• Java8流Stream的创建和操作

    1. 流的创建方法

    import java.util.stream.*;

public class StreamOf {
  public static void main(String[] args) {
    Stream.of(
      new Bubble(1), new Bubble(2), new Bubble(3))
      .forEach(System.out::println);
    Stream.of("It's ", "a ", "wonderful ",
      "day ", "for ", "pie!")
      .forEach(System.out::print);
    System.out.println();
    Stream.of(3.14159, 2.718, 1.618)
      .forEach(System.out::println);
  }
}
/* Output:
Bubble(1)
Bubble(2)
Bubble(3)
It's a wonderful day for pie!
3.14159
2.718
1.618
*/

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    1.1 随机数流

    import java.util.*;
import java.util.stream.*;

public class RandomGenerators {
  public static <T> void show(Stream<T> stream) {
    stream
      .limit(4)
      .forEach(System.out::println);
    System.out.println("++++++++");
  }
  public static void main(String[] args) {
    Random rand = new Random(47);
    show(rand.ints().boxed());
    show(rand.longs().boxed());
    show(rand.doubles().boxed());
    // Control the lower and upper bounds:
    show(rand.ints(10, 20).boxed());
    show(rand.longs(50, 100).boxed());
    show(rand.doubles(20, 30).boxed());
    // Control the stream size:
    show(rand.ints(2).boxed());
    show(rand.longs(2).boxed());
    show(rand.doubles(2).boxed());
    // Control the stream size and bounds:
    show(rand.ints(3, 3, 9).boxed());
    show(rand.longs(3, 12, 22).boxed());
    show(rand.doubles(3, 11.5, 12.3).boxed());
  }
}

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    1.2 int类型流

    import static java.util.stream.IntStream.*;

public class Ranges {
  public static void main(String[] args) {
    // The traditional way:
    int result = 0;
    for(int i = 10; i < 20; i++)
      result += i;
    System.out.println(result);

    // for-in with a range:
    result = 0;
    for(int i : range(10, 20).toArray())
      result += i;
    System.out.println(result);

    // Use streams:
    System.out.println(range(10, 20).sum());
  }
}

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    1.3 generate()

    import java.util.*;
import java.util.function.*;
import java.util.stream.*;

public class Generator implements Supplier<String> {
  Random rand = new Random(47);
  char[] letters =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
  @Override public String get() {
    return "" + letters[rand.nextInt(letters.length)];
  }
  public static void main(String[] args) {
    String word = Stream.generate(new Generator())
      .limit(30)
      .collect(Collectors.joining());
    System.out.println(word);
  }
}

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    1.4 iterate()

    import java.util.stream.*;

public class Fibonacci {
  int x = 1;
  Stream<Integer> numbers() {
    return Stream.iterate(0, i -> {
      int result = x + i;
      x = i;
      return result;
    });
  }
  public static void main(String[] args) {
    new Fibonacci().numbers()
      .skip(20) // Don't use the first 20
      .limit(10) // Then take 10 of them
      .forEach(System.out::println);
  }
}

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    1.5 流的建造者模式

    import java.io.*;
import java.nio.file.*;
import java.util.stream.*;

public class FileToWordsBuilder {
  Stream.Builder<String> builder = Stream.builder();
  public FileToWordsBuilder(String filePath)
  throws Exception {
    Files.lines(Paths.get(filePath))
      .skip(1) // Skip the comment line at the beginning
      .forEach(line -> {
        for(String w : line.split("[ .?,]+"))
          builder.add(w);
      });
  }
  Stream<String> stream() { return builder.build(); }
  public static void
  main(String[] args) throws Exception {
    new FileToWordsBuilder("Cheese.dat").stream()
      .limit(7)
      .map(w -> w + " ")
      .forEach(System.out::print);
  }
}

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    1.6 Arrays

    import java.util.*;
import onjava.Operation;

public class MetalWork2 {
  public static void main(String[] args) {
    Arrays.stream(new Operation[] {
      () -> Operation.show("Heat"),
      () -> Operation.show("Hammer"),
      () -> Operation.show("Twist"),
      () -> Operation.show("Anneal")
    }).forEach(Operation::execute);
  }
}

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    1.7 正则表达式

    import java.io.*;
import java.nio.file.*;
import java.util.stream.*;
import java.util.regex.Pattern;

public class FileToWordsRegexp {
  private String all;
  public FileToWordsRegexp(String filePath)
  throws Exception {
    all = Files.lines(Paths.get(filePath))
      .skip(1) // First (comment) line
      .collect(Collectors.joining(" "));
  }
  public Stream<String> stream() {
    return Pattern
      .compile("[ .,?]+").splitAsStream(all);
  }
  public static void
  main(String[] args) throws Exception {
    FileToWordsRegexp fw =
      new FileToWordsRegexp("Cheese.dat");
    fw.stream()
      .limit(7)
      .map(w -> w + " ")
      .forEach(System.out::print);
    fw.stream()
      .skip(7)
      .limit(2)
      .map(w -> w + " ")
      .forEach(System.out::print);
  }
}

