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There are some missing import statements in the Color class. Look at what classes the compiler can not find definitions for, look in the API doc and find what package those classes are in and add an...

The three classes are ok.

Have you executed the code? What is supposed to be changed? The ball bounces and deforms when I execute it.

The Circle class has lost the indentation of the nested statements.

Is the "ground" the side at maximum y value? The bounces on the other three sides are normal.