Java different objects in a single method

Question

I Know there are a bunch of questions related with this topic. Most of the answers states "use a interface" or "create a generic". Tried both and didnt work =( dont know what I'm doing wrong.

Here is the problem: I have two classes: Subjects and Courses. In the main section, I want a function that will receive an id (provided by the user) and look into, for example, a subjectList, trying to find if the subject is there, if yes , return its index. The logic is the same for both Courses and Subjects, so I'm trying to let this function become a bit more generic.

 public static void main(String[] args) {
        
             //Objects created along the code are stored here
             ArrayList<Course> courseList = new ArrayList<>();   
             ArrayList<Subject> subjectList = new ArrayList<>();
        
            public static Integer findObjectById(int id, ArrayList<IStudyDetails> object_list) {
                for (int i = 0; i < object_list.size(); i++) {
                    if (id == object_list.get(i).getId()) {
                        return i;
                    }
                }
                return null;
            }
    int index_subject = findObjectById(subject_id,subjectList);
    }

Here is the Interface: I tried to create this after look into some Stack Overflow related topics.

public interface IStudyDetails{
    int getId();

}

Here is the Course class: (I did hide most of the constructors/gets and setters)

public class Course implements IStudyDetails {
    private int id;
    private String name;
    public int getId() {return id;}}

Here is the Subject class (I did hide most of the constructors/gets and setters)

public class Subject implements IStudyDetails {
    private int id;
    private String name;
    public int getId() {return id;}}

THe error I'm receiving is:

java: incompatible types: java.util.ArrayList<entities.Subject> cannot be converted to java.util.ArrayList<entities.IStudyDetails>

@shmosel can you make this an answer and explain ? Also ,is changing `subjectList` and `courseList` type to `ArrayList` wrong ? — Yasser CHENIK, Jan 27 '22 at 01:42
@YasserCHENIK See also [What is PECS (Producer Extends Consumer Super)?](https://stackoverflow.com/a/2723538/2970947) — Elliott Frisch, Jan 27 '22 at 01:52
@shmosel wow, it worked! I agree with YasserChenik, could you explain a bit better in this specific context? — digolira2, Jan 27 '22 at 02:12

score 1 · Answer 1 · edited Jan 27 '22 at 02:22

public static void main(String[] args) {
    //Objects created along the code are stored here
    List<IStudyDetails> courseList = new ArrayList<>();
    List<IStudyDetails> subjectList = new ArrayList<>();
    Subject subject = new Subject(0001, "John");
    Subject subject1 = new Subject(1001, "Tom");
    subjectList.add(subject);
    subjectList.add(subject1);
    int subject_id = 1001;
    int index_subject = findObjectById(subject_id,subjectList);
}
public static Integer findObjectById(int id, List<IStudyDetails> object_list) {
    for (int i = 0; i < object_list.size(); i++) {
        if (id == object_list.get(i).getId()) {
            System.out.println(i);
            return i;
        }
    }
    return null;
}

I Just change the List Type, Cause both Course and Subject are implemented from IStudyDetails, so you need to use IStudyDetails as a type to create the list and it can work.

Java different objects in a single method

1 Answers1