java.util.streams

1. Proud (stream)umožňuje na principu roury agregované zpracování dat-objektů či int, long, double. Proud je definován sekvencí metod a je tzv. líný – nepracuje napřed do zásoby, nýbrž až po iniciaci terminální operace.Zdroj lze využít jen jednou. Tok zajišťuje jediné běžné vlákno. java.util.streams source - - - intermediate - - - terminal RESULT SUCTION Collection c.stream() Arrays.stream( ) java.nio.file.File.lines( ) bufferedRdr.lines( ) Stream.of( … ) Stream.iterate( , ) IntStream.range( , ) LongStream.range( , ) Stream.generate( Supplier s ) Stream.empty() “ abc“ .chars() .split() Stream.concat(-> | # , -> | # ) Random.ints( ) BitSet filter(Predicate p ) sorted( [Comparator c]) map( Function f ) flatMap( Function f ) peek( Consumer c ) distinct() limit(long n ) skip (long n ) mapToInt(ToIntFunction f) flatMapToInt(Function f) onClose( ) parallel( ) unordered( ) voidforEach(Consumer c ) voidforEachOrdered( Consumer c ) collect( -> ) booleananyMatch( Predicate p ) boolean{all|none}Match( Predicate p ) long count( ) reduce( -> | # ) toArray( ) {max|min}( Comparator c ) booleanfindAny() booleanfindFirst() iterator() spliterator() Lokesh Gupta: http://howtodoinjava.com/2014/04/13/java-8-tutorial-streams-by-examples http://howtodoinjava.com/2014/05/04/read-file-line-by-line-in-java-8-streams-of-lines-example PJV04

2. java.util.streams pro int/long/double BaseStream Stream Collector AutoCloseable <T,S..> <T> <T,A,R> Characteristics IntStream Stream Support Collectors + …… asDoubleStream() average() boxed() mapToLong( -> ) mapToObj( -> ) staticrange( , ) staticrangeClosed( , ) sum() summaryStatistics() toArray() static static …… stream( , ) intStream( , ) averagingInt( -> | # ) collectingAndThen( , ) counting() groupingBy( ) groupingByConcurrent( ) joining( ) mapping( , ) {max|min}By( ->| # ) partitioningBy( ) reducing( , ) summarizingInt( -> | # ) summingInt( -> | # ) toCollection(-> | # ) toConcurrentMap(-> | # ) toList() toMap(-> | # ) toSet() Int alter Long Double PJV04

3. .stream.BaseStream<T,S extendsBaseStream<T,S>> boolean isParallel( ) // static methods only Iterator<T> iterator( ) S onClose( Runnable closeHandler ) S parallel( ) S sequential( ) Spliterator<T> spliterator( ) S unordered( ) PJV25

4. Interfejs java.util.stream.Stream<T> 1/2 short circuitting je potomkemBaseStream<T, Stream<T>> static <T> Stream<T> concat( Stream<? extends T> a, Stream<? extends T> b ) static <T> Stream<T> empty( ) static <T> Stream<T> generate( Supplier<T> s ) static <T> Stream<T> iterate( T seed, UnaryOperator f ) s, f(s), f(f(s)), … static <T> Stream<T> of( T t ), of( T … values ) – hodnota či pole static <T> Stream.Builder<T> build( ) booleanallMatch( ), noneMatch( ), anyMatch( ) <R,A> R collect( Collector<? super T, R> collector ) long count( ) Stream<T> distinct( ) - propustí jenunikátníhodnoty Stream<T> filter( Predicate< ? super T > predicate ) Optional<T> findAny( ), findFirst( ) flatMap( … ) voidforEach( Consumer< ? super T > action ) PJV25

5. Interfejs java.util.stream.Stream<T> 2/2 Stream<T> limit( longmaxSize ) <R> Stream<R> map( Function<? super T,? extends R> mapper) IntStream mapToInt( ToIntFunction<? super T> mapper) Optional<T> max( ), min( ) Stream<T> peek( Consumer<? super T> action ) - pro diagnostiku Optional<T>reduce( BinaryOperator accumulator ) - kumulace T reduce( T identity, BinaryOperator accumulator ) Stream<T> skip(long n ) - přeskočení Stream<T> sorted( ), sorted( Comparator<? super T> comparator ) Object[ ] toArray( ) PJV25

6. java.util.stream. StreamSupport public final class StreamSupport { // static methods only IntStreamintStream( Spliterator.ofIntspliterator, boolean parallel ) IntStreamintStream( Supplier <? extendsSpliterator.ofInt> supplier, intcharacteristics, boolean parallel ) <T> Stream<T> stream( Spliterator<T> spliterator, boolean parallel ) <T> Stream<T> stream( Supplier <? extends Spliterator<T>> supplier, intcharacteristics, boolean parallel ) } PJV25

