Stream API 例子

Java8 中Stream类中源码方法例子

package com.h3c;import java.util.Arrays;import java.util.Collections;import java.util.Comparator;import java.util.HashMap;import java.util.List;import java.util.Map;import java.util.Random;import java.util.function.Supplier;import java.util.stream.Collectors;import java.util.stream.Stream.Builder;public class Stream {public static void main(String[] args) {List<String> list = Arrays.asList("a", "bb", "ccc", "dddd", "eeeee", "ffffff");List<String> list1 = Arrays.asList("a", "bb", "ccc", "a", "bb", "ccc");List<String> list2 = Arrays.asList("dd", "da", "ac", "cc", "ee", "ea");/*** filter 过滤   length()>=2 返回值是Stream<String>类型*/List<String> collect1 = list.stream().filter(s -> s.length() >= 2).collect(Collectors.toList());System.out.println(collect1);/*** map 转换    [ length()>=2 ] 返回值为 Stream<Boolean>*/List<Integer> collect2 = list.stream().map(s -> s.length()).collect(Collectors.toList());System.out.println(collect2);/*** mapToInt 转换方法结果为int  返回值是IntStream类型* 同样的方法  mapToLong mapToDouble*/int[] array = list.stream().mapToInt(s -> s.length()).toArray();double mapToInt = list.stream().mapToInt(s -> s.length()).average().getAsDouble();System.out.println(array.length);System.out.println(mapToInt);/*** flatMap 转换方法 结果必须为stream   返回值是stream*/List<String> collect3 = list.stream().flatMap(s -> list.stream()).collect(Collectors.toList());System.out.println(collect3);/*** flatMapToInt 转换方法结果为 IntStream  返回值是IntStream* 类似 flatMapToLong flatMapToDouble*/int[] array1 = list.stream().flatMapToInt(s -> list.stream().mapToInt(ss -> ss.length())).toArray();System.out.println(array1.length);/*** distinct 去重    和sql中distinct一样  返回值为Stream*/Object[] array2 = list1.stream().distinct().toArray();System.out.println(array2.length);/*** sorted 排序方法   对list进行排序* java8 提供直接java.util.List中的sort方法，而不是旧的Collections.sort*///实例方法的引用和Comparator.comparingCollections.sort(list2, Comparator.comparing(String::toString));List<String> collect4 = list2.stream().sorted().collect(Collectors.toList());System.out.println(collect4);//排序反转  reversed()Comparator<String> comparator = (h1, h2) -> h1.compareTo(h2);list2.sort(comparator);list2.sort(comparator.reversed());System.out.println(list2);/*** sorted(Comparator<? super T> comparator) 排序方法*/Comparator<String> comparator1 = (h1, h2) -> h1.compareTo(h2);List<String> collect5 = list2.stream().sorted(comparator1.reversed()).collect(Collectors.toList());System.out.println(collect5);/*** peek 用来<偷看一眼>的方法     不会改变list*/List<String> collect6 = list.stream().peek(string -> System.out.println("偷看一眼" + string)).collect(Collectors.toList());List<String> collect7 = list.stream().peek(string -> string.concat("!")).map(string -> string.concat("!")).collect(Collectors.toList());System.out.println(collect6);System.out.println(collect7);/*** limit 限制多少个*/List<String> collect8 = list.stream().limit(4).collect(Collectors.toList());System.out.println(collect8);/*** skip 跳过*/List<String> collect = list.stream().skip(2).collect(Collectors.toList());System.out.println(collect);/*** forEach  循环  终止语句，返回值void*/list.stream().forEach(System.out::println);list.forEach(System.out::println);/*** forEachOrdered  排序循环   终止语句，返回值void*/list2.stream().forEachOrdered(System.out::println);/*** toArray 转成数组*/Object[] array3 = list.stream().toArray();Object[] array4 = list.toArray();System.out.println(array3);System.out.println(array4);/*** reduce 归约   前面的a+后面的b+后面的b+...