The Sequence library is a leaner alternative to sequential Java 8 Streams, used in similar ways but with a lighter step, and with better integration with the rest of Java.
It aims to be roughly feature complete with sequential Streams, with additional convenience methods for advanced traversal and transformation. In particular it allows easier collecting into common Collections without Collectors, better handling of Maps with Pair and Map.Entry as first-class citizens, tighter integration with the rest of Java by being implemented in terms of Iterable, and advanced partitioning, mapping and filtering methods, for example allowing you to peek at previous or next elements to make decisions during traversal. Sequences go to great lengths to be as lazy and late-evaluating as possible, with minimal overhead.
Sequences use Java 8 lambdas in much the same way as Streams do, but is based on readily available Iterables instead of a black box pipeline, and is built for convenience and compatibility with the rest of Java. It's for programmers wanting to perform common data processing tasks on moderately sized collections. If you need parallel iteration or are processing over 1 million or so entries, you might benefit from using a parallel Stream instead.
{@code Listevens = Sequence.of(1, 2, 3, 4, 5, 6, 7, 8, 9) .filter(x -> x % 2 == 0) .map(Object::toString) .toList(); assertThat(evens, contains("2", "4", "6", "8")); }
See also: {@link org.d2ab.sequence.Sequence#of(Object...)}, {@link org.d2ab.sequence.Sequence#from(Iterable)}
The main Sequence package is {@link org.d2ab.sequence} where all the sequences reside. There are seven kinds of Sequences, each dealing with a different type of entry. The first is the regular {@link org.d2ab.sequence.Sequence} which is the general purpose stream of items. {@link org.d2ab.sequence.EntrySequence} and {@link org.d2ab.sequence.BiSequence} work directly on the constituent components of {@link java.util.Map.Entry} and {@link org.d2ab.util.Pair} objects. The last four are primitive sequences dealing with {@code char}, {@code int}, {@code long} and {@code double} primitives; {@link org.d2ab.sequence.CharSeq}, {@link org.d2ab.sequence.IntSequence}, {@link org.d2ab.sequence.LongSequence}, and {@link org.d2ab.sequence.DoubleSequence}. These work much the same as the regular {@link org.d2ab.sequence.Sequence} except they're adapted to work directly on primitives.
Because each {@link org.d2ab.sequence.Sequence} is an {@link Iterable} you can re-use them safely after you have already traversed them, as long as they're not backed by an {@link java.util.Iterator} or {@link java.util.stream.Stream} which can only be traversed once.
{@code Sequencesingulars = Sequence.range(1, 9); // Digits 1..9 // using sequence of ints 1..9 first time to get odd numbers between 1 and 9 Sequence odds = singulars.step(2); assertThat(odds, contains(1, 3, 5, 7, 9)); // re-using the same sequence again to get squares of numbers between 4 and 8 Sequence squares = singulars.startingFrom(4).endingAt(8).map(i -> i * i); assertThat(squares, contains(16, 25, 36, 49, 64)); }
Also because each {@link org.d2ab.sequence.Sequence} is an {@link Iterable} they work beautifully in foreach loops:
{@code Sequencesequence = Sequence.ints().limit(5); int expected = 1; for (int each : sequence) assertThat(each, is(expected++)); assertThat(expected, is(6)); }
Because Sequence is a {@link FunctionalInterface} requiring only the {@link Iterable#iterator()} method to be implemented, it's very easy to create your own full-fledged {@link org.d2ab.sequence.Sequence} instances that can be operated on like any other {@link org.d2ab.sequence.Sequence} through the default methods on the interface that carry the bulk of the burden. In fact, this is how `Sequence's` own factory methods work. You could consider all of `Sequence` to be a smarter version of `Iterable`.
{@code Listlist = Arrays.asList(1, 2, 3, 4, 5); // Sequence as @FunctionalInterface of list's Iterator Sequence sequence = list::iterator; // Operate on sequence as any other sequence using default methods Sequence transformed = sequence.map(Object::toString); assertThat(transformed.limit(3), contains("1", "2", "3")); }
Sequences can be created from Iterators or Streams but can then only be passed over once.
{@code Iteratoriterator = Arrays.asList(1, 2, 3, 4, 5).iterator(); Sequence sequence = Sequence.once(iterator); assertThat(sequence, contains(1, 2, 3, 4, 5)); assertThat(sequence, is(emptyIterable())); }
See also: {@link org.d2ab.sequence.Sequence#once(Iterator)}, {@link org.d2ab.sequence.Sequence#once(Stream)}
If you have an Iterator or Stream and wish to convert it to a full-fledged multi-iterable Sequence, use the caching methods on Sequence.
