Question on implentation inheritance

[Kavya Das]

Can we design a super class to have a instance variable whose type is one of its sub classes?

Logically, this may sound stupid, but programmatically its possible right?

I have an interface Say UserInterface. I have 2 implementations of this interface UserImpl1 and UserImpl2.

I have to invoke the method from one of these impl based on some logic. And in some cases, I may have to call the methods from both the impl one after the other.

So, what I have done is I wrote one more class, Say UserImpl3 that implements the interface and the clients would be invoking the methods on this class. Methods in this class will have the logic to determine whether the method from the UserImpl1 has to be called or UserImpl2 or from both.

I am somewhat not comfortable writing the UserImpl3 at the same level as the UserImpl1 and UserImpl3.

I thought of gng for the adapter pattern with the hierarchy as UserImpl3 (only this implements the interface now) as the super class and UserImpl1 and UserImpl2 as its subclasses. So, the question is can we have the super class call the methods from the sub classes by defining the sub class objects as its instance variables?

Please advice.

[Venkata Kumar]

Can we design a super class to have a instance variable whose type is one of its sub classes?

Yes a super class can have one of its sub classes as instance variable.

I am somewhat not comfortable writing the UserImpl3 at the same level as the UserImpl1 and UserImpl3.

For the given interface UserInterface you may have 3 implementations UserImpl1 , UserImpl2 , UserImpl3. UserImpl3 can have instance variables UserImpl1 and UserImpl2

interface UserInterface(){
  public void method1();
}

class UserImpl1 implements UserInterface{
  public void method1(){
    // implementation code 
 }
}

class UserImpl2 implements UserInterface{
  public void method1(){
    // implementation code 
 }
}

class UserImpl3 implements UserInterface{
     UserInterface userImpl1;
     UserInterface  userImpl2;
     public UserImpl3(){
        userImpl1 = new UserImpl1();
        userImpl2 = new UserImpl2(); 
     }  
   
    public void method1(){
       // logic to call either one of the userImpl1,userImpl2 methods or both
    }
}

So, the question is can we have the super class call the methods from the sub classes by defining the sub class objects as its instance variables?

Yes super class can have sub classes as instance variables and call their methods.

[Madhavi Yeduguri Sandinti]

If we see the examples of Adapter pattern we can observe the "sub class objects as super class instance variables"