Calling method that exists in child classes but not in parent class

Question

public class Parent {
    ....
}

public class Child1 extends Parent {
    ....
    public void foo() {
        ....
    }
}

public class Child2 extends Parent {
    ....
    public void foo() {
        ....
    }
}

Here method foo() only exists in the Child classes and CAN NOT be added to the Parent class (not even abstract method). In this situation when I want to call the foo() method on obj which is Parent class's reference then I need to use intanceof with multiple if..else which I want to avoid.

Parent obj = ...// Object of one of the child classes
obj.foo();

EDIT: I Need to use type of obj as Parent only. Else I will not be able to call methods on obj which exists in Parent class.

---

My Solution: The approach that I am thinking is to define an interface say FooInterface with foo() method and let all the child classes implement it, then I could just type cast the obj to that interface and call foo() method like this:

if(obj instanceof FooInterface){
    ((FooInterface)obj).foo();
}

Is there a better approach ? Or any improvement to this one?

Edea

Whenever a method is common in the child, it should be place in parent. Why u r not putting that method in abstract class?? – Mitrewort

I CAN NOT change Parent class, as I do not manage it. – Edea

Then use interface which is stated in the solution given below in the answers. – Mitrewort

Can you introduce an intermediary class between Parent and Child? – Gui

Answer 1

The approach that I am finally taking is to define an interface say FooInterface with foo() method and let all the child classes implement it, then I could just type cast the obj to that interface and call foo() method like this:

Parent obj = ...// Object of one of the child classes
.....
if(obj instanceof FooInterface){
    ((FooInterface)obj).foo();
}

Edea

Answer 2

You can't do it with parent object reference until an unless method is declared in parent class/interface itself.

You have to downcast it to child class because parent class/interface doesn't have any knowledge about the child class other than the contract defined between them.

Here contract means abstract methods.

---

you can try in this way where there is no need to put a check it.

FooInterface sc =new Child1();
sc.foo();

...

interface FooInterface{
    void foo();
}

public class Parent {

}

public class Child1 extends Parent implements FooInterface{

    public void foo() {

    }
}

public class Child2 extends Parent implements FooInterface{

    public void foo() {

    }
}

Addia

With this solution I will not be able to call methods on sc which exists in Parent class. I edited my question to mention this. Thanks. – Edea

have you made Child1 implements FooInterface same for Child2? – Addia

This is what I mentioned in My Solution, only difference is that in my solution, reference is of type Parent and thus I need to use instanceof once to see if it is of type FooInterface. – Edea

so you don't want to change your design then its not possible as I told you parent class/interface doesn't have any knowledge about the child class. how would parent class/interface know that child has created x methods? – Addia

How about my solution that I had mentioned in the question? it does solve my problem. But is good approach is what I want to know? – Edea

Answer 3

The polymorphism is applied on object reference, not a type. When you call

FooInterface obj = ...// Object of one of the child classes
obj.foo(); 

the child class method foo() is called.

Titanomachy

Look at the OP question again. Parent doesn't have any foo() method. – Addia

How can you call a method on the reference of interface where interface has not declared that method? Look at my post. – Addia

@Addia Now you should read OP question again, the interface define a method foo. – Titanomachy

Read this line not solution part Here method foo() only exists in the Child classes and CAN NOT be added to the Parent class (not even abstract method) – Addia

@Roman C, the interface is part of my solution And what am asking is whether my solution is good or is there a better way to do it. – Edea

@Addia read more that OP notes: The approach that I am thinking is to define an interface say FooInterface with foo() – Titanomachy

@Addia What a code review? Are you not a java programmer? Or anything wrong with the format the answer? Or you just kidding me out there? – Titanomachy

@RomanC your answer is best suited for this condition. +1 for this approach. – Addia

@Edea One more comment to you, when designing your classes consider coding to interfaces, not classes. – Titanomachy

@RomanC, With this solution I will not be able to call methods on obj which exists in Parent class. Yes, I understand advantage of code to interface but this is legacy code and I CAN NOT modify Parent class. – Edea

@Edea if you need a parent class then typecast it directly or use a method in the interface that return a parent type. Why there isn't this in the question that you want to use parent? See you have another incomplete answer from %Braj. – Titanomachy

Answer 4

If you want to typecast only then there is no need of adding interface. You can typecast it to your desired class and call the method. Example

public class HelloWorld {
    public static void main(String args[]) throws FileNotFoundException {
        SuperClass sc =new Child1();
        if(sc instanceof Child1)//Do same for Child2
        ((Child1)sc).foo();
    }
}

class SuperClass {

}

class Child1 extends SuperClass{
    public void foo(){
        System.out.println("From child1");
    }
}

class Child2 extends SuperClass{
    public void foo(){
        System.out.println("From child2");
    }
}

Output : From child1

Submerge

Well, that's what I think is not a good approach and hence don't want to use it. If there are multiple child classes then it would be ugly. Isn't it ? – Edea

Answer 5

You could implement an AbstractChild inheriting from Parent and then extend this class instead of Parent:

public class Parent {
    ....
}

public abstract class AbstractChild extends Parent{

    public abstract void foo();

}



public class Child1 extends AbstractChild {
    ....
    public void foo() {
        ....
    }
}

public class Child2 extends AbstractChild {
    ....
    public void foo() {
        ....
    }
}

So you need to only check if your instance is instanceof AbstractChild.

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