Everything that's wrong with Java in a single class(plus.google.com)

  public class Student implements Comparable<Student> {
    @NotNull public List<Test> getTests() { ... }
    @NotNull public String getLastName() { ... }
    @Override public int compareTo(Student that) {
      return getLastName().compareTo(that.getLastName());
    }
  }

  List<Student> studentsWhoHaveNotTakenAtLeastOneTest() {
    TreeSet<Student> studentsMissingAtLeastOneTest = new TreeSet<Student>();

    for(Student student : getStudents()) {
      boolean allTestsWereTaken = true;
      for(Test test : student.getTests()) {
        allTestsWereTaken &= test.isTaken();
      }

      if (!allTestsWereTaken) {
        studentsMissingAtLeastOneTest.add(student);
      }
    }

    return new ArrayList<Student>(studentsMissingAtLeastOneTest);
  }