{@code Iteratoriterator = Arrays.asList(1, 2, 3, 4, 5).iterator(); Sequence cached = Sequence.cache(iterator); assertThat(cached, contains(1, 2, 3, 4, 5)); assertThat(cached, contains(1, 2, 3, 4, 5)); }
See also: {@link org.d2ab.sequence.Sequence#cache(Iterable)}, {@link org.d2ab.sequence.Sequence#cache(Iterator)}, {@link org.d2ab.sequence.Sequence#cache(Stream)}
Sequences have full support for updating the underlying collection where possible, both through Iterator#remove() and by modifying the underlying collection directly in between iterations.
{@code Listlist = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5)); Sequence.from(list).filter(x -> x % 2 != 0).clear(); assertThat(list, contains(2, 4)); }
{@code Listlist = new ArrayList<>(Arrays.asList(1, 2, 3, 4, 5)); Sequence sequence = Sequence.from(list).filter(x -> x % 2 == 0); assertThat(sequence, contains(2, 4)); list.add(6); assertThat(sequence, contains(2, 4, 6)); }
See also: {@link org.d2ab.sequence.Sequence#clear()}
Sequences interoperate beautifully with Stream, through the once(Stream) and .stream() methods.
{@code Sequencepaired = Sequence.once(Stream.of("a", "b", "c", "d")).pairs().flatten(); assertThat(paired.stream().collect(Collectors.toList()), contains("a", "b", "b", "c", "c", "d")); }
See also: {@link org.d2ab.sequence.Sequence#once(Stream)}, {@link org.d2ab.sequence.Sequence#cache(Stream)}, {@link org.d2ab.sequence.Sequence#stream()}
There is full support for infinite recursive Sequences, including termination at a known value.
{@code Sequencefibonacci = BiSequence.recurse(0, 1, (i, j) -> Pair.of(j, i + j)) .toSequence((i, j) -> i) .endingAt(34); assertThat(fibonacci, contains(0, 1, 1, 2, 3, 5, 8, 13, 21, 34)); }
{@code Exception exception = new IllegalStateException(new IllegalArgumentException(new NullPointerException())); SequenceexceptionAndCauses = Sequence.recurse(exception, Throwable::getCause).untilNull(); assertThat(exceptionAndCauses, contains(instanceOf(IllegalStateException.class), instanceOf(IllegalArgumentException.class), instanceOf(NullPointerException.class))); StringBuilder builder = new StringBuilder(); exceptionAndCauses.last(IllegalArgumentException.class).ifPresent(builder::append); assertThat(builder.toString(), is("java.lang.IllegalArgumentException: java.lang.NullPointerException")); }
{@code Iteratordelimiter = Sequence.of("").append(Sequence.of(", ").repeat()).iterator(); StringBuilder joined = new StringBuilder(); for (String number : Arrays.asList("One", "Two", "Three")) joined.append(delimiter.next()).append(number); assertThat(joined.toString(), is("One, Two, Three")); }
{@code CharSeq hexGenerator = CharSeq.random("0-9", "A-F").limit(8); String hexNumber1 = hexGenerator.asString(); String hexNumber2 = hexGenerator.asString(); assertTrue(hexNumber1.matches("[0-9A-F]{8}")); assertTrue(hexNumber2.matches("[0-9A-F]{8}")); assertThat(hexNumber1, is(not(hexNumber2))); }
See also:
The standard reduction operations are available as per Stream:
{@code Sequencethirteen = Sequence.longs().limit(13); long factorial = thirteen.reduce(1L, (r, i) -> r * i); assertThat(factorial, is(6227020800L)); }
See also: {@link org.d2ab.sequence.Sequence#reduce(BinaryOperator)}, {@link org.d2ab.sequence.Sequence#reduce(Object, BinaryOperator)}
Maps are handled as Sequences of Entry, with special transformation methods that convert to/from Maps.
{@code Sequencekeys = Sequence.of(1, 2, 3); Sequence values = Sequence.of("1", "2", "3"); Map map = keys.interleave(values).toMap(); assertThat(map, is(equalTo(Maps.builder(1, "1").put(2, "2").put(3, "3").build()))); }
See also:
You can also map Entry Sequences to Pairs which allows more expressive transformation and filtering.
{@code Mapmap = Maps.builder("1", 1).put("2", 2).put("3", 3).put("4", 4).build(); Sequence > sequence = Sequence.from(map) .map(Pair::from) .filter(p -> p.test((s, i) -> i != 2)) .map(p -> p.map((s, i) -> Pair.of(s + " x 2", i * 2))); assertThat(sequence.toMap(), is(equalTo(Maps.builder("1 x 2", 2).put("3 x 2", 6).put("4 x 2", 8).build()))); }
See also: {@link org.d2ab.util.Pair}
You can also work directly on Entry keys and values using EntrySequence.
