Java Stream Map和flatmap及集合处理

编程中多数要对集合进行各种操作，获取Map、Set、List等。
Map

比如一个对象List，获取所有人的名字集合

    @Test
    public void should_can_get_name_map() {
        PersonInfo kaka = new PersonInfo("Kaka", 22);
        PersonInfo hustzw = new PersonInfo("Hustzw", 24);

        List personInfos = Lists.newArrayList(kaka, hustzw);

        List nameList = personInfos.stream().map(PersonInfo::getName).collect(Collectors.toList());

        assertThat(nameList).contains("Kaka");
    }

HashMap

构建一个属性和其本身的映射，比如根据人名找到人。

    @Test
    public void should_can_get_name_info_map() {
        PersonInfo kaka = new PersonInfo("Kaka", 22);
        PersonInfo hustzw = new PersonInfo("Hustzw", 24);

        List personInfos = Lists.newArrayList(kaka, hustzw);

        // 注意， 这里 key 不能重复，否则报错
        Map nameInfoMap1 = personInfos.stream().collect(Collectors.toMap(PersonInfo::getName, x -> x));
        Map nameInfoMap2 = personInfos.stream().collect(Collectors.toMap(PersonInfo::getName, Function.identity()));// 效果一样

        assertThat(nameInfoMap1).containsKeys("Kaka", "Hustzw");
        assertThat(nameInfoMap2).containsKeys("Kaka", "Hustzw");
    }

Set

同样是获取集合

    @Test
    public void should_can_get_set() {
        PersonInfo kaka = new PersonInfo("Kaka", 22);
        PersonInfo hustzw = new PersonInfo("Hustzw", 24);

        List personInfos = Lists.newArrayList(kaka, hustzw);

        // 注意， 这里 key 不能重复，否则报错
        Set adultsSet = personInfos.stream().filter(x -> x.getAge() >= 18).collect(Collectors.toSet());

        assertThat(adultsSet).contains(kaka, hustzw);
        assertThat(adultsSet).contains(hustzw, kaka);
    }

flatmap

Java8的stream中提供flatmap也是用于构建Map，不过区别的是它会把结果打平。看个例子：

有两个List，想对所有元素按年龄分类。如果利用循环可能需要两层循环。

利用 flatemap后，打平后很容易对其操作。

代码：

    @Test
    public void should_can_get_name_info_flat_map() {
        PersonInfo kaka = new PersonInfo("Kaka", 22);
        PersonInfo hustzw = new PersonInfo("Hustzw", 22);
        PersonInfo zhangSan = new PersonInfo("ZhangSan", 14);
        PersonInfo lisi = new PersonInfo("LiSi", 14);

        List firstInfos = Lists.newArrayList(kaka, zhangSan, zhangSan);
        List secondInfos = Lists.newArrayList(hustzw, lisi);

        List> complexInfos = Lists.newArrayList(firstInfos, secondInfos);

        // 按年龄分组，不会去重
        Map> map = complexInfos.stream().flatMap(l -> l.stream()).collect(Collectors.groupingBy(PersonInfo::getAge));

        // 如果需要去重复
        Map> distinctMap = complexInfos.stream().flatMap(l -> l.stream())// 重新生成一个Stream对象取而代之
                .distinct().collect(Collectors.groupingBy(PersonInfo::getAge));

        assertThat(map).containsKey(14);
        assertThat(map.get(14).size()).isEqualTo(3);
        assertThat(distinctMap.get(14).size()).isEqualTo(2);
    }

集合处理

当然还有一些用于对集合过滤、统计的。看个例子：

@Data
@Builder
@AllArgsConstructor
public class Work {

    private String name;

    private String department;

    private Integer age;
}

下边对该类型的集合进行处理：

public class MyTest {

    private List personList;

    @Before
    public void setup() {

        personList = Lists.newArrayList();
        personList.add(Work.builder().name("zhangsan").department("IT").age(20).build());
        personList.add(Work.builder().name("lisi").department("IT").age(22).build());
        personList.add(Work.builder().name("wangwu").department("HR").age(28).build());
        personList.add(Work.builder().name("zhaoliu").department("Purchasing").age(23).build());
        personList.add(Work.builder().name("liuqi").department("IT").age(22).build());
    }

    @Test
    public void should_can_match() {
        // 是否全部 20-30 岁
        boolean allYoung = personList.stream().allMatch(work -> (20 <= work.getAge() && work.getAge() <= 30));

        // 是否存在 age = 22的
        boolean anyTwentyFive = personList.stream().anyMatch(work -> 22 == work.getAge());

        // 没有 age = 25的
        boolean noneTwentyFive = personList.stream().noneMatch(work -> 25 == work.getAge());

        // 查找第一个满足条件的
        Optional firstTwentyTwo = personList.stream().filter(work -> 22 <= work.getAge()).findFirst();

        // 查找任何一个满足条件的，注意不一定是第一个
        Optional anyTwentyTwo = personList.stream().filter(work -> 22 <= work.getAge()).findAny();
    }

    @Test
    public void should_can_filter_it_work() {
        // 年龄集合
        List ageList = personList.stream().map(Work::getAge).distinct().collect(Collectors.toList());

        //  最大年龄
        Optional maxAgeWork = personList.stream().collect(Collectors.maxBy((s1, s2) -> s1.getAge() - s2.getAge()));
        // 或者
        Optional maxAgeWork2 = personList.stream().collect(Collectors.maxBy(Comparator.comparing(Work::getAge)));

        // 最小年龄
        Optional minAgeWork = personList.stream().collect(Collectors.minBy(Comparator.comparing(Work::getAge)));

