如何在 Java 8 中使用流将几个字段收集到一个列表中？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/42028143/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How to use stream in Java 8 to collect a couple of fields into one list?
提问by user2620644
For example I have class Personwith nameand surnamefields.
例如，我有一个Person带有name和surname字段的类。
I want to collect a Listof String(names and surnames all together) from Listof Person, but it seems that I can't use map twice per one list or can't use stream twice per list.
我想从of收集Listof String（姓名和姓氏全部），但似乎我不能为每个列表使用两次 map 或不能为每个列表使用两次流。ListPerson
My code is:
我的代码是：
persons.stream()
.map(Person::getName)
.collect(Collectors.toSet())
.stream().map(Person::getSurname)
.collect(Collectors.toList())
but it keeps telling me that Person::getSurnamenon-static method can't be referenced from static context.
但它一直告诉我Person::getSurname不能从静态上下文中引用非静态方法。
What am I doing wrong?
我究竟做错了什么？
回答by Landei
To get both names and surnames in the same list, you could do this:
要在同一个列表中同时获取姓名和姓氏，您可以执行以下操作：
List<String> set = persons.stream()
.flatMap(p -> Stream.of(p.getName(),p.getSurname()))
.collect(Collectors.toList());
回答by Matthieu Saleta
When you're doing :
当你在做：
persons.stream().map(Person::getName).collect(Collectors.toSet())
The result is a Set<String>that contains only the nameof the persons.
Then you're recreating a stream from this Setand not from your List<Person> persons.
结果是Set<String>只包含name的persons。然后你从这个Set而不是从你的List<Person> persons.
That's why you can not use Person::getSurnameto map this Set.
这就是为什么你不能Person::getSurname用来映射 this Set。
The solution from @Alexis C. :
persons.stream().flatMap(p -> Stream.of(p.getName(), p.getSurname()).collect(Collectors.toSet())must do the job.
@Alexis C. 的解决方案：
persons.stream().flatMap(p -> Stream.of(p.getName(), p.getSurname()).collect(Collectors.toSet())必须完成这项工作。
回答by matejetz
Your code should look something like that:
你的代码应该是这样的：
persons.stream()
.map(person -> person.getName() + " " + person.getSurname)
.collect(Collectors.toList());
回答by Jayen Chondigara
if person has first name and middle name optional then use below code
如果人的名字和中间名是可选的，则使用下面的代码
return Stream.of(Optional.ofNullable(person)
.map(Person::getFirstName)
.orElse(null),
Optional.ofNullable(person)
.map(Person::getMiddleName)
.orElse(null))
.filter(Objects::nonNull)
.collect(Collectors.joining(SPACE));
