Let's go a little more indepth with what goes on for that method. For the time being we'll treat it like you only have 1 of them and it's named randomBall.

In your updateGame method you could have this code:

[source lang="java"]//This line has randomBall create and return a Rectangle that encloses it ('it' being the one ball
//that we have stored in the variable randomBall)
Rectangle randomBallRectangle = randomBall.getBallBounds();

Rectangle mainBallRectangle = mainBall.getBallBounds();

if (mainBallRectangle.intersects(randomBallRectangle))
{
//Do relevant stuff for a collision
}[/source]

It's in our updateGame method so that on every pass through our updateGame method, we get new rectangles from mainBall and randomBall. That means if they've moved, we're still getting up to date information because we're asking for it again every update before making the comparison.

Also note that I declared both of those Rectangle variables here inside the updateGame method. That means when the updateGame method ends, these variables are gone (they've gone out of scope) and then they are created anew next time through the updateGame method. This works in this particular design, but if I wanted to do something like be able to draw those rectangles (maybe for debugging purposes, or whatever), this wouldn't work because they wouldn't exist anymore by the time I got to the paint method. To get that kind of functionality, I would declare the variables in the game class (but not in any method) and then just assign to them each time through the updateGame method.

A good design question for you to think about is when you get the rectangle of one of your multiple random balls, what do you want to do with it? Do you want to compare it immediately against the mainBallRectangle? Do you want to store it in an array so you can later go through that array of Rectangles to see if each collides with mainBallRectangle? Since the random balls don't move, do you want to get their Rectangle as soon as the ball is created and store it in an array (that would mean you wouldn't need to update each time through the updateGame method)? Do you have a different approach in mind?

Hey, for now all that matters is that you've chosen a design so now you can progress towards making that design a reality. We all think differently and fortunately computer programming leaves lots of room for alternate approaches to solving a problem.

So, for that approach, you'll need to declare an array of Rectangles.
And then when it comes time to create a new Ball (the stuff that happens inside that particular if statement), you'll want to do something like (not saying this is all the code you'll need, but this code will handle the portion we're talking about now):

[source lang="java"]Ball newBall = new Ball(newX, newY, 20, 20);
randomBalls.add(newBall);
randomBallRectangles.add(newBall.getBallBounds());[/source]
You'll still need to update the mainBall's rectangle each time in the updateGame method because it moves so you need to get the most up to date location.

Then you'll want to put together a loop that compares the mainBallRectangle against each rectangle in the randomBallRectangles array. And you'll want to do something to mark when a collision has happened.

does this work for what i want?

That does work for what you want. It would probably be a little better design to use your getBallBounds method, but they get you the same results in this case.

right below the checkCollision(); in the update game method

By doing this, you're making BallRectangle (variables should start with a lowercase letter ala ballRectangle) hold more and more rectangles. You're adding a new rectangle to it every time through the updateGame method (to get an idea of how often and fast that happens, add a debug statement at the beginning of the method that says something simple like "Running updateGame").

I think for the design approach you chose last night, a better approach is to go at it like you were earlier in that post.. add one rectangle every time you add one new ball.

Given what new code you've shown, I'm not sure why the program freezes without giving you any kind of error or anything. You'll have to show more code to get some thoughts on why that happens.

what is the reason for needing both of the lines of code? how come just the first line wont work?

Those 2 lines do 2 different things and you need both of them.

One of them (public ArrayList BallRectangle;) declares your variable. It gives it an access modifier (public), a data type (ArrayList) and a name (BallRectangle).
The other line (BallRectangle = new ArrayList();) assigns a value to your variable. It constructs an object (new ArrayList()) and then assigns the result of that statement (in this case, a newly reserved block of memory for an ArrayList) to the variable on the left side of the equal sign (BallRectangle).

If you declared the variable and then tried to use it without assigning a value to it, the variable would have its default value. For data types that are not primitive data types (primitive data types: http://docs.oracle.c.../datatypes.html), that default value is null. This is where something should come full circle for you and register as something you've definitely seen before... If you try to call a method or access a field on a null object, you get a NullPointerException. I know you've seen that exception once or twice. And that process is the reason.
Declare: public ArrayList ballList;
[s]Initialize/Assign: ballList = new ArrayList();[/s]
Use: ballList.add(new Ball());

If you go straight from declaring to using, the Use line would throw a NullPointerException.