*/List<Integer> ints = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);String reduce = list.stream().reduce((a, b) -> a + b.length()).get();String reduce2 = list.stream().reduce("reduce ", (a, b) -> a + b.length());Integer reduce3 = ints.stream().reduce(11, (a, b) -> a + b);System.out.println(reduce);System.out.println(reduce2);System.out.println(reduce3);/*** collect*/List<String> collect9 = list.stream().collect(Collectors.toList());System.out.println(collect9);/*** min 最小的 参数是Comparator*/String min = list.stream().min((h1, h2) -> h1.compareTo(h2)).get();System.out.println(min);/*** max 最大的 参数是Comparator*/String max = list.stream().max((h1, h2) -> h1.compareTo(h2)).get();System.out.println(max);/*** count 元素的总数*/long count = list.stream().count();System.out.println(count);/*** anyMatch 任何一个 返回值boolean*/boolean anyMatch = list.stream().anyMatch(s -> s.length() > 7);System.out.println(anyMatch);/*** allMatch 所有的 返回值boolean*/boolean allMatch = list.stream().allMatch(s -> s.length() > 1);System.out.println(allMatch);/*** noneMatch 没有一个 返回值boolean*/boolean noneMatch = list.stream().noneMatch(s -> s.length() > 7);System.out.println(noneMatch);/*** findFirst 找到第一个 返回值Optional<T>*/String string = list.stream().filter(s->s.length()>2).findFirst().get();System.out.println(string);/*** findAny*/String string2 = list.stream().filter(s -> s.length() > 3).peek(s -> System.out.println(s)).findAny().get();System.out.println(string2);/*** 创建stream方法  Stream的静态方法** builder static方法  返回值Builder*/java.util.stream.Stream.Builder<String> builder1 = java.util.stream.Stream.builder();builder1.accept("ss");builder1.add("xxx");Builder<String> add = builder1.add("xxx1");java.util.stream.Stream<String> stream1 = add.build();java.util.stream.Stream<String> stream = builder1.build();List<String> collect10 = stream.collect(Collectors.toList());List<String> collect11 = stream1.collect(Collectors.toList());System.out.println(collect10);System.out.println(collect11);/*** empty  创建一个不含任何元素的Stream*/java.util.stream.Stream<Object> empty = java.util.stream.Stream.empty();List<Object> collect16 = empty.collect(Collectors.toList());System.out.println(collect16);/*** of 转换成String的静态发放 ，里面可以是 数组，list，*/Map<String,String> map = new HashMap<String,String>();map.put("1", "a");map.put("2", "b");map.put("3", "c");List<String> collect13 = java.util.stream.Stream.of("ssss,sss,ss,1111".split(",")).collect(Collectors.toList());List<List<String>> collect14 = java.util.stream.Stream.of(list,list2,list1).collect(Collectors.toList());List<Map<String,String>> collect15 = java.util.stream.Stream.of(map,map,map).collect(Collectors.toList());System.out.println(collect13);System.out.println(collect14);System.out.println(collect15);/*** iterate 循环1到n 必须要有limit限制    和reduce类似*/java.util.stream.Stream.iter4000ate(0 , n -> n +3).limit(10).forEach(System.out::println);/*** generate  创建无限Stream的静态方法*/java.util.stream.Stream<Double> echos = java.util.stream.Stream.generate(Math::random);System.out.println(echos.findFirst().get()+"======");Random seed = new Random();Supplier<Integer> random = seed::nextInt;java.util.stream.Stream.generate(random).limit(10).forEach(System.out::println);/*** concat   static方法   拼接stream  返回值stream*/java.util.stream.Stream<String> concat = java.util.stream.Stream.concat(list.stream(), list1.stream());List<String> collect12 = concat.collect(Collectors.toList());System.out.println(collect12);/*** parallel  并行流*/list.stream().parallel().forEach(System.out::println);list.stream().forEach(System.out::println);}}