java.util.stream.ReferencePipeline$3@396555a1

is the outcome of trying to print the Stream you have created until the map operation performed.

You are looking to collect the user names mapped until now while joining then with a \n delimiter.

String users = memberNames.stream()
        .map(user -> String.format("The user name is %s and his lastname is %s", user.name, user.lastname))
        .collect(Collectors.joining("\n"));
System.out.println(users); // to print them all at once

If I correctly understand your task, you need this stream:

    projects.stream()
            .map(Project::getGroups)
            .flatMap(Collection::stream)
            .map(Group::getStudents)
            .flatMap(Collection::stream)
            .map(Student::getName)
            .collect(toList());

With this source List:

    List<Project> projects = asList(
            new Project(0L, "Title", 5, 2, asList(
                    new Group(0L, "", asList(
                            new Student(1, "Smith"),
                            new Student(2, "Jackson"),
                            new Student(3, "Norris")
                    ))
            )
            )
    );

I got the output:

[Smith, Jackson, Norris]

You can grab the stream of the map's entry set, then map each entry to the string representation you want, joining them in a single string using Collectors.joining(CharSequence delimiter).

import static java.util.stream.Collectors.joining;

String s = attributes.entrySet()
                     .stream()
                     .map(e -> e.getKey()+"="+e.getValue())
                     .collect(joining("&"));

But since the entry's toString() already output its content in the format key=value, you can call its toString method directly:

String s = attributes.entrySet()
                     .stream()
                     .map(Object::toString)
                     .collect(joining("&"));