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    2. 流的中间操作

    2.1 跟踪和调试

    class Peeking  {
  public static void
  main(String[] args) throws Exception {
    FileToWords.stream("Cheese.dat")
      .skip(21)
      .limit(4)
      .map(w -> w + " ")
      .peek(System.out::print)
      .map(String::toUpperCase)
      .peek(System.out::print)
      .map(String::toLowerCase)
      .forEach(System.out::print);
  }
}

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    2.2 排序

    import java.util.*;

public class SortedComparator {
  public static void
  main(String[] args) throws Exception {
    FileToWords.stream("Cheese.dat")
      .skip(10)
      .limit(10)
      .sorted(Comparator.reverseOrder())
      .map(w -> w + " ")
      .forEach(System.out::print);
  }
}


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    2.3 移除元素

    import java.util.stream.*;
import static java.util.stream.LongStream.*;

public class Prime {
  public static boolean isPrime(long n) {
    return rangeClosed(2, (long)Math.sqrt(n))
      .noneMatch(i -> n % i == 0);
  }
  public LongStream numbers() {
    return iterate(2, i -> i + 1)
      .filter(Prime::isPrime);
  }
  public static void main(String[] args) {
    new Prime().numbers()
      .limit(10)
      .forEach(n -> System.out.format("%d ", n));
    System.out.println();
    new Prime().numbers()
      .skip(90)
      .limit(10)
      .forEach(n -> System.out.format("%d ", n));
  }
}

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    2.5 应用函数到每个元素

    import java.util.*;
import java.util.stream.*;

class Numbered {
  final int n;
  Numbered(int n) { this.n = n; }
  @Override public String toString() {
    return "Numbered(" + n + ")";
  }
}

class FunctionMap2 {
  public static void main(String[] args) {
    Stream.of(1, 5, 7, 9, 11, 13)
      .map(Numbered::new)
      .forEach(System.out::println);
  }
}
class FunctionMap3 {
  public static void main(String[] args) {
    Stream.of("5", "7", "9")
      .mapToInt(Integer::parseInt)
      .forEach(n -> System.out.format("%d ", n));
    System.out.println();
    Stream.of("17", "19", "23")
      .mapToLong(Long::parseLong)
      .forEach(n -> System.out.format("%d ", n));
    System.out.println();
    Stream.of("17", "1.9", ".23")
      .mapToDouble(Double::parseDouble)
      .forEach(n -> System.out.format("%f ", n));
  }
}

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    2.6 在map()中组合流

    import java.util.stream.*;

public class StreamOfStreams {
  public static void main(String[] args) {
    Stream.of(1, 2, 3)
      .map(i -> Stream.of("Gonzo", "Kermit", "Beaker"))
      .map(e-> e.getClass().getName())
      .forEach(System.out::println);
  }
}

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    import java.util.*;
import java.util.stream.*;

public class StreamOfRandoms {
  static Random rand = new Random(47);
  public static void main(String[] args) {
    Stream.of(1, 2, 3, 4, 5)
      .flatMapToInt(i -> IntStream.concat(
        rand.ints(0, 100).limit(i), IntStream.of(-1)))
      .forEach(n -> System.out.format("%d ", n));
  }
}

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    import java.nio.file.*;
import java.util.stream.*;
import java.util.regex.Pattern;

public class FileToWords {
  public static Stream<String> stream(String filePath)
  throws Exception {
    return Files.lines(Paths.get(filePath))
      .skip(1) // First (comment) line
      .flatMap(line ->
        Pattern.compile("\\W+").splitAsStream(line));
  }
}

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    3. 终端或订阅操作

    3.1 输出为数组

    • toArray()
    • toArray(generator): 特殊情况下，生成自定义类型的数组
    import java.util.*;
import java.util.stream.*;

public class RandInts {
  private static int[] rints =
    new Random(47).ints(0, 1000).limit(100).toArray();
  public static IntStream rands() {
    return Arrays.stream(rints);
  }
}

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    3.2 循环

    • forEach(Consumer)：在Collection中可以改变值，但是stream不应该修改值。因为流式要求non-interfering。
    • forEachOrdered(Consumer): 保证forEach按照原始流顺序返回
      Collection.forEach和Collection.stream().forEach区别：
      https://www.baeldung.com/java-collection-stream-foreach

