Probably similar to the question, Why can outer Java classes access inner class private members? or Access to superclass private fields using the super keyword in a subclass .
But there are some differences: the children class can access the private members of their parent (and only the nearest parent) class.
Given the sample code below:
public class T {
private int t;
class T1 {
private int t1;
public void test() {
System.out.println(t);
}
}
class T2 extends T1 {
private int t2;
public void test() {
System.out.println(t);
System.out.println(super.t1);
System.out.println(this.t2);
}
}
class T3 extends T2 {
public void test() {
System.out.println(t);
System.out.println(super.t1); // NG: t1 Compile error! Why?
System.out.println(super.t2); // OK: t2 OK
}
}
}
Clever example! But it's actually a somewhat boring explanation - there's no visibility problem, you simply have no way of referring to t1
directly from T3
because super.super
isn't allowed.
T2
can't access its own t1
field directly since it's private (and child classes don't inherit their parent's private fields), but super
is effectively an instance of T1
and since it's in the same class T2
can refer to the private fields of super
. There's just no mechanism for T3
to address the private fields of its grandparent class T1
directly.
Both of these compile just fine inside T3
, which demonstrates that a T3
can access its grandparent's private
fields:
System.out.println(((T1)this).t1);
System.out.println(new T1().t1);
Conversely this doesn't compile in either T2
or T3
:
System.out.println(t1);
If super.super
were allowed you'd be able to do this from T3
:
System.out.println(super.super.t1);
if I'd define 3 classes,
A
,B
,C
,A
having a protected fieldt1
andB
would inherit fromA
andC
fromB
,C
could refer toA
st1
by invokingsuper.t1
because it´s visible here. logically shouldn't the same apply to inner classes inheritance even if the field are private, because these private members should be visible due to being in the same class?
(I'm going to stick with OP's T1
, T2
, and T3
class names for simplicity)
If t1
were protected
there'd be no problem - T3
could refer to the t1
field directly just like any subclass. The issue arises with private
because a class has no awareness of its parent classes' private
fields, and therefore can't reference them directly, even though in practice they are visible. That's why you have to use super.t1
from T2
, in order to even refer to the field in question.
Even though as far as a T3
is concerned it has no t1
field it has access into T1
s private
fields by being in the same outer class. Since that's the case all you need to do is cast this
to a T1
and you have a way to refer to the private field. The super.t1
call in T2
is (in essence) casting this
into a T1
letting us refer to its fields.