通过stream将 List<A>转化为List<B> 。 其中B类中存在某个方法
public B fromA(A a);
具体代码如下
package cn.zwy; import java.util.ArrayList; import java.util.List; import java.util.stream.Collectors; public class StreamTest { public static void main(String[] args) { List<String> names = new ArrayList<>(); names.add("zhou"); names.add("wen"); List<User> users = names.stream().map(new User()::setName).collect(Collectors.toList()); System.out.println(users); } static class User { private String name; public String getName() { return name; } public User setName(String name) { this.name = name; return this; } @Override public String toString() { return "User{" + "name=‘" + name + ‘\‘‘ + ‘}‘; } } }
此处代码有误:
List<User> users = names.stream().map(new User()::setName).collect(Collectors.toList());
输出结果如下:
[User{name=‘wen‘}, User{name=‘wen‘}]
官网描述：
Method References
You use lambda expressions to create anonymous methods. Sometimes, however, a lambda expression does nothing but call an existing method. In those cases, it’s often clearer to refer to the existing method by name. Method references enable you to do this; they are compact, easy-to-read lambda expressions for methods that already have a name.
上步的代码等效于
User user = new User(); List<User> users = names.stream().map(user::setName).collect(Collectors.toList());
map内并不是每个数据进来就执行new User().setName方法。该方式只是注册一个对象，并一直调用该方法
正确处理方式应该为:
List<User> users = names.stream().map(name->new User().setName(name)).collect(Collectors.toList());
或者
static class User { private String name; public User() { } public User(String name) { this.name = name; } public String getName() { return name; } public User setName(String name) { this.name = name; return this; } @Override public String toString() { return "User{" + "name=‘" + name + ‘\‘‘ + ‘}‘; } }
List<User> users = names.stream().map(User::new).collect(Collectors.toList());
map这个有点诡异，对于lambda的说明有点模棱两可
以上
原文：https://www.cnblogs.com/zhouwenyang/p/14926194.html