7. Finální třída java.util.stream.Collectors její statickémetody vracejí kolektory pro praktické redukční operace: <T> Collector<T, ?, Double> averagingInt( ToIntFunction<? super<T> mpr) <T> Collector<T, ?, Long> counting() collectingAndThen <T,K> Collector<T,?,Map<K,List<T>>> groupingBy(Function<? super T> Collector<CharSeq,?,String> joining( CharSequencedeleimiter ) <T,U,A,R> Collector<T,?,R> mapping(Function< > mpr,Collector<> stream) <T> Collector<T,?,Optional<T>> max/minBy(Comparator<? super T> cmp) <T> Collector<T,?,Map<Boolean,D>>partitioningBy(Predicate<?superT> p. <T> Collector<T,?,Optional<T>> reducing( BinaryOperator op ) <T> Collector<T,?,IntSum~> summarizingInt(ToIntFunction<? super<T> m) <T> Collector<T,?,Integer>summingInt(ToIntFunction< ? super T > mpr) <T,C extends Collection <T>>Collector<T,?,C>toCollection( Supplier<C ) <T> Collector<T,?,List<T>> toList(), toSet() toMap toConcurrentMap PJV25

8. Příklady Integer[ ] a = { 0,1,2,3,4,5,6,7,8,9 }; Stream<Integer> s1 = Stream.of( a );// proud z pole s1.sort( ).forEach( w -> { System.out.println(w); } ); // sort a výpis longcount = s1.collect( Collectors.counting( ) ); // počet položek Stream<Integer> s2 = Stream.of( a ).limit( 5 ); // jen prvních pět položek doubleavg = s2.collect( Collectors.averagingInt( x -> x ) ); // průměr Stream<Person> s3 = kolekceOsob.stream( ); // proud z kolekce doubleavg = s3.collect( Collectors.averagingDouble( p -> p.getPlat( ) ) ); List b = Arrays.asList( "A","BB","CCC" ); // tvorbakolekce Stream<String> s4 = b.stream( ); // proud z kolekce doubleavg = s4.collect( Collectors.averagingInt( String::length ) ); Optional w = b.stream( ).reduce( ( u, v) -> u=u+"-"+v); PJV04

9. Příklady vytvoření statistiky IntConsumerst = newIntSummaryStatistics( ); Integer[ ]a = { 0,1,2,3,4,5,6,7,8,9 }; for (intk : a) st.accept(k); Collection<Integer> b = Arrays.asList( 0,1,2,3,4,5,6,7,8,9 ); Spliterator<Integer> sp = b.spliterator( ); sp.forEachRemaining( st::accept ); Collector<Integer, ?, IntSummaryStatistics> coSt = Collectors.summarizingInt( x -> x ); IntSummaryStatisticsiss = b.stream( ).collect(coSt); Tisk vytvořené statistiky: System.out.println( b.stream( ).collect(Collectors.summarizingInt( x -> x ) ) ); PJV04

10. Příklad: statistika znaků List<String> Stream<String> IntStream String[] p={ "AAA", "BB", "C" }; int[] stat=newint[128]; Arrays.asList(p) // tvorba statistiky .stream().forEach( s -> s.chars().forEach(n-> stat[n]++)); for (int i = 0; i < stat.length; i++) // tisk statistiky if(stat[i]>0) System.out.println( (char)i+" "+stat[i]); PJV04

11. Příklad grouping a partitioning Budiž Collection col = Arrays.asList("A","BB","CCC","DDD","EE","F"); Stream<String> stream = col.stream(); Map<Integer, List<String>> map = stream .collect( Collectors.groupingBy( x->x.length( ) ) ); System.out.println(map ); // ~{ 1=[A, F], 2=[BB, EE], 3=[CCC, DDD] } Stream<String> stream = col.stream(); Map<Boolean, List<String>> map= stream .collect( Collectors.partitioningBy(t-> t.length( )%2==0)); System.out.println(map ); // ~{ false=[A, CCC, DDD, F], true=[BB, EE] } PJV04

12. Příklady redukce Budiž Collection col = Arrays.asList("A","BB","CCC","DDD","EE","F"); Stream<String> stream = col.stream(); Optional<Integer> opt = stream .map(t-> t.length()) .reduce( ( a, e ) -> a + e ); System.out.println( opt ); // ~ Optional[12] • OptionalInt opt = IntStream.iterate( 111, i -> --i ).limit( 5 ) .reduce( ( acu, elem ) -> Math.min( acu, elem ) ); System.out.println( opt ); // ~ Optional[107] System.out.println( IntStream.iterate( 1, i -> i+1 ).limit( 5 ) .reduce( ( a, e ) -> a * e ).getAsInt( ) ); // = 120 = 5 ! PJV04

13. Příklad konverze proudu a redukce ekvivalentní možnosti např. možnosti Stream<Integer> s=list.stream(); double result = s.filter( x -> true) // čirá propust .map(R -> ++R) // inkrementace hodnot .map( r -> new Double(r) ) .map( r -> Double.valueOf(r) ) // konverze do Double .map( r -> Double.valueOf ) .map(Double::new ) .collect( Collectors.summingDouble( Double::doubleValue ) ) ; .collect( Collectors.averagingDouble( r -> r.doubleValue() ) ); .collect( Collectors.averagingDouble( r -> r ) ); System.out.println( result ); PJV04

14. Příkladspliteratoru s externími zdroji ekvivalentní možnosti Stream<String> s= java.nio.file.File.lines(Paths.get(".", "data.txt")); newBufferedReader( newFileReader( "data.txt" ) ).lines( ); Spliterator<String> sp = s.limit( 42 ) // jentrochudat .spliterator( ); Cons cons = new Cons( ); // konzumující objekt while ( sp.tryAdvance( cons::accept ) ) ; class Cons implements Consumer<String> { public void accept( String t ) { System.out.print( t ); } } PJV04