家长班级公开成员召集的儿童班级私有成员

Question

What is happening in the code below? Please explain the output:

class Parent{
    private void fun(){
        System.out.println("parent fun\n");
    }
    public void accessFun(){
        System.out.println(this);
        this.fun();
    }
}

class Child extends Parent{
    private void fun(){
        System.out.println("child fun");
    }
}

class Test{
    public static void main(String[] args) {
        Child a = new Child();
        Parent b = new Parent();
        Parent c = new Child();
        a.accessFun();
        b.accessFun();
        c.accessFun();
    }
}

Output:

Child@7960847b

parent fun

Parent@3b192d32

parent fun

Child@16f65612

parent fun

Why the line this.fun() is not giving compile-time error?

I think fun is a private member in Child class and therefore can't be accessed from outside the Child class(from public member of it's Parent class).

Why Parent class version of fun() is being called by this.fun()? Note this refers to child class object.

Answer 1

Private members are not inherited.

I think this might be the key point that you are missing here. What this means is that Child.fun does not override Parent.fun . They are just two plain old methods that has nothing to do with each other.

When you call accessFun , control always goes into this bit of code in Parent :

public void accessFun(){
    System.out.println(this);
    this.fun(); <---- here
}

Now, since we are now inside Parent , we can access fun . And since Parent.fun is not overridden , it calls Parent.fun and not Child.fun .

I think fun is a private member in child class and therefore can't be accessed from outside the class(may even from public member of it's Parent class).

That is a complete misunderstanding. if private members can't be accessed from outside of the class, not even through public methods, then they will be much less useful. Why even have them in the first place?

"Private members can only be accessed by members declared in the same class" is probably a better thing to remember.

Answer 2

private fun() in Parent is called by public accessFun() of same Parent class. And public accessFun() is called in main() of Test class.
Here accessFun() is public so it can be called from any where.
For example just like Pojo classes private members are accessed through public setters and getters

Answer 3

The fun method in parent is private and the child fun() method is not overriding the parent fun() method. But the accessFun method is public so it can be called from the child class and is publicly accessible.

So when you call accessFun() method it will be calling the parent class fun() method. Not the child class fun() method as it is private. That is why you wont get an error.

Answer 4

I think fun is a private member in child class and therefore can't be accessed from outside the class(may even from public member of it's Parent class).

Sometimes everything comes from "outside". The main call comes from outside for example.

If a private member can not be accessed from outside, it would always be dead code and this makes no sense.

The difference is the understanding from direct-access and indirect-access.

In all cases you call the method non-directly via the method accessFun() .