Assigning value to variable within Range

Question

The Problem:
Range of values permitted is 10 < x < 30. x starts at 20. Variable x increments by 5, on a button click, until it reaches 30, at which point it should decrement by 5, until it reaches 10, before moving up the range again... and so on.
I'm sure this is quite simple, but it's been bugging me for hours... the following simple increments by 5.
How would I include range constraint. please some help.
void jButton_mouseClicked(MouseEvent e) {
int r = aBall.radius();
aBall.rad = r + 5;
repaint();
}

Answer 1

You're right, perhaps, nasch_.
When I was starting out, I learned a lot from looking at code and then figuring out how to adapt it. Perhaps I have denied the OP this opportunity.
To learn more from the code I posted, OP, don't just copy and paste it. As nasch_ was pointing out, you absolutely must learn how to translate requirements in English (or language of your choice) into code. If-statements ar crucial elements. Look at the code and figure out how it matches your requirements. Think about what you have to do logically - When the size would be less than 10, you need to grow instead of shrink... etc. Write out (or at least think out) every scenario that your code has to react to. Size would be in range, size would be too large, size would be too small, ball is growing, ball is shrinking, etc. Make every possible combination and figure out how you must react.
When your logic is hammered out like that, in natural language, start converting to logic. You see that your code has to remember whether your Ball is growing or shrinking. Your Ball should know it's minimum and maximum size (which my code ignores, making it less flexible). Turn your scenarios into If-statements. Your first code could have way too many If-elses, but get it to work, and then refine it by combining and nesting statements.
Don't ever just copy and paste code. You won't learn how to do anything yourself. Learn from code you see. Take this opportunity to not just solve the immediate problem, but to realize that this is an area of your skills that needs improvement and follow through on improving it.
-- end soapbox