Check if an object is of the same class as this?

Question

I have a base class for multiple data object types in Java. I want to create an equals method in the base class, that works directly when inherited.

Equality is determined by the two objects

belonging to subclasses of the base class. This is easily achievable using
```
if (!(anObject instanceof BaseClass))
    return false;
```
having the same ID. The ID field is defined by the base class so we can here test that.
```
if (this.id != ((BaseClass) anObject).id)
    return false;
```
belonging to the same class. This is where I have the problem. Two objects may be of different types (and so be in different lists), but have the same ID. I have to be able to distinguish them. How can I do this?

You can use `Object.getClass().equals(this.getClass())` for this purpose (assuming you meant the class type instead of a class attribute in your specific entities). — Luiggi Mendoza, Nov 18 '13 at 16:01
Effective Java makes it pretty clear: trying to extend value objects and maintain the `equals` contract is pretty much hopeless. — Louis Wasserman, Nov 18 '13 at 16:39

score 9 · Accepted Answer · answered Nov 18 '13 at 16:03

9

Use

this.getClass() == anotherObject.getClass()

instead of instanceof. This will return true only if the two object belong to the same class (it's safe to check if class objects are equal by reference). And after that you may compare the id.

answered Nov 18 '13 at 16:03

user2660000

332
2
3

V G · Answer 2 · 2013-11-18T16:12:37.923

1

You should read this article for problems when implementing equals.

To make it short: use this.getClass()==other.getClass() instead of instanceof, because otherwise the equals() relationship will not be transitive (superInstance.equals(subInstance) will be true, but subInstance.equals(superInstance) will be false).

edited Nov 18 '13 at 16:12

answered Nov 18 '13 at 16:07

V G

18,822
6
51
89

This is true; in my case the base class is however abstract so there can be no instance of it. All data objects belong to subclasses. – PurkkaKoodari Nov 18 '13 at 17:53
I think you meant commutative, not transitive. – Nom1fan May 30 '22 at 11:59

user3000162 · Answer 3 · 2013-11-18T16:22:02.260

0

If I understand you question correctly, you need a way to differentiate two objects of same Class with same id. If so, for this you may use toString() which gives unique representation of objects unless they are string objects. Of course, you must not have overridden toString() in your base class. example:You can use this, only for the third case that you mentioned.

if (this.toString()!= anObject.toString())
    return false;

edited Nov 18 '13 at 16:22

answered Nov 18 '13 at 16:14

user3000162

16
1

No, my question was about two objects with same ID but _different_ class. – PurkkaKoodari Nov 18 '13 at 17:50

score -1 · Answer 4 · answered Nov 18 '13 at 16:05

You can do that with Class.isInstance() method. In your base class, do this.

public static boolean isAnInstance(Object obj)
{
    return BaseClass.class.isInstance(obj);
}

Then you can check

if (BaseClass.isAnInstance(object))
{
    // Class of object is 'BaseClass' or
    // it extends the 'BaseClass'
}

Hope this helps.

score -1 · Answer 5 · edited May 23 '17 at 10:32

-1

The best practice to know objects equality is to override hashcode() and equals() if you maitain objects in a collection

you can refer Why do we have to override the equals() method in Java?

edited May 23 '17 at 10:32

Community

1
1

answered Nov 18 '13 at 16:06

ksv

37
5

Check if an object is of the same class as this?

5 Answers5