        // 部门年龄求和
        Integer sumAgeForIT = personList.stream().filter(work -> "IT".equalsIgnoreCase(work.getDepartment())).collect(Collectors.summingInt(Work::getAge));

        // 平均年龄
        Double avgAge = personList.stream().collect(Collectors.averagingInt(Work::getAge));

        // 名字拼接
        String nameList = personList.stream().map(Work::getName).collect(Collectors.joining(","));

        // 按部门group by
        Map> deparmentWorkMap = personList.stream().collect(Collectors.groupingBy(Work::getDepartment));

        // 按部门group by并计数
        Map deparmentWorkCountMap = personList.stream().collect(Collectors.groupingBy(Work::getDepartment, Collectors.counting()));

        // 按部门、年龄两级group by
        Map>> deparmentAgeMap = personList.stream().collect(Collectors.groupingBy(Work::getDepartment, Collectors.groupingBy(Work::getAge)));

        // 一分为二：IT部分和非IT部门
        Map> partition = personList.stream().collect(Collectors.partitioningBy(work -> "IT".equals(work.getDepartment())));

    }

    @Test
    public void should_can_statistic_int() {
        List primes = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
        IntSummaryStatistics stats = primes.stream().mapToInt(x -> x).summaryStatistics();

        System.out.println(stats);
    }


    // 还有一些复杂
    @Test
    public void should_can_sort_by_asc() {
        Map maxAgeWork = personList.stream().collect(Collectors.toMap(Work::getName, Function.identity(), BinaryOperator.maxBy(Comparator.comparing(Work::getAge))));

        maxAgeWork.forEach((name, work) -> {
            System.out.println("name is [" + name + "], age is [" + work.getAge() + "]");
        });
    }

    // TODO summaryStatistics 方法可以帮我们获取:最大值，最小值，和，平均值信息
    // 返回值为(Int/Long/Double)的 SummaryStatistics

    // 获取每个部门的统计信息
    @Test
    public void should_can_get_group_by_age() {
        Map result = personList.stream().collect(Collectors.groupingBy(Work::getDepartment, Collectors.summarizingInt(Work::getAge)));

        System.out.println("Department | Statistics");
        for (Map.Entry entry : result.entrySet()) {
            System.out.printf("%10s : %70s\n", entry.getKey(), entry.getValue().toString());
        }
    }
}

返回值Int/Long/Double对应的统计方法分别是IntSummaryStatistics、LongSummaryStatistics、DoubleSummaryStatistics。

值得说的是：summaryStatistics 方法帮我们获取:最大值，最小值，和，平均值信息。即简单的数据类型的统计，就像这样：

Department | Statistics
Purchasing : IntSummaryStatistics{count=1, sum=23, min=23, average=23.000000, max=23}
        HR : IntSummaryStatistics{count=1, sum=28, min=28, average=28.000000, max=28}
        IT : IntSummaryStatistics{count=3, sum=64, min=20, average=21.333333, max=22}

apache的SummaryStatistics

如果想要一些复杂的统计，可以考虑apache的commons-math3的SummaryStatistics。

compile "org.apache.commons:commons-math3:3.5"

SummaryStatistics 可计算数组中最小值，最大值，平均值，几何平均，和，平方和，标准差，方差

DescriptiveStatistics 计算数组中最小值，最大值，平均值，几何平均，和，平方和，标准差，方差，百分位数，偏态，峰度，中位数。

Frequency 统计列表中的某些数据出现的频次

    @Test
    public void should_calculate_summary() {

        SummaryStatistics statistics = new SummaryStatistics();
        personList.stream().forEach(p -> statistics.addValue(p.getAge()));

        System.out.println(statistics);
        System.out.printf("%30s: %30s\n", "统计项", "数据");
        System.out.printf("%30s: %30s\n", "数据个数", statistics.getN());
        System.out.printf("%30s: %30s\n", "最小值", statistics.getMin());
        System.out.printf("%30s: %30s\n", "最大值", statistics.getMax());
        System.out.printf("%30s: %30s\n", "平均值", statistics.getMean());
        System.out.printf("%30s: %30s\n", "几何平均值", statistics.getGeometricMean());
        System.out.printf("%30s: %30s\n", "方差", statistics.getVariance());
        System.out.printf("%30s: %30s\n", "总体方差", statistics.getPopulationVariance());
        System.out.printf("%30s: %30s\n", "二阶矩", statistics.getSecondMoment());
        System.out.printf("%30s: %30s\n", "二阶矩", statistics.getSecondMoment());
        System.out.printf("%30s: %30s\n", "平方和", statistics.getSumsq());
        System.out.printf("%30s: %30s\n", "标准差", statistics.getStandardDeviation());
        System.out.printf("%30s: %30s\n", "对数和", statistics.getSumOfLogs());

        Frequency frequency = new Frequency();
        personList.stream().forEach(p -> frequency.addValue(p.getAge()));
        System.out.println(frequency);

        DescriptiveStatistics descriptiveStatistics = new DescriptiveStatistics();
        personList.stream().forEach(p -> descriptiveStatistics.addValue(p.getAge()));
        System.out.println(descriptiveStatistics);

        // SimpleRegression 线性模型最小二乘回归
        // OLSMultipleLinearRegression和GLSMultipleLinearRegression：提供最小二乘回归拟合线性模型
        // NaturalRanking：排名转换
        // Covariance：协方差相关
    }

还有
SimpleRegression：线性模型最小二乘回归
OLSMultipleLinearRegression、GLSMultipleLinearRegression：提供最小二乘回归拟合线性模型。
NaturalRanking：排名转换。
Covariance：协方差相关
有兴趣的可以自己尝试下