Java Stream - 在过滤器中抛出异常并返回一个列表

Question

I have created a Java stream:

List<Student> getStudentWithSameGrade(List<Student> students, Grade grade) {
    return students.stream().filter(s -> s.getGradeId().equals(grade.getGid())).collect(Collectors.toList());
}

The above code returns a List<Student> that satisfies the filter check but I want to throw an exception for a student/list of students that don't satisfy this check. I tried doing something like this,

return students.stream().filter(s -> s.getGradeId().equals(grade.getGid())).findAny()
                .orElseThrow(() -> new EntityNotFoundException(ENTITY_NOT_FOUND));

However, the above code snippet returns a single student object instead of a list. I am relatively new to the Stream concept so would really appreciate some help on this.

Answer 1

The easiest way would be simply to check:

List<Student> getStudentWithSameGrade(List<Student> students, Grade grade) {
    List<Student> students = students.stream().filter(s -> s.getGradeId().equals(grade.getGid())).collect(Collectors.toList());
    if (students.isEmpty()) throw new EntityNotFoundException(ENTITY_NOT_FOUND);
    else return students;
}

If you really need to stick to stream api, you could define your own collector:

public static <T> Collector<T, ?, T> toNonEmptyList() {
    return Collectors.collectingAndThen(
        Collectors.toList(),
        list -> {
            if (list.isEmpty()) {
                throw new IllegalStateException();
            }
            return list;
        }
    );
}

And then:

return students.stream()
    .filter(s -> s.getGradeId().equals(grade.getGid()))
    .collect(toNonEmptyList());