{@code Maporiginal = Maps.builder("1", 1).put("2", 2).put("3", 3).put("4", 4).build(); EntrySequence oddsInverted = EntrySequence.from(original) .filter((k, v) -> v % 2 != 0) .map((k, v) -> Maps.entry(v, k)); assertThat(oddsInverted.toMap(), is(equalTo(Maps.builder(1, "1").put(3, "3").build()))); }
See also: {@link org.d2ab.sequence.EntrySequence}
When iterating over sequences of Pairs of item, BiSequence provides native operators and transformations:
{@code BiSequencepresidents = BiSequence.ofPairs("Abraham Lincoln", 1861, "Richard Nixon", 1969, "George Bush", 2001, "Barack Obama", 2005); Sequence joinedOffice = presidents.toSequence((n, y) -> n + " (" + y + ")"); assertThat(joinedOffice, contains("Abraham Lincoln (1861)", "Richard Nixon (1969)", "George Bush (2001)", "Barack Obama (2005)")); }
See also: {@link org.d2ab.sequence.BiSequence}
There are also primitive versions of Sequence for char, int, long and double processing: CharSeq, IntSequence, LongSequence and DoubleSequence.
{@code CharSeq snakeCase = CharSeq.from("Hello Lexicon").map(c -> (c == ' ') ? '_' : c).map(Character::toLowerCase); assertThat(snakeCase.asString(), is("hello_lexicon")); }
{@code IntSequence squares = IntSequence.positive().map(i -> i * i); assertThat(squares.limit(5), contains(1, 4, 9, 16, 25)); }
{@code LongSequence negativeOdds = LongSequence.negative().step(2); assertThat(negativeOdds.limit(5), contains(-1L, -3L, -5L, -7L, -9L)); }
{@code DoubleSequence squareRoots = IntSequence.positive().toDoubles().map(Math::sqrt); assertThat(squareRoots.limit(3), contains(sqrt(1), sqrt(2), sqrt(3))); }
See also: {@link org.d2ab.sequence.CharSeq}, {@link org.d2ab.sequence.IntSequence}, {@link org.d2ab.sequence.LongSequence}, {@link org.d2ab.sequence.DoubleSequence}
Sequences also have mapping and filtering methods that peek on the previous and next elements:
{@code CharSeq titleCase = CharSeq.from("hello_lexicon") .mapBack('_', (p, c) -> p == '_' ? toUpperCase(c) : c) .map(c -> (c == '_') ? ' ' : c); assertThat(titleCase.asString(), is("Hello Lexicon")); }
See also:
Both regular and primitive Sequences have advanced windowing and partitioning methods, allowing you to divide up Sequences in various ways, including a partitioning method that uses a BiPredicate to determine which two elements to create a batch between.
{@code Sequence> batched = Sequence.of(1, 2, 3, 4, 5, 6, 7, 8, 9).batch(3); assertThat(batched, contains(contains(1, 2, 3), contains(4, 5, 6), contains(7, 8, 9))); }
{@code String vowels = "aeoiuy"; SequenceconsonantsVowels = CharSeq.from("terrain") .batch((a, b) -> (vowels.indexOf(a) == -1) != (vowels.indexOf(b) == -1)) .map(CharSeq::asString); assertThat(consonantsVowels, contains("t", "e", "rr", "ai", "n")); }
See also:
Primitive sequences can be read from `Readers` or `InputStreams` into a `CharSeq` or `IntSequence` respective. These can also be converted back to `Readers` and `InputStreams` respectively, allowing for filtering or transformation of these streams.
{@code Reader reader = new StringReader("hello world\ngoodbye world\n"); SequencetitleCase = CharSeq.read(reader) .mapBack('\n', (p, n) -> p == '\n' || p == ' ' ? Character.toUpperCase(n) : n) .split('\n') .map(phrase -> phrase.append('!')) .map(CharSeq::asString); assertThat(titleCase, contains("Hello World!", "Goodbye World!")); reader.close(); }
{@code Reader original = new StringReader("hello world\ngoodbye world\n"); BufferedReader transformed = new BufferedReader(CharSeq.read(original).map(Character::toUpperCase).asReader()); assertThat(transformed.readLine(), is("HELLO WORLD")); assertThat(transformed.readLine(), is("GOODBYE WORLD")); transformed.close(); original.close(); }
{@code InputStream inputStream = new ByteArrayInputStream(new byte[]{0xD, 0xE, 0xA, 0xD, 0xB, 0xE, 0xE, 0xF}); String hexString = IntSequence.read(inputStream) .toSequence(Integer::toHexString) .map(String::toUpperCase) .join(); assertThat(hexString, is("DEADBEEF")); inputStream.close(); }
See also: {@link org.d2ab.sequence.CharSeq#read(java.io.Reader)}, {@link org.d2ab.sequence.IntSequence#read(java.io.InputStream)}