Java casting objects

Question

I have two classes: Teacher and Coordinator. Coordinator extends Teacher.

Basically I wanted to make an instance variable that could take in either a Teacher instance or Coordinator instance.

I know this is easily possible by just writing Teacher exp = new Teacher() or Teacher exp = new Coordinator(), however when I do this, I am only able to access Teacher methods and properties when I apply new Coordinator() or new Teacher() to the reference variable which is normal. My question is that is there another way where I only use one instance variable that can be assigned to Teacher OR Coordinator objects and use this variable to call any property/method of the object that was assigned to the object reference variable?

You do realize that classes can be extend from an existing class. How will your already written code can predict methods from the classes that you haven't extended? reflection API is the only sane mechanism. Other options are to write interfaces which are guaranteed to specify required protocol of the class. — Usman Saleem
– Usman Saleem, Commented Dec 27, 2012 at 23:47

Dennis · Accepted Answer · 2012-12-27 23:58:05Z

8

You can check if exp is a Coordinator then cast and call its function accordingly:

if(exp instanceof Coordinator) {
    ((Coordinator)exp).foo();
}

If you have several things you want to do, you can create a variable in a local scope and use it repeatedly:

if(exp instanceof Coordinator) {
    Coordinator expCoord = (Coordinator)exp;
    expCoord.foo();
    expCoord.bar();
    expCoord.baz();
}

edited Dec 27, 2012 at 23:58

answered Dec 27, 2012 at 23:47

Dennis

32.7k12 gold badges66 silver badges79 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Apoorv Kansal Over a year ago

So after doing this, will Teacher exp (Is assigned to a coordinator object) become Coordinator exp permanently?

Dennis Over a year ago

No. It only works for that expression in the first example or that scope in the second.

Lee Meador · Accepted Answer · 2012-12-27 23:48:26Z

1

No way to do that unless all the methods you want to call are present in the parent class.

You can cast what's in a Teacher variable to be a Coordinator but it will fail if what's in the Teacher variable isn't really a Coordinator object instance.

((Coordinator)exp).coordinate();

answered Dec 27, 2012 at 23:48

Lee Meador

13k2 gold badges38 silver badges43 bronze badges

Comments

tskuzzy · Accepted Answer · 2012-12-27 23:47:42Z

0

Whenever you want to call a method of Coordinator, you would have to cast the object to a Coordinator first.

((Coordinator)myTeacher).coordinatorMethod();

answered Dec 27, 2012 at 23:47

tskuzzy

36.6k15 gold badges78 silver badges145 bronze badges

Comments

m0s · Accepted Answer · 2012-12-27 23:47:54Z

0

No, because of type safety. But you can always do:

Teacher exp = new Coordinator()
((Coordinator)exp).someCoordinatorMethodOrProperty()

answered Dec 27, 2012 at 23:47

m0s

4,3009 gold badges43 silver badges64 bronze badges

Comments

Surendra · Accepted Answer · 2012-12-29 17:43:56Z

0

Yes you are correct,

Here same concept implements as parent child relationship Since Instance of child class can access the property of child as well as parents both, but the instance of parents class access only the property of child class...

That is why it is happening.

answered Dec 29, 2012 at 17:43

Surendra

293 bronze badges

Collectives™ on Stack Overflow

Java casting objects

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

Comments

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related