Can you access the private methods, instance variables also using this method?
Yes.
Can you invoke private methods also?
Yes.
And finally, what is the use of such mechanisms when we can access private methods, constructors and instances.
Dosen't it breach security?
Access modifiers are not security mechanisms. If you genuinely don't want calls to particular methods
you should secure them accordingly, don't rely on access modifiers.
Access modifiers are closer to guides to simplify APIs for other developers. Its not saying you really can't call this code, rather you really should be calling this code.