一、前言

对于聚合统计可以参考下面博客。
Java8 Stream（12）Collectors 分组统计 详解
开发中，List转Map是非常常见的操作，例如：

• List<T> 转 Map<String, T>
• List<T> 转 Map<String, List<T>>
• List<T> 转 Map<String, Count>
• List<Map<String, Object>> 转 Map<String, List<Map<String, Object>>>

先造点数据：

pigs.add(new Pig(1, "猪爸爸", 31, "M", false));
pigs.add(new Pig(2, "猪妈妈", 28, "F", true));
pigs.add(new Pig(3, "乔治", 2, "M", false));
pigs.add(new Pig(4, "佩奇", 5, "F", false));

二、List转Map

1. List<T> 转 Map<String, List<T>>

使用groupingBy根据gender属性分组

Map<String, List<Pig>> groupByGender = pigs.stream()
        .collect(
                Collectors.groupingBy(Pig::getGender)
        );

groupByGender.forEach((k, v) -> {
    System.out.println(k + " -> " + v);
});

结果如下：

F -> [Pig(id=2, name=猪妈妈, age=28, gender=F, valid=true), Pig(id=4, name=佩奇, age=5, gender=F, valid=false)]
M -> [Pig(id=1, name=猪爸爸, age=31, gender=M, valid=false), Pig(id=3, name=乔治, age=2, gender=M, valid=false)]

2. List<T> 转 Map<ID, T>

使用Collectors.toMap转换，如果id相同，取第1个

Map<Integer, Pig> mapById = pigs.stream()
        .collect(
                Collectors.toMap(
                        Pig::getId,
                        a -> a,
                        (k1, k2) -> k1)//如果id相同，取第1个
        );
mapById.forEach((k, v) -> {
    System.out.println(k + " -> " + v.toString());
});

结果如下：

1 -> Pig(id=1, name=猪爸爸, age=31, gender=M, valid=false)
2 -> Pig(id=2, name=猪妈妈, age=28, gender=F, valid=true)
3 -> Pig(id=3, name=乔治, age=2, gender=M, valid=false)
4 -> Pig(id=4, name=佩奇, age=5, gender=F, valid=false)

3. List<T> 转 Map<String, Count>

Map<String, Long> groupByGenderThenCount = pigs.stream()
        .collect(
                Collectors.groupingBy(
                        Pig::getGender,
                        Collectors.counting()
                )
        );

groupByGenderThenCount.forEach((k, v) -> {
    System.out.println(k + " -> " + v);
});

结果如下：

F -> 2
M -> 2

4. List<T> 转 Map<ID, String>

Map<Integer, String> genderNameMap = pigs.stream()
        .collect(
                Collectors.toMap(
                        Pig::getId,
                        pig -> pig.getName() == null ? "" : pig.getName(),
                        (oldValue, newValue) -> newValue)
        );

genderNameMap.forEach((k, v) -> {
    System.out.println(k + " -> " + v.toString());
});

结果如下：

1 -> 猪爸爸
2 -> 猪妈妈
3 -> 乔治
4 -> 佩奇

5. 使用partitioningBy分区

Map<Boolean, List<Pig>> partitioningByValid =
        pigs.stream()
                .collect(partitioningBy(Pig::isValid));

partitioningByValid.forEach((k, v) -> {
    System.out.println(k + " -> " + v.toString());
});

结果如下：

false -> [Pig(id=1, name=猪爸爸, age=31, gender=M, valid=false), Pig(id=3, name=乔治, age=2, gender=M, valid=false), Pig(id=4, name=佩奇, age=5, gender=F, valid=false)]
true -> [Pig(id=2, name=猪妈妈, age=28, gender=F, valid=true)]

6. List<Map>排序

System.out.println("List<Map> 根据age降序, 再根据sex降序，sex默认M");
List<Map> sorted = userMaps.stream().sorted((map1, map2) -> {
    int age1 = (int) map1.getOrDefault("age", 0);
    int age2 = (int) map2.getOrDefault("age", 0);
    if (age1 == age2) {
        String sex1 = (String) map1.getOrDefault("sex", "M");
        String sex2 = (String) map2.getOrDefault("sex", "M");
        return sex2.compareTo(sex1);
    } else {
        return age2 - age1;
    }
}).collect(Collectors.toList());//记住一定要加上这句

sorted.forEach(System.out::println);

System.out.println("直接在List上排序");
userMaps.sort((map1, map2) -> {
    int age1 = (int) map1.getOrDefault("age", 0);
    int age2 = (int) map2.getOrDefault("age", 0);
    if (age1 == age2) {
        String sex1 = (String) map1.getOrDefault("sex", "M");
        String sex2 = (String) map2.getOrDefault("sex", "M");
        return sex2.compareTo(sex1);
    } else {
        return age2 - age1;
    }
});

7. List多字段排序，NULL值排序

System.out.println("按照ID升序");
users.sort(comparing(User::getId, Comparator.nullsFirst(String::compareTo)));
users.forEach(System.out::println);

System.out.println("按照ID降序");
users.sort(comparing(User::getId, Comparator.nullsFirst(String::compareTo)).reversed());
users.forEach(System.out::println);

System.out.println("按照age升序，NULL在前面");
users.sort(comparing(User::getAge, Comparator.nullsFirst(Integer::compareTo)));
users.forEach(System.out::println);

System.out.println("按照age升序，NULL在后面");
users.sort(comparing(User::getAge, Comparator.nullsLast(Integer::compareTo)));
users.forEach(System.out::println);

System.out.println("按照age降序，NULL在前面");
users.sort(comparing(User::getAge, Comparator.nullsLast(Integer::compareTo)).reversed());
users.forEach(System.out::println);

System.out.println("按照age降序，NULL在后面");
users.sort(comparing(User::getAge, Comparator.nullsFirst(Integer::compareTo)).reversed());
users.forEach(System.out::println);

System.out.println("按照age升序，再按照sex升序，NULL在前面");
users.sort(
        comparing(User::getAge, Comparator.nullsFirst(Integer::compareTo))
                .thenComparing(User::getSex, Comparator.nullsFirst(String::compareTo))

);
users.forEach(System.out::println);

System.out.println("按照age升序，再按照sex降序，NULL在前面");
users.sort(
        comparing(User::getAge, Comparator.nullsFirst(Integer::compareTo))
                .thenComparing(comparing(User::getSex, Comparator.nullsLast(String::compareTo)).reversed() )

);
users.forEach(System.out::println);

System.out.println("按照age降序，再按照sex升序，NULL在前面");
users.sort(
        comparing(User::getAge, Comparator.nullsLast(Integer::compareTo)).reversed()
                .thenComparing(User::getSex, Comparator.nullsFirst(String::compareTo))

);
users.forEach(System.out::println);

System.out.println("按照age降序，再按照sex降序，NULL在前面");
users.sort(
        comparing(User::getAge, Comparator.nullsLast(Integer::compareTo))
                .thenComparing(User::getSex, Comparator.nullsLast(String::compareTo))
                .reversed()

);
users.forEach(System.out::println);