Instantiate abstract class using class of variable

Question

For obvious reasons, we cannot instantiate an abstract class directly like so:

AbstractObj obj = new AbstractObj();

Where AbstractObj is a class of the form:

public abstract class AbstractObj {
//... Body omitted 
}

However if we have extending classes, such as the following:

public class ConcreteObj extends AbstractObj {
//... Body omitted
}

public class AnotherObj extends AbstractObj {
//... Body omitted
}

Is it possible to instantiate an object in the following manner? This determines which constructor has to be used based on the class of the variable passed in. Assume for now that o1 and o2 are guaranteed to be of the same type.

protected AbstractObj computeDiff(AbstractObj o1, AbstractObj o2){
    AbstractObj delta = ...?
}

For example, in the above, if o1 is of type ConcreteObj, is there a way to recognise at runtime whether or not it is of this type and use the appropriate constructor?

you want to call a constructor after constructing the object? — Stultuske
– Stultuske, Commented Mar 12, 2019 at 10:41
You can do it with : o1.getClass(). Then, if it has a default constructor, you can call it with o1.getClass().newInstance(). — Arnaud Denoyelle
– Arnaud Denoyelle, Commented Mar 12, 2019 at 10:41
Abstract classes can not be instantiated but you can instantiate its concretions. — GaripTipici
– GaripTipici, Commented Mar 12, 2019 at 10:41
You can use instanceof to check whether o1 is indeed a ConcreteObj. — Mena
– Mena, Commented Mar 12, 2019 at 10:42

Arnaud Denoyelle · Accepted Answer · 2019-03-12 10:45:32Z

1

Here :

protected AbstractObj computeDiff(AbstractObj o1, AbstractObj o2){
   AbstractObj delta = ...?
}

You can get the concrete class of o1 with o1.getClass(). Then, if this concrete class has a default constructor, you can call it with Class.newInstance() :

protected AbstractObj computeDiff(AbstractObj o1, AbstractObj o2) throws IllegalAccessException, InstantiationException{
   AbstractObj delta = o1.getClass().newInstance();
}

answered Mar 12, 2019 at 10:45

Arnaud Denoyelle

31.5k18 gold badges106 silver badges161 bronze badges

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

2 Comments

Jesper Over a year ago

This will work, but it does require that all subclasses of AbstractObj have a no-args constructor.

Arnaud Denoyelle Over a year ago

@Jesper Yes, it is often a requirement for classes dealing with reflection.

GaripTipici · Accepted Answer · 2019-03-12 10:44:44Z

0

You can use instanceof statement. You can cast o1 to ConcreteObj and use this class' methods or fields.

if(o1 instanceof ConcreteObj){
  //((ConcreteObj)o1).someMethod();
}

answered Mar 12, 2019 at 10:44

GaripTipici

5201 gold badge9 silver badges27 bronze badges

Comments

Torben · Accepted Answer · 2019-03-12 11:32:07Z

This is definitely code smell. Do not use instanceof. Do not use reflection.

Absolutely do not continue on this path.

All your AbstractObj instances implement a common set of operations. Since your computeDiff operates on AbstractObjs, it must not depend on any special features of the different implmentations it receives.

Therefore it does not matter what type of an object the method receives or returns, as long as they all adhere to the AbstractObj definition. You can return an anonymous class that extends AbstractObj if you want or any other subclass. You can even create a specific subclass for just this purpose. But whatever you return, it must not return anything more than AbstractObj.

Declaring the parameters and the return value as AbstractObj is a contract you sign with whoever calls your method. Don't break your contracts.

Collectives™ on Stack Overflow

Instantiate abstract class using class of variable

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related