    3.3 集合

    • collect(Collector)
    • collect(Supplier, BiConsumer, BiConsumer): 第一个参数Supplier创建一个新的结果集合，第二个参数BiConsumer将下一个元素收集到结果集合，第三个参数BiConsumer用于将两个结果集合合并起来。
    import java.util.*;
import java.nio.file.*;
import java.util.stream.*;
public class TreeSetOfWords {
  public static void
  main(String[] args) throws Exception {
    Set<String> words2 =
      Files.lines(Paths.get("TreeSetOfWords.java"))
        .flatMap(s -> Arrays.stream(s.split("\\W+")))
        .filter(s -> !s.matches("\\d+")) // No numbers
        .map(String::trim)
        .filter(s -> s.length() > 2)
        .limit(100)
        .collect(Collectors.toCollection(TreeSet::new));
    System.out.println(words2);
  }
}

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    import java.util.*;
import java.util.stream.*;

class Pair {
  public final Character c;
  public final Integer i;
  Pair(Character c, Integer i) {
    this.c = c;
    this.i = i;
  }
  public Character getC() { return c; }
  public Integer getI() { return i; }
  @Override public String toString() {
    return "Pair(" + c + ", " + i + ")";
  }
}

class RandomPair {
  Random rand = new Random(47);
  // An infinite iterator of random capital letters:
  Iterator<Character> capChars = rand.ints(65,91)
    .mapToObj(i -> (char)i)
    .iterator();
  public Stream<Pair> stream() {
    return rand.ints(100, 1000).distinct()
      .mapToObj(i -> new Pair(capChars.next(), i));
  }
}

public class MapCollector {
  public static void main(String[] args) {
    Map<Integer, Character> map =
      new RandomPair().stream()
        .limit(8)
        .collect(
          Collectors.toMap(Pair::getI, Pair::getC));
    System.out.println(map);
  }
}

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    3.4 组合

    • reduce(BinaryOperator)
    • reduce(identity, BinaryOperator)
    • reduce(identity, BiFunction, BinaryOperator)
    import java.util.*;
import java.util.stream.*;

class Frobnitz {
  int size;
  Frobnitz(int sz) { size = sz; }
  @Override public String toString() {
    return "Frobnitz(" + size + ")";
  }
  // Generator:
  static Random rand = new Random(47);
  static final int BOUND = 100;
  static Frobnitz supply() {
    return new Frobnitz(rand.nextInt(BOUND));
  }
}

public class Reduce {
  public static void main(String[] args) {
    Stream.generate(Frobnitz::supply)
      .limit(10)
      .peek(System.out::println)
      .reduce((fr0, fr1) -> fr0.size < 50 ? fr0 : fr1)
      .ifPresent(System.out::println);
  }
}

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    3.5 匹配

    • allMatch(Predicate)
    • anyMatch(Predicate)
    • noneMatch(Predicate)
    import java.util.stream.*;
import java.util.function.*;
import static streams.RandInts.*;

interface Matcher extends
  BiPredicate<Stream<Integer>, Predicate<Integer>> {}

public class Matching {
  static void show(Matcher match, int val) {
    System.out.println(
      match.test(
        IntStream.rangeClosed(1, 9)
          .boxed()
          .peek(n -> System.out.format("%d ", n)),
        n -> n < val));
  }
  public static void main(String[] args) {
    show(Stream::allMatch, 10);
    show(Stream::allMatch, 4);
    show(Stream::anyMatch, 2);
    show(Stream::anyMatch, 0);
    show(Stream::noneMatch, 5);
    show(Stream::noneMatch, 0);
  }
}

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    3.6 查找

    • findFirst()
    • findAny()
    import java.util.*;
import java.util.stream.*;
import static streams.RandInts.*;

public class SelectElement {
  public static void main(String[] args) {
    System.out.println(rands().findFirst().getAsInt());
    System.out.println(
      rands().parallel().findFirst().getAsInt());
    System.out.println(rands().findAny().getAsInt());
    System.out.println(
      rands().parallel().findAny().getAsInt());
  }
}

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    3.7 信息

    • count()
    • max(Comparator)
    • min(Comparator)
    import java.util.stream.*;
import java.util.function.*;

public class Informational {
  public static void
  main(String[] args) throws Exception {
      System.out.println(
        FileToWords.stream("Cheese.dat").count());
      System.out.println(
        FileToWords.stream("Cheese.dat")
        .min(String.CASE_INSENSITIVE_ORDER)
        .orElse("NONE"));
      System.out.println(
        FileToWords.stream("Cheese.dat")
        .max(String.CASE_INSENSITIVE_ORDER)
        .orElse("NONE"));
  }
}

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    3.8 数字流信息

    • avarage()
    • max() min()
    • sum()
    • summaryStatistics()
    import java.util.stream.*;
import static streams.RandInts.*;

public class NumericStreamInfo {
  public static void main(String[] args) {
    System.out.println(rands().average().getAsDouble());
    System.out.println(rands().max().getAsInt());
    System.out.println(rands().min().getAsInt());
    System.out.println(rands().sum());
    System.out.println(rands().summaryStatistics());
  }
}

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  • 原文地址：https://blog.csdn.net/Fire_to_cheat_/article/details/126837265