Thread: Question regarding superclasses & subclasses

Hello

I'm trying to understand some basics of classes that extends from other classes. I wonder if I could get some help.

Assume I have the following class

public class Person{
	private String name;
	private int age;
	private String adress;
	public Person(String n, int ag, String adr){
		name = n;
		age = ag;
		adress = adr;
	}
}

and this one below

public class Student extends Person{
	private double gpa;
	public Student(String n, int ag, String adr, double gpa) {
		super(n, ag, adr);
		this.gpa = gpa;
	}
}

What's the difference between these two lines of code below? The parameters are not interesting though, I'm just trying to figure out what they are instances of.

Student student = new Student("name1", 20, "adress1", 50);
Person student2 = new Student("name", 21, "adress2", 50);

I mean in the first one it's Student student = new Student(...). On the second line there is [B]Person[/B] student2 = new Student(...).

Is student2 a "Student" or "Person"?
Thanks in advance.

Best regards,
Robin.

student2 is a variable of type Person, which holds a Student object which happens to also be a Person (due to polymorphism).

You can check this quite handily using the instanceof method:

if(student2 instanceof Student)
{
    System.out.println("student2 holds a Student object");
}
if(student2 instanceof Person)
{
    System.out.println("student2 holds a Person object");
}

Student is a Person, but a Person is not necessarily a Student. A Person could be a Teacher, Staff, etc.

In general there's no way for the compiler to know before-hand if a Person variable does indeed hold a Student object.

For example:

Person p;
if(Math.random() > 0.5)
{
    p = new Student();
}
else
{
    p = new Person();
}
// is p a Student? Don't know, must assume p is a Person.

The compiler therefore must assume the variable holds a Person object. Since we've established that a Student is a Person, it is perfectly valid to treat every Student as a Person. That means you don't get access to any of the special features of a Student without special handling.

Person p;
if(Math.random() > 0.5)
{
    p = new Student();
}
else
{
    p = new Person();
}
if(p instanceof Student)
{
    // we now know p is a Student
    Student s = (Student) p;
    // s is guaranteed to be hold a Student, we can access features specific to to Students
    s.getNumber();
}
else
{
    System.out.println("p is not a Student");
}