Inheritance concepts

[Vivek Raj]

public class Super{
	private void doStuff(){
		System.out.println("In super");
	}
	public static void main(String[] args){
		Super s = new Sub();
		s.doStuff();
	}
}

class Sub extends Super{
	
}

In the above code doStuff() is private in Super. But main method is in the class Super and the ref variable 's' is also of type Super, so compiler sees this as perfect method invocation.
However as per K&B instance method invocation is virtual and happens at run-time based on the actual object the ref variable is holding (Sub object in this case) and Sub has not inherited the private method from Super.

But still its calling the private method and printing "In Super". How is this happening?

[Sidharth Khattri]

Vivek Raj wrote:

public class Super{
	private void doStuff(){
		System.out.println("In super");
	}
	public static void main(String[] args){
		Super s = new Sub();
		s.doStuff();
	}
}

class Sub extends Super{
	
}

In the above code doStuff() is private in Super. But main method is in the class Super and the ref variable 's' is also of type Super, so compiler sees this as perfect method invocation.
However as per K&B instance method invocation is virtual and happens at run-time based on the actual object the ref variable is holding (Sub object in this case) and Sub has not inherited the private method from Super.

But still its calling the private method and printing "In Super". How is this happening?

Dynamic call to a method only occur when the methods are overridden. Methods with access modifier private can never be overidden. In your code, the method invocation does not follow the dynamic invocation rule of method override, hence the type of the reference variable i.e Super decide which method signature will be invoked, in your case, it is the doStuff() in Super class.

[Muhammad Khojaye]

Private methods never overridden as Sidharth mentioned. I have tried to modified your example, run and see what happens,

public class Super {

    private String doStuff() {
        return "In super";
    }

    public void printSuper() {
        System.out.println(doStuff());
    }
    public static void main(String[] args) {
        new Super().printSuper();
        new Sub().printSuper();
    }
}

class Sub extends Super {

    public String doStuff() {
        return "In sub";
    }
}

[Vivek Raj]

its printing
In Super
In Super

A new thing for me. Thank you for modifying the code to make it more tricky.

This means that private Super methods can be called on Sub object, provided the calling method is in super class? Am I correct?

[Muhammad Khojaye]

Vivek Raj wrote:
A new thing for me. Thank you for modifying the code to make it more tricky.

You are welcome.

Vivek Raj wrote:
This means that private Super methods can be called on Sub object, provided the calling method is in super class? Am I correct?

Private method cannot be called directly from object of subclass since it can't be overridden. In the example, public printSuper method has been called of base class which implicitly invoke the private method. I hope it helps.