    .sorted()
    .forEach(System.out::println);

Wrong usage of Comparator

Comparator.reverseOrder() does not implement the Comparator interface.

stream
    .sorted(Comparator::reverseOrder)  //does not compile!

Terminal Operations

FindAny() / findFirst()

public Optional<T> findAny()
public Optional<T> findFirst()

The findFirst() method always returns the first element on an ordered stream, regardless if it is serial or parallel.

Ordered and Unordered Stream

How to make an ordered stream unordered:

    IntStream.rangeClosed(1, 10)
        .boxed()
        .unordered()
        .parallel()
        .forEach(n -> System.out.print(n + " "));
  }

Matching

public boolean anyMatch(Predicate <? super T> predicate)
public boolean allMatch(Predicate <? super T> predicate)
public boolean noneMatch(Predicate <? super T> predicate)

min

For Stream<T>

Stream<String> stream = Stream.of("a", "ab", "abc");
//min() requires a Comparator
Optional<String> min = stream.min((o1, o2) -> o1.length() - o2.length());

For IntStream

IntStream rangeClosed = IntStream.rangeClosed(0, 9);
OptionalInt optional = rangeClosed.min();

Reduce

public T reduce(T identity, BinaryOperator<T> accumulator)
 
public Optional<T> reduce(BinaryOperator<T> accumulator)
 
public <U> U reduce(U identity,
   BiFunction<U,? super T,U> accumulator,
   BinaryOperator<U> combiner)

Collectors

PartitioningBy

Map<Boolean, List<String>> map = ohMy.collect(
   Collectors.partitioningBy(s -> s.length() <= 5));

It requires a Predicate<T>
CollectorsPartitioningBy

GroupingBy

Overloaded with one, two or three arguments.

Map<Integer, List<String>> map = ohMy.collect(
   Collectors.groupingBy(String::length));

It requires a Function<T,V>
CollectorsGroupingBy

Counting

Note that it returns a Long.

Long cnt = IntStream.rangeClosed(1, 10)
    .boxed()
    .collect(Collectors.counting());  //10

Teeing

It has three parameters.

public static <T,R1,R2,R> Collector<T,?,R> teeing(Collector<? super T,?,R1> downstream1,
 Collector<? super T,?,R2> downstream2,
 BiFunction<? super R1,? super R2,R> merger)

Example:

record DataReport(long count, long sum) {}
  
DataReport dataReport = stream.collect(Collectors.teeing(
        Collectors.counting(),
        Collectors.summingLong(DataWrapper::value),
        DataReport::new));

Teeing

Spliterator

Stream<String> toys = ...
var spliterator = toys.spliterator();
var batch = spliterator.trySplit();  //batch contains the first two: Toy A, Toy B

var more = batch.tryAdvance(x -> {}); //we remove Toy A from batch but it still contains Toy B
System.out.println(more);  //true - as it still contains Toy B
spliterator.tryAdvance(System.out::println); //here we print the first of the 2nd group: Toy C
spliterator.forEachRemaining(System.out::println);

from Collection

List<String> words = Arrays.asList("Hello", "World", "Java", "Programming");
Spliterator<String> spliterator = words.spliterator();

from Stream

Stream<String> fruitStream = Stream.of("Apple", "Banana", "Orange", "Grape", "Kiwi");
//here applied on the stream
Spliterator<String> spliterator = fruitStream.spliterator();

Spliterator