Java how to stream a list inside a list for an object?

So I have lists stored inside of another list.
And inside of that lists there a different objects.
I want to get a specific object based on an attribute.

Human robert = building.getRoomList()
    .stream()
    .filter(room -> room.getEmployeeList()
         .stream()
         .filter(human -> human.getName().equals("Robert")))
    .findFirst().get();

Why doesn't it work?

Try this:

Human robert = building.getRoomList().stream()
    .map(Room::getRoomList)
    .flatMap(Collection::stream)
    .filter(human -> human.getName().equals("Robert"))
    .findFirst()
    .orElseThrow(() -> new RuntimeException("Robert not found!"));

You can do it using Stream.flatMap:

Optional<Human> robert = building.getRoomList().stream()
    .flatMap(room -> room.getEmployeeList().stream())
    .filter(human -> human.getName().equals("Robert"))
    .findFirst();

robert.ifPresent(r -> /* do something with Robert */);

Stream.flatMap maps each room to a stream of humans and the returned stream consists of all the humans of all the rooms, flattened (i.e. not as sublists or substreams).

> Why doesn't it work?

Stream::filter requires a Predicate<T> and returns the very same Stream<T>. As long as you nest filter in filter, this would never work because the outer one requires Predicate<T> while the inner one returns Stream<T> which is clearly not compatible.

You might want to use flatMap:

Human robert = building.getRoomList()
    .stream()                                     // Stream<Room>
    .flatMap(room -> room                         // Stream<Human> flatmapped from:
        .getEmployeeList()                        // .. for each Room get List<Human>
        .stream()                                 // .. for each Room have Stream<Human>
        .filter(human -> human.getName()          // .. for each Room keep the one named
                              .equals("Robert"))) // .. Robert
    .findFirst()                                  // find the first Robert, the Human 
    .orElse(null);                                // or result in null

Remember the predicate human -> human.getName().equals("Robert")) is not null-safe neither when human is null nor when its name is null. For the null-safety, flat-map like this:

    .flatMap(room -> room.getEmployeeList()
                         .stream()
                         .filter(Objects::nonNull)
                         .filter(human -> "Robert".equals(human.getName())))

You are doing .findFirst() on RoomList only but you need to do .findFirst() on inner list means EmployeeList of every Room also and transform Room into Human using .map(), like

building.getRoomList()
.stream()
.map(room -> room.getEmployeeList()
                 .stream()
                 .filter(human -> human.getName().equals("Robert"))
                 .findFirst().get())
.findFirst().get();

Which can be simplified using flatMap to flatten then EmployeeList then get findFirst

building.getRoomList()
        .flatMap(e -> room.getEmployeeList().stream())
        .filter(human -> human.getName().equals("Robert"))
        .findFirst().get();

Note: It's better to use .orElse() rather calling .get()