Robot Schreenshot doesn't work in game

Hi all!
I'm scripting a Java program that, using the Robot Class, takes a screenshot and saves it.
The problem is that when i'm in a game, for exemple Call Of Duty 4, the screen is all black.
Is there another class or how can i solve problem?
Waiting your illumination ^^
Have a nice day

Quote:

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the screen is all black

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If the game has an all black screen, is that what the java program captures?
How does your java program execute while the game program is executing?
What OS are you running on? Does the PrtScn button work as expected?