I dont think you can buddy. you do not have access to the memory
where the image on the screen  is stored.

except if you want to take a screenshot. Use awt.Robot for that

Can an external program be written for capturing the screen  image at that
instance..
e.g. a C/C++ program ?

I don't quite grasp what you are asking. Can you please clarify your
question?

in java  its not possible to access memory locations directly and store the
screen rendered from the video memory point of view.
but this is possible in C.

probably for ur requirement, a C/C++ program can be written which does
exactly that. store the image somewhere and re-render it when needed...

just an idea to implement that...

If I understand correctly, you say it isnt possible to take a
screenshot with java, but I beg to differ. There is a class  added
from java  1.3 and onwards called Robot. It is in the package  java.awt
Now this class is capable of overiding low-level input such as from a
keyboard or mouse. Eg it can make the cursor jump around on the
screen >=) There is also the capability of taking a screenshot. But
you need the right permissions to create a new Robot. Check the
documentation for more info.

Sorry If my language was confusing, I did not mean to say that its not
possible in java.
In fact I have not been working with java  since last year and a half.

I just tried to provide an alternate idea.. just that.

If its possible in java, all the more better..