🚀 go-pugleaf

Vladimir Olenin wrote:

> After construction of MainClass is competed i can fire an action. But
> then i get null pointer exception in inner class someAdapter of B class
> when i try to use subA class to getSomeObject.
> When i checked this out, it turned out that subA in B class is null
> indeed while subA in A class is not null.
>

Vladimir,

they key to understanding what's happening is in your comment, "// !!! NOTE
that subA is constructed outside A constructor", or rather 'Note that a is
NOT fully initialized until a.init(...) is called, and therefore subA is not
initialized until a.init(...) is called.'

So, according to your MainClass, the order of operations you're getting is

    a constructs, including subA gets initialized to null (default value for
class member reference value)
    b constructs, copies (the current) subA from a to initialize its b.subA
to null.
        [Now, both a.subA and b.subA are null.]
    THEN (once all the above MainClass initializations are done)
    MainClass() gets called resulting in a.subA getting properly initialized
(albeit too late for b.subA, above).
        [Now, a.subA is 'new A()', but b.subA is still null since it's never
been reinitialized after its construction.]
    b.subA is still null... including when your ActionListener gets called
resulting in a NullPointerException..

To fix this problem, you need to fully initialize a.subA at a's
construction, or you need to construct b after a.init().  One way or
another, you need to ensure a.subA is fully initialized before b copies
a.subA for itself (i.e., into b.subA).

I hope this explanation helps.

Good luck,

= Joe =


> class MainClass{
>   A a = new A();
>   B b = new B(a);
>
>   MainClass(){
>     a.init(b);
>     ...
>   }
>