Android : Polymorphic in AIDL inout param

for example, there are two classes

class B extends A { public int x; };

in AIDL:

public void func( inout A params ); // the function does not change B.x value

then there is the following code:

B objB;
objB.x = 99;
IObject.func( objB );

when the code exectued, objB.x will be changed( not 99 ) since the "func" function only returned the part A of objB, objB.x will get a default value( most are 0 ).

My question:

How to make objB.x be constant if "func" function does not change B.x value?