Styjun

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?

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.

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() 

 

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();

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