我设法写了Collector.groupingBy(Function, groupingBy(classifier, toList())
Map<String, Map<Person.Sex, List<Person>>> groupByYearThenSex = personList.stream().collect(
groupingBy(p ->{
if(p.getYear() > 2015) return "New members";
else if(p.getYear() < 2009) return "Senior members";
else return "Normal members";
},
groupingBy(Person::getSex)
)
);
groupByYearThenSex.forEach((String y, Map<Person.Sex, List<Person>> m) ->{
System.out.println("\n" + y);
m.forEach((Person.Sex s, List<Person> p) -> {
System.out.println(" " + s);
p.forEach((Person prsn) -> {
System.out.println(" " + prsn.toString());
});
});
});
对应的结果是:
New members
MALE
Person{name: Gawel, age: 23, sex: MALE}
Normal members
MALE
Person{name: Patryk, age: 34, sex: MALE}
Person{name: Pawel, age: 21, sex: MALE}
Person{name: Bolek, age: 12, sex: MALE}
Person{name: Lolek, age: 12, sex: MALE}
FEMALE
Person{name: Jola, age: 70, sex: FEMALE}
Person{name: Ala, age: 25, sex: FEMALE}
Senior members
FEMALE
Person{name: Iwona, age: 34, sex: FEMALE}
Person{name: Oliwia, age: 17, sex: FEMALE}
现在我正在尝试生成仅列出最旧成员的多级 map ,如下所示:
New members
MALE
Person{name: Gawel, age: 23, sex: MALE}
Normal members
MALE
Person{name: Patryk, age: 34, sex: MALE}
FEMALE
Person{name: Jola, age: 70, sex: FEMALE}
Senior members
FEMALE
Person{name: Iwona, age: 34, sex: FEMALE}
下面的代码不起作用。我得到“推断变量 RR 具有不兼容的边界错误,我认为 Function<R,RR>
=> collectingAndThen(reducing(), Optional)
。有什么方法可以减少人员流以获取最旧的人员并获得上面的输出?
Map<String, Map<Person.Sex, List<Person>>> groupByYearThenSexOldest = personList.stream().collect(
groupingBy(p ->{
if(p.getYear() > 2015) return "New members";
else if(p.getYear() < 2009) return "Senior members";
else return "Normal members";
},
groupingBy(Person::getSex, collectingAndThen(
reducing((p1, p2) -> p1.getAge() > p2.getAge() ? p1 : p2),
Optional::get)
)
)
);
最佳答案
问题出在 map 的类型参数上。它应该给你一个 Map<String, Map<Person.Sex, Person>>
而不是Map<String, Map<Person.Sex, List<Person>>>
因为您正在减少列表。
但是你可以结合第二个groupingBy
收集器 maxBy
和collectingAndThen
:
groupingBy(Person::getSex, collectingAndThen(maxBy(Comparator.comparingInt(Person::getAge)), Optional::get)
有时,如果您需要按多个字段进行分组,则可以使用对字段进行分组的列表(假设它们具有适当的 equals 和 hashcode 方法)。例如:
Function<Person, String> groupingYear = p -> {
if (p.getYear() > 2015) return "New members";
else if (p.getYear() < 2009) return "Senior members";
else return "Normal members";
};
Map<Object, Optional<Person>> map2 =
personList.stream()
.collect(groupingBy(p -> Arrays.asList(groupingYear.apply(p), p.getSex()), maxBy(Comparator.comparingInt(Person::getAge))));
给出:
[Normal members, MALE] => Optional[Person{name='Patryk', age=34, sex=MALE}]
[Senior members, FEMALE] => Optional[Person{name='Iwona', age=34, sex=FEMALE}]
[Normal members, FEMALE] => Optional[Person{name='Jola', age=70, sex=FEMALE}]
[New members, MALE] => Optional[Person{name='Gawel', age=23, sex=MALE}]
但是您可以从使用 toMap
开始收集器并比较当 2 个值分组在一起时您提供的合并函数中的年龄:
Map<Object, Person> map =
personList.stream()
.collect(toMap(p -> Arrays.asList(groupingYear.apply(p), p.getSex()),
p -> p,
(p1, p2) -> {
if(p1.getAge() > p2.getAge()) return p1;
else return p2;
},
HashMap::new));
您有一个Object
作为您 Map
中的关键(真正的类型很糟糕而且写起来很长),但是你可以做的是创建一个专用的类 MemberStatusAndSex
组合两个属性(而不是 List
- 不要忘记为此类实现 equals 和 hashcode),并获得 Map<MemberStatusAndSex, Person>
在最后。
关于java - 使用stream()collect()groupingBy()和reducing()的多级映射,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/39665281/