Find (custom) objects in list from another list

So first of all in the description You say that

A train has the attributes Id, Color, Wagon.

But, later in the example:

finalList = trains.stream() .filter(tr -> wagons.stream() .anyMatch(wg ->tr.getWagons() .foreach(trWg -> trWg.getId().equals(wg.getId()))));

Therefore I assume that the first one was true, and for some reason your train can have only one wagon for now

There are several solutions to this "problem":

First and the cleanest method in my opinion:

List<Wagon> wagons = new ArrayList<>();

    wagons.add(new Wagon(0L, new ArrayList<>(), "0"));
    wagons.add(new Wagon(1L, new ArrayList<>(), "1"));

    List<Train> trains = new ArrayList<>();

    trains.add(new Train(0L, wagons.get(0), "Yellow"));
    trains.add(new Train(1L, wagons.get(1), "Blue"));
    trains.add(new Train(2L, new Wagon(12L, new ArrayList<>(), "not in wagon list"), "Blue"));


    List<Train> finalList = trains.stream().
            filter(train -> wagons.contains(train.getWagon()))
            .collect(Collectors.toList());

    finalList.forEach(System.out::println);

And ofc POJO'S

@Getter
@AllArgsConstructor
@ToString
class Train {
    private Long id;
    private Wagon wagon;
    private String color;
}

@Getter
@ToString
@AllArgsConstructor
@EqualsAndHashCode
class Wagon {
    private Long id;
    private List<String> seats;
    private String type;
}

But you have to remember here to implement equals() for Wagon

Second method without the need of implementing the equals() is:

  List<Train> finalList2 = trains.stream()
            .filter(train -> wagons
                    .stream()
                    .anyMatch(wagon -> wagon.getId().equals(train.getWagon().getId())))
            .collect(Collectors.toList());

Now assume that your train can have more than one wagon...

@Getter
@AllArgsConstructor
@ToString
class Train {
    private Long id;
    private List<Wagon> wagons;
    private String color;
}

And the code:

List<Wagon> wagons = new ArrayList<>();

wagons.add(new Wagon(0L, new ArrayList<>(), "0"));
wagons.add(new Wagon(1L, new ArrayList<>(), "1"));
List<Wagon> wagons2 = new ArrayList<>();

wagons2.add(new Wagon(11L, new ArrayList<>(), "11"));
wagons2.add(new Wagon(12L, new ArrayList<>(), "12"));

List<Train> trains = new ArrayList<>();

trains.add(new Train(0L, wagons,"Yellow"));
trains.add(new Train(1L, wagons2, "Blue"));
trains.add(new Train(2L, Arrays.asList(new Wagon(132L, new ArrayList<>(),"not in wagon list")), "Blue"));

   List<Train> finalList3 = trains.stream()
            .filter(train -> train.getWagons().containsAll(wagons))
            .collect(Collectors.toList());

    finalList3.forEach(System.out::println);

As long as you assume, that you are interested only in trains containing exact set of wagons. And equals() is necessary for this method too!

I have no idea what your POJO's are since you didn't post them so all I can do is assume from your list of attributes that you've posted. This is one way to do it.

All we're doing is we're looping through all trains and filtering by whether or not the List of Wagon objects contains a wagon with the same id as the wagon from the train. We're then collecting to a List.

    List<Train> trains = Arrays.asList(
            new Train(1, 0, new Wagon(0, 1, 0)), 
            new Train(2, 0, new Wagon(1, 1, 0)));

    List<Wagon> wagons = Arrays.asList(new Wagon(0, 1, 0));

    List<Train> trainsFromWagons = trains.stream().filter(train -> wagons.stream().anyMatch(wagon -> train.wagon.id == wagon.id))
            .collect(Collectors.toList());