Java 8 Stream: How to get a new list from one list property in a list - java-8

Question

I'm new to java 8 and meet a problem trouble me a lot:
I've a List like below:
List<objMain>
class objMain{
Long rid;
List<objUser> list;
String rname;
}
class objUser{
String userid;
}
now,I want get a new List like below:
List<objUserMain>
class objUserMain{
Long rid;
String rname;
String userid;
}
How can I do this by java 8 stream? Thanks anyone answer me.

Answer 1

It seems as though you want to map each objMain instance into an objUserMain type.
In order to accomplish the task at hand, you'll need to utilise flatMap along with map then collect to a list implementation.
Assuming you have getters and setter where necessary then you can perform the following logic to get the required result.
List<objUserMain> result =
objMainsList.stream()
.flatMap(obj -> obj.getList().stream().map(e -> {
objUserMain user = new objUserMain();
user.setRid(obj.getRid());
user.setRname(obj.getRname());
user.setUserid(e.getUserid());
return user;
})).collect(Collectors.toList());

Answer 2

You can do this using streams the following way
public static List convert(List existing, Function func) {
return existing.stream().map(func).collect(Collectors.toList());
}
The above method will help you to convert your list from one object type to another. The parameters to this is the initial object you want to convert and the method you want to use for conversion. Import the following in you main class
import java.util.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;
import java.util.function.*;
Now call the method for conversion the following way in the main class that you are trying to convert
List result=convert(newList,
l->{
objUserMain r=new objUserMain();
r.rid=l.rid;
r.rname=l.rname;
r.userid=l.list.get(0).userid;
return r;});
System.out.println(result.get(0).rid);
System.out.println(result.get(0).rname);
System.out.println(result.get(0).userid);
The above is a mixture of lambda functions and streams to allow you to convert object from one type to another. Let me know if you have any queries then I am happy to help. Happy coding.