Java Casting Abstract Between Two Concrete Classes

Question

I am implementing a factory pattern to create two concrete classes in an abstract which has the factory methods overloaded (see below):

public abstract class User {
    ...
    public static User make(int id, String name) {
        return new Admin(id, name); 
    }

    public static User make(int id, int student_id, String name) {
        return new Student(id, student_id, name); 
    }
}

Here is the factory calls:

ArrayList<User> users = new ArrayList<>(
    Arrays.asList(
        User.make(1000, "Andy"),    // makes new Admin
        User.make(1001, 101001, "Bob")    // makes new Student
    )
);

Here is the Admin class:

public class Admin extends User {
    ...
    // constructor
    protected Admin(int id, String name) {
        super(id, name);
    }
    ...
}

Here is the Student class:

public class Student extends User {
    ...
    // constructor
    protected Student(int id, int student_id, String name) {
        super(id, name);
        this.student_id = student_id;
    }
    ...
}

Each of these concretes are placed into a User ArrayList. I have a function (below) which loops through the list and does runtime inference to call particular methods unique to each concrete; however I am getting a ClassCastException error in my IDE stating Admin cannot be cast to Student.

Full exception message is: Exception in thread "main" java.lang.ClassCastException: presentation_layer.Admin cannot be cast to presentation_layer.Student

public class App {
    ...
    public static void main(String[] args) {
        ArrayList<User> users = new ArrayList<>(
            Arrays.asList(
                User.make(1000, "Andy"),        // makes new Admin
                User.make(1001, 101001, "Bob")  // makes new Student
            )
        );

        users.forEach((u) -> {
            if (u instanceof Admin)) {
                System.out.println("hello admin");
                ((Admin)u).anAdminFunc();
            } else if (u instanceof Student)) {
                System.out.println("hello student");
                ((Student)u).aStudentFunc();
            }
        });
    }
    ...
}

When I comment out the concrete method calls, the respective print statements output as expected with no errors; however, when trying to use these unique method calls between each loop iteration I get the casting error. Can you please advise how this can be addressed and what I am doing wrong, with either my approach at inference or my approach at a factory pattern?

Answer 1

use instanceof instead. also, you might want to rethink your use of inheritance if you find yourself doing lots of casting