Create a common object for two classes

I have 2 classes (A and B) which will be used according to functional rules. Exemple:

if (case 1) 
 use objectA
if (case 2)
 use objectB

In that case, I am looking for the best way, to create a common object which contains the right attributes. that means if I have case 1, My objectCommon = objectA, if case 2 objectCommon = objectB.