Java inheritance: How can you find which subclass an object belongs to?

Question

I wasn't quite sure how to word it in the title, but here is the use case.

I have a class Test. Test has an attribute called Letter as so

public class Test() {
    Letter x;
}

Letter can be one of several subclasses.

class A()
class B()
class C()

Now suppose that in a class (let's call it driver), I have an instance of Test. I want to figure out whether this Test's letter is A, B, C, etc. so that I can access attribute unique to the child class. See:

public class Driver() {
    Test t;
}

If I use t.getClass(), will I get Class.Test, or will I get the child class (e.g. Class.A)? Is it possible for the Driver class to know x's subclass?

Is it possible to create a method like:

public Class getSubclassFromLetter(Letter x) {
    // Find subclass from the letter
}

*"so that I can access attribute unique to the child class."* Then your design is bad. One of the most important OO principles is *information hiding* which basically means that no other class knows what properties an object has. — Timothy Truckle, Mar 23 '17 at 20:31
Possible duplicate of [How to find out the subclass from the base class instance?](http://stackoverflow.com/questions/2856122/how-to-find-out-the-subclass-from-the-base-class-instance) — Mike Nakis, Mar 23 '17 at 22:00

score 2 · Accepted Answer · answered Mar 23 '17 at 20:35

2

You can use instanceof. It is not really a good practice to do that, but you can do the following:

if (x instanceof A) {
    //TODO whatever related to A
}

answered Mar 23 '17 at 20:35

Peyman Mahdian

522
4
15

score 1 · Answer 2 · answered Mar 24 '17 at 04:22

using either instanceof or Class#getClass()

A returned = getA();

if (returned instanceof B) { .. }
else if (returned instanceof C) { .. }

getClass() would return either of: A.class, B.class, C.class

That said, sometimes it is considered that using instanceof or getClass() is a bad practice.

Java inheritance: How can you find which subclass an object belongs to?

2 Answers2