You were right that it's not an SCJP-related post.
I detect confusion in this thread. I also detect poor style. Please start your class names with CAPITAL letters LikeThis. And don't start variable names with capital letters. Then you can see which is the Type (Starts With Capital Letter) and which is the primitive (int boolean char short long float double or byte) or the variable (doesn't start with capital letters). Also avoid using "A" "B" or "XYZ" as names; they are not easy to read.
Also make sure to indent your code and use the code button. If you did those things, your posts would be easier to read.
In the first example there is a field called "obj" which is of type "a", and Kshitij Dogra demonstrated how you can use that variable. Creating methods which use that variable and a constructor to set the variable would be more useful design; that would show how the class can use its included data.