如何使用 Java 8 Stream 从某些类属性中获取列表？
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How can I get a List from some class properties with Java 8 Stream?
提问by NCNecros
I have a List<Person>. I need to get a Listfrom a property of Person.
我有一个List<Person>. 我需要List从Person.
For example, I have a Personclass:
例如，我有一个Person类：
class Person
{
private String name;
private String birthDate;
public String getName() {
return name;
}
public String getBirthDate() {
return birthDate;
}
Person(String name) {
this.name = name;
}
}
List<Person> personList = new ArrayList<>();
personList.add(new Person("David"));
personList.add(new Person("Joe"));
personList.add(new Person("Michel"));
personList.add(new Person("Barak"));
I want to get a list of names with the StreamAPI, like this:
我想通过StreamAPI获取名称列表，如下所示：
List<String> names = personList.stream().somecode().collect(Collectors.toList());
names.stream().forEach(System.out::println);
#David
#Joe
#Michel
#Barak
This code doesn't work:
此代码不起作用：
public class Main
{
public static void main(String[] args)
{
List<Person> personList = new ArrayList<>();
Person person = new Person("Иван");
person.getFriends().addAll(Arrays.asList("Друг 1", "Друг 2", "Друг 3"));
personList.add(person);
person = new Person("Федор");
person.getFriends().addAll(Arrays.asList("Друг 4", "Друг 5", "Друг 6"));
personList.add(person);
person = new Person("Алексей");
person.getFriends().addAll(Arrays.asList("Друг 7", "Друг 8", "Друг 9"));
personList.add(person);
person = new Person("Константин");
person.getFriends().addAll(Arrays.asList("Друг 10", "Друг 11", "Друг 12"));
List<String> friens = personList.stream().map(e->e.getFriends()).collect(Collectors.toList());
friends.stream().forEach(System.out::println);
//Друг 1
//Друг 2
//Друг 3
//Друг 4
//...
}
}
class Person
{
String name;
List<String> friends;
Person(String name) {
this.name = name;
}
public String getName() {
return name;
}
public List<String> getFriends() {
return friends;
}
}
How can I get a Listof a property with StreamAPI?
如何List使用StreamAPI获取属性？
采纳答案by Eran
You can use map:
您可以使用map：
List<String> names =
personList.stream()
.map(Person::getName)
.collect(Collectors.toList());
EDIT :
编辑 ：
In order to combine the Lists of friend names, you need to use flatMap:
为了组合好友姓名列表，您需要使用flatMap：
List<String> friendNames =
personList.stream()
.flatMap(e->e.getFriends().stream())
.collect(Collectors.toList());
