Two Different types of Arrays into the first Array

I think the approach you should be taking is something akin to this :

class Student{ 
   private String name, ID;
   private int gpa, age;
   //More personalia stuff...

   private ArrayList<Subject> subjects;

   public void addSubject (Subject new_subject) { subjects.add(new_subject); }
   public void addSubjects (List <Subject> subjects_list) {
       for (Subject s : subjects_list) addSubject(s);
   }

   //Getters 
   public int getGPA() { return gpa; }
   public int getAge() { return age; }
   public String getName() { return name; }

   //.. More personalia getters

   //Setters 
   public void setGPA(int new_gpa) { gpa = new_gpa; }



 }

class Subject { 
    String description;
    String ID;

}

Now if you have a list of students say :

 ArrayList <Student> all_students;

Here you can go through the list and add subjects to any or all of them.

Edit : added some getters and setters that makes it clearer how you access object info.

Edit 2 to address your updated post : There are a number of issues with your code. Let's go through them from the most high-level stuff to low (not the same as importance mind you)

1) Class names A class should not be called "Main". It is unclear what the class is. Instead, call it what it is : a student-subject-registry system. class StudentRegister should be just fine.

2) Class contents You included the class Subject as an inner class in Student. This does not make sense, as a subject is not a part of a student (in literal terms in real life). This is not the worst part of it though; having Subject as an inner class will make you create loads of identical subjects for each and every student. This is completely redundant.

Now you also included a list of all students in every Student object. Is this intentional? If so, it seems a bit misguided. Student A does not need to, and probably will never know about all other students. Not to mention, everytime you add a new student N, you'll need to add N to all student objects that currently exist. That is redundant as well.

A proposed solution

Let's think about what you know as a student. You know :

• your age (private int age)
• your GPA (private int GPA)
• your ID (private String ID)
• your name (private String name)
• the list of subjects you take (private List subjects)

Note that you as a student have no control over what the course description and ID is

All those private variables are private because those are things that you as a student disclose on request of someone. It is akin to someone asking you "What's your name?" or "What's your age?" etc. This is what you accomplish by implementing getters like getName() , getAge().

Now let's consider the part in italic. When someone asks you what courses you are taking, you simply refer to the list of courses that are common for you and other students that are enrolled at the universities. You do not create your own courses that nobody else knows about. Therefore, a Subject should not be an inner class in your Student class. It is instead a referance (i.e a pointer) to a course object that is offered by the university (continue reading for further explanation)

Now consider a course object. That course object does not need to know about :

• the list of students that are enrolled in a given year
• any other subjects

Any given course object needs to know :

• it's course description
• course ID
• a string representation (i.e what you've implemented as toString() )

Finally, in your StudentRegistry class, you should have two lists now : - a list of students enrolled at the university : List <Student> all_students; - a list of all subjects/courses offered : `List all_subjects;

Remember that each students have methods for adding subjects? You simply refer to the course objects that exist in the studentRegistry and ask each student to add those to their subject list. No new subject objects will be created. Any and all subjects in every student's study plan will refer to objects in the all_subjects list contained in the StudentRegistry class