Java - Abstract classes and concrete classes

Question

I'm working on my assignment but I got confused with the abstract classes and concrete classes, and I get error from my program...

Suppose there is an abstract class Person and a concrete subclass Student:

abstract class Person{
    private String name;
    public Person(String s){
          name = s;
    }
    public String getName() {
          return name;
    }
    public abstract void doAction(Person other);
}

class Student extends Person{
    public Student(String name){
          super(name);
    }
    public void doAction(Person other){
          System.out.println("This person's name is " + getName());
    }
}

Then I implement a main function to test it but I got an error...:

public class TestMain {
    public static void main(String[] args) {
        Person person;
        Student student;

        person = new Student("Sam");
        student = person;
        student.doAction(person);
    }
}

It is said student = person receiving an error saying that "Error: Incompatible types: Person cannot be converted to Student". What's wrong with that actually and why...? Does anyone can explain this...?

Answer 1

A Student is a Person, but not every Person is a Student.

If you have a variable of type Person, you can't assign its value to a variable of type Student because, in general, that might not be safe.

If you are certain that it's definitely a Student (e.g. you use an instanceof check, or you have reasoned about the code and thus "know"), you can cast the variable; but one of the central ideas on object-oriented programming is that you shouldn't need to care about the specific subclass.

There are two ways round this:

• Assign the new Student() to the Student variable first, and then assign that value to the Person variable:

  student = new Student("Sam");
  person = student;
  student.doAction(person);
  

  This is fine because every Student is a Person, so a Person variable can be assigned the value of a Student variable.

• Forgo the student variable entirely, since you only need a reference to a Person on which to call doAction, not specifically a Student:

  person = new Student("Sam");
  person.doAction(person);
  

Answer 2

During run-time the person variable can refer to instances of Person which are not instances of Student. Therefore the assignment student = person; is not allowed.

You have to check the run-time type of person and perform a cast in order for the assignment to work (well, the type check is not mandatory, but recommended, in order to avoid potential ClassCastException):

if (person instanceof Student)
    student = (Student) person;