Intersection of Two Lists Objects in java 8

Question

An intersection of Two Lists Objects in java 8. Can some tell me what am I doing wrong?

List<Student> originalStudent = new ArrayList<>();
List<Student> newStudent = new ArrayList<>();

List<Student> intersectListStudent = new LinkedList<>()

originalStudent.add(new Student("William", "Tyndale",1));
originalStudent.add(new Student("Jonathan", "Edwards",2));
originalStudent.add(new Student("Martin", "Luther"),3);

newStudent.add(new Student("Jonathan", "Edwards",2));
newStudent.add(new Student("James", "Tyndale",4));
newStudent.add(new Student("Roger", "Moore",5));


originalStudent.forEach(n ->
        newStudent.stream()
                .filter(db -> !n.getName().equals(db.getName()) &&
                        !n.getLastName().equals(db.getLastName()))
                    .forEach(student-> intersectListStudent .add(student)));

Answer 1

Can some tell me what am I doing wrong?

You violate the Side-effects principle of java-stream which in a nutshell says that a stream shouldn't modify another collection while performing the actions through the pipelines. I haven't tested your code, however, this is not a way you should treat streams.

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How to do it better?

Simply use the List::contains in the filter's predicate to get rid of the unique values.

List<Student> students = originalStudent.stream()
                                        .filter(newStudent::contains)
                                        .collect(Collectors.toList());

This solution (understand the method List::contains) is based on the implemented equality comparison using Object::equals. Hence, there is needed to override the very same method in the class Student.

Edit: Please, be aware that that automatically overriding the Object::equals will mind the id to the equality computation. Therefore the equality will be based on the name and surname only. (thanks to @nullpointer).

Without the Object::equals overridden?

You have to perform the comparison in the filter using another stream and the method Stream::anyMatch which returns true if the predicate is qualified.

List<Student> students = originalStudent.stream()
              .filter(os -> newStudent.stream()                    // filter
                  .anyMatch(ns ->                                  // compare both
                       os.getName().equals(ns.getName() &&         // name
                       os.getLastName().equals(ns.getLastName()))) // last name
              .collect(Collectors.toList());

Answer 2

What you can do is construct a SortedSet<Student> from the two concatenated lists originalStudent and newStudent. The sorted set uses a Comparator.comparing(Student::getName).thenComparing(Student::getLastName) as its comparator.

Stream.concat(originalStudent.stream(), newStudent.stream())
    .collect(Collectors.toCollection(() -> new TreeSet<>(
        Comparator.comparing(Student::getFname)
            .thenComparing(Student::getLname))
    ))