java

        自java更新到jdk8后，添加了lambda表达式及java.util.Stream,以下便是对此进行说明
Stream对数据进行处理，但对原来的数据不进行更改

java创建stream的几种方法

        From a Collection via the stream() and parallelStream() methods

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        From an array via Arrays.stream(Object[])

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int[] array = {1, 2, 3, 4, 5}

Stream stream = Arrays.stream(array)

        From static factory methods on the stream classes, such as Stream.of(Object[]), IntStream.range(int, int) or Stream.iterate(Object, UnaryOperator)

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int[] array = {1, 2, 3, 4, 5}

Stream stream = Stream.of(array)

Stream stream = IntStream.range(3,9)      //stream = {3, 4, 5, 6, 7, 8, 9}

        The lines of a file can be obtained from BufferedReader.lines()

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BufferedReader bufferedReader = new BufferedReader(new FileReader("E:\\tmdb_5000_movies2.json"));

Stream stream = bufferedReader.lines()

        Streams of file paths can be obtained from methods in Files

        Streams of random numbers can be obtained from Random.ints()

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Random.ints().asLongStream()

        Numerous other stream-bearing methods in the JDK, including BitSet.stream(), Pattern.splitAsStream(java.lang.CharSequence), and JarFile.stream()

filter

        对stream的对象内的内容进行对其内部的所有数据进行循环，过滤不符合条件的数据，不会带来副作用

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int[] array = {1, 2, 3, 4, 5}

Stream stream = Stream.of(array)

stream.filter(e -> e / 2 == 0).forEach(System.out::println)  // --> {2, 4}

map

        获得stream对象中的数据进行操作，对其内部的所有数据进行循环操作，若用它进行过滤，将会带来副作用，不仅能返回值，同时返回可过滤后的结果Boolean值

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int[] array = {1, 2, 3, 4, 5}

Stream stream = Stream.of(array)

stream.filter(e -> e / 2 == 0)
.map(e -> e*2)
.forEach(System.out::println)  // --> {4, 8}

reduce

        该函数处于stream终端，用了reduce函数后不可再用stream的其他方法，若需要使用stream的方法时需要重新stream化.
        在java8中reduce方法重载有三个，三个方法中的参数数量不同

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# supplier为reduce方法开始的参数，同时决定着返回值的类型
# 当stream为并行化时才会调用combiner
stream.parallel().reduce(Supplier<R> supplier, BiConsumer<R, ? super T> accumulator, BiConsumer<R, R> combiner)

stream.reduce(Supplier<R> supplier, BiConsumer<R, ? super T> accumulator, BiConsumer<R, R> combiner)

stream.reduce(BiConsumer<R, ? super T> accumulator)

        
        

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stream.filter(e -> e / 2 == 0)
.map(e -> e*2)
.reduce()

javaScript

iterator

iterable

最后更新： 2019年07月06日 16:45

原始链接： https://chenadminchen.github.io/2019/07/06/java-python-javaScript-filter-map-reduce/

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