java8 流操作

0  创建流

    public void test1(){
        List<String> list = new ArrayList<>();
        Stream<String> stream = list.stream(); 

        Integer[] nums = new Integer[];
        Stream<Integer> stream1 = Arrays.stream(nums);

        Stream<Integer> stream2 = Stream.of(,,,,,);

        Stream<Integer> stream3 = Stream.iterate(, (x) -> x + ).limit();
        stream3.forEach(System.out::println);

        Stream<Double> stream4 = Stream.generate(Math::random).limit();
        stream4.forEach(System.out::println);
    }

1  合并多个流

    //合并多个流
    private void merge() {
        List<String> list = new ArrayList<String>();
        list.add("I am a boy");
        list.add("I love the girl");
        list.add("But the girl loves another girl");

        List<String> result = list.stream() //得到流
        .map(line->line.split(" "))  //将每个元素转成String[]
        .flatMap(Arrays::stream)    //将每个String[] 转成流 ，并合并为一个大流
        .distinct()    //对元素去重
        .collect(Collectors.toList());   //终止流得到集合
         result.forEach(System.out::println);   //打印
    }

2  是否匹配任一元素

    //是否匹配任一元素
    private void anyMatch() {

        List<Person> persons = Arrays.asList(new Person("zhangsan", ),new Person("lisi", ));
        boolean isHas = persons.stream().anyMatch(e -> e.getAge() == );
        System.out.println("isHas : "+isHas);
    }

3  是否所有元素都匹配

    //是否所有元素都匹配
    private void allMatch() {

        List<Person> persons = Arrays.asList(new Person("zhangsan", ),new Person("lisi", ));
        boolean bool = persons.stream().allMatch(e -> e.getAge() == );
        System.out.println("isHas : "+bool);
    }

4  是否所有元素都不匹配

    //是否所有元素都不匹配
    private void  noneMatch() {

        List<Person> persons = Arrays.asList(new Person("zhangsan", ),new Person("lisi", ));
        boolean bool = persons.stream().noneMatch(e -> e.getAge() == );
        System.out.println("isHas : "+bool);
    }

5 新增容器Optional ，可以避免空指针异常

    private void  orElse() {
        Optional<Integer> optional2 = Optional.ofNullable(null);
        System.out.println( optional2.orElse());
    }

6  归并  reduce

    //归并
    private void  testReduce() {
        List<Person> persons = Arrays.asList(new Person("zhangsan", ),new Person("lisi", ));
        //reduce 不设置初始值
        Optional<Integer> optional = persons.stream().map(e -> e.getAge()).reduce((a,b) -> a + b);
        System.out.println(optional.get());
        //reduce 设置初始值
        Integer ageTotal = persons.stream().map(e -> e.getAge()).reduce(,(a,b) -> a + b);
        System.out.println(ageTotal);
    }

7  map  filter等操作

    private void commonTest() {
        List<Integer> numbers = Arrays.asList(, , , , , , );
        //forEach 遍历
        numbers.stream().forEach(System.out::println);

        //map 映射每个元素到对应的结果
        List<Integer> squaresList = numbers.stream().map(i -> i * i).distinct().collect(Collectors.toList());
        squaresList.stream().forEach(System.out::println);

        //filter 过滤
        List<Integer> filetrList = numbers.stream().filter(x -> x>).collect(Collectors.toList());
        filetrList.stream().forEach(System.out::println);

        //limit  截取多少个
        numbers.stream().limit().forEach(System.out::println);

        //sorted  排序
        numbers.stream().sorted().forEach(System.out::println);

        //summaryStatistics  统计
        IntSummaryStatistics statistics = numbers.stream().mapToInt(x -> x).summaryStatistics();
        System.out.println(statistics.getMax());
        System.out.println(statistics.getMin());
    }

8  收集器

    //使用收集器
    private void collector() {

        List<Person> persons = Arrays.asList(new Person("zhangsan", ),
                new Person("lisi", ),new Person("wangwu", ),
                new Person("zhaoliu", ),new Person("qianqi", ));
        //统计个数
        Long count = persons.stream().collect(Collectors.counting());
        System.out.println(count);

        //计算最大值
        Optional<Person> optional = persons.stream().collect(Collectors.maxBy(Comparator.comparingInt(Person::getAge)));
        System.out.println(optional.get().getName());

        //求和
        Integer sum = persons.stream().collect(Collectors.summingInt(Person::getAge));
        System.out.println(sum);

        //求平均值
        Double average = persons.stream().collect(Collectors.averagingInt(Person::getAge));
        System.out.println(average);

        //统计 最大值  , 最小值  , 平均值  , 和  , 个数
        IntSummaryStatistics summary = persons.stream().collect(Collectors.summarizingInt(Person::getAge));
        System.out.println(summary.getMax());

        //连接字符串
        String names = persons.stream().map(x -> x.getName()).collect(Collectors.joining(" | "));
        System.out.println(names);

        //一般归约
        Integer sumAge = persons.stream().collect(Collectors.reducing(, Person::getAge, (x , y) -> x + y ));
        System.out.println(sumAge);

        //分组
        Map<String, List<Person>> map = persons.stream().collect(Collectors.groupingBy((person) -> {
            if(person.getAge() <  ) {
                return  "small";
            }else if(person.getAge() < ) {
                return  "middle";
            }else {
                return  "larger";
            }
        }));
        map.get("middle").forEach( x -> System.out.println(x.getName()));

        //分组并统计
       Map<String, Long> map2  = persons.stream().collect(Collectors.groupingBy((person) -> {
            if(person.getAge() <  ) {
                return  "small";
            }else if(person.getAge() < ) {
                return  "middle";
            }else {
                return  "larger";
            }
        },
        Collectors.counting()
        ));
        map2.values().stream().forEach(System.out::println);

       //分区是分组的特殊情况
        Map<Boolean, List<Person>> map3 = persons.stream().collect(Collectors.partitioningBy(x -> x.getAge() > ));
        map3.get(false).forEach( x -> System.out.println(x.getName()));

    }

9  map 的使用，转换类型

    //map 可以将一个类型的集合转变为其他类型的集合
    public void testMap() {
        List<Integer> foo = Arrays.asList(,,,,);
        List<Score> scores = foo.stream().map(e -> new Score(e))
                .collect(Collectors.toList());
    }
    class Score {
        int value ;
        Score(int value){
            this.value = value;
        }
    }

 

10 对流的操作

skip
filter 从流中排除某些元素。
map
flatMap
limit 截断流，使其元素不超过给定数量。
sort 排序
distinct 筛选，通过流所生成元素的 hashCode() 和 equals() 去除重复元素

allMatch——检查是否匹配所有元素
anyMatch——检查是否至少匹配一个元素
noneMatch——都不匹配或者stream为空时候，返回true
findFirst——返回第一个元素
findAny——返回当前流中的任意元素
count——返回流中元素的总个数
max——返回流中最大值
min——返回流中最小值