Collections

Collectors.teeing()

Old long count = items.stream().count(); double sum = items.stream() .mapToDouble(Item::price) .sum(); var result = new Stats(count, sum);

Modern var result = items.stream().collect( Collectors.teeing( Collectors.counting(), Collectors.summingDouble(Item::price), Stats::new ) );

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JDK 12+ learn more →

Collections

Typed stream toArray

Old List<String> list = getNames(); List<String> filtered = new ArrayList<>(); for (String n : list) { if (n.length() > 3) { filtered.add(n); } } String[] arr = filtered.toArray(new String[0]);

Modern String[] arr = getNames().stream() .filter(n -> n.length() > 3) .toArray(String[]::new);

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JDK 8+ learn more →

Collections

Unmodifiable collectors

Old List<String> list = stream.collect( Collectors.collectingAndThen( Collectors.toList(), Collections::unmodifiableList ) );

Modern List<String> list = stream.toList();

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JDK 16+ learn more →

Strings

String chars as stream

Old for (int i = 0; i < str.length(); i++) { char c = str.charAt(i); if (Character.isDigit(c)) { process(c); } }

Modern str.chars() .filter(Character::isDigit) .forEach(c -> process((char) c));

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JDK 9+ learn more →

Streams

Stream.ofNullable()

Old Stream<String> s = val != null ? Stream.of(val) : Stream.empty();

Modern Stream<String> s = Stream.ofNullable(val);

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JDK 9+ learn more →

Streams

Stream.iterate() with predicate

Old Stream.iterate(1, n -> n * 2) .limit(10) .forEach(System.out::println); // can't stop at a condition

Modern Stream.iterate( 1, n -> n < 1000, n -> n * 2 ).forEach(IO::println); // stops when n >= 1000

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JDK 9+ learn more →

Streams

Stream takeWhile / dropWhile

Old List<Integer> result = new ArrayList<>(); for (int n : sorted) { if (n >= 100) break; result.add(n); } // no stream equivalent in Java 8

Modern var result = sorted.stream() .takeWhile(n -> n < 100) .toList(); // or: .dropWhile(n -> n < 10)

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JDK 9+ learn more →

Streams

Collectors.flatMapping()

Old // Flatten within a grouping collector // Required complex custom collector Map<String, Set<String>> tagsByDept = // no clean way in Java 8

Modern var tagsByDept = employees.stream() .collect(groupingBy( Emp::dept, flatMapping( e -> e.tags().stream(), toSet() ) ));

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JDK 9+ learn more →

Streams

Stream.toList()

Old List<String> result = stream .filter(s -> s.length() > 3) .collect(Collectors.toList());

Modern List<String> result = stream .filter(s -> s.length() > 3) .toList();

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JDK 16+ learn more →

Streams

Stream.mapMulti()

Old stream.flatMap(order -> order.items().stream() .map(item -> new OrderItem( order.id(), item) ) );

Modern stream.<OrderItem>mapMulti( (order, downstream) -> { for (var item : order.items()) downstream.accept( new OrderItem(order.id(), item)); } );

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JDK 16+ learn more →

Streams

Stream gatherers

Old // Sliding window: manual implementation List<List<T>> windows = new ArrayList<>(); for (int i = 0; i <= list.size()-3; i++) { windows.add( list.subList(i, i + 3)); }

Modern var windows = stream .gather( Gatherers.windowSliding(3) ) .toList();

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JDK 24+ learn more →

Streams

Predicate.not() for negation

Old List<String> nonEmpty = list.stream() .filter(s -> !s.isBlank()) .collect(Collectors.toList());

Modern List<String> nonEmpty = list.stream() .filter(Predicate.not(String::isBlank)) .collect(Collectors.toList());

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JDK 11+ learn more →