Check if parent class is instance of child class

Question

I have one class defined like this:

class Car {

}

And many other defined like this:

class Audi extends Car {

}

class Seat extends Car {

}

class Mercedes extends Car {

}

class Opel extends Car {

}

...

I have a situation where I receive a list of all these cars which is defined like this:

List<Car> cars = new ArrayList<Car>();

In that list there are many different cars so how can I find out which one is Audi, which one is Seat etc?

I've tried to do this for cars for which I know they are of type Audi:

Audi audi = (Audi) cars.get(0);

This throws ClassCastException. How to handle this situation?

You can use instanceof, but that subverts OOP principles. What are you trying to do? — Adrian Leonhard, Feb 19 '15 at 21:57
"*how can I find out which one is Audi, which one is Seat*" why do you even want to do it? Doesn't your `Car` class have already all method you need from car? — Pshemo, Feb 19 '15 at 22:00
@AdrianLeonhard, I am trying to get the data for specific car from a list of cars. I am receiving that list from another place and need to process it. — user3339562, Feb 19 '15 at 22:00
@Pshemo, no. Each specific car has some extra variables with it's getters and setters. — user3339562, Feb 19 '15 at 22:00
OK, in that case `instanceof` is operator you are looking for. But still I am interested what data could have `Seat` which `Audi` or other cars shouldn't have. — Pshemo, Feb 19 '15 at 22:02

score 8 · Accepted Answer · edited May 23 '17 at 11:47

You can use instanceof:

Car car = cars.get(0);
if (car instanceof Audi) {
    // It's an Audi, now you can safely cast it
    Audi audi = (Audi) car;
    // ...do whatever needs to be done with the Audi
}

However, in practice you should use instanceof sparingly - it goes against object oriented programming principles. See, for example: Is This Use of the "instanceof" Operator Considered Bad Design?

score 3 · Answer 2 · answered Feb 19 '15 at 21:57

3

Obvious or not, this will do the trick:

Car car = cars.get(0);
if(car instanceof Audi) {
  Audi audi = (Audi) car;
}

answered Feb 19 '15 at 21:57

Tobias

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Dmytro · Answer 3 · 2015-02-19T22:10:23.417

3

You can use instanceof before casting. For example:

Car someCar = cars.get(0);
Audi audi = null;
if(someCar instanceof Audi){
audi = (Audi) someCar;
}
if(audi != null){
//...

But likely it's a bad idea, because generics was introduced to avoid using casting and instanceof.

edited Feb 19 '15 at 22:10

answered Feb 19 '15 at 21:58

Dmytro

1,850
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score 2 · Answer 4 · answered Feb 19 '15 at 21:57

2

Check with instanceof, for example:

car instanceof Audi

This returns true if variable car is an instance of Audi, otherwise returns false.

answered Feb 19 '15 at 21:57

Peter Pei Guo

7,770
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Check if parent class is instance of child class

4 Answers4