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Did you look up the what the API doc says about that error? The problem is explained there.
Java Platform SE 7

Please copy the full text of the error message and paste it here. It has important info about the error.

Did you look up the what the API doc says about that error?

Don't use a new statement. Assign the value of the passed reference to a class instance variable so it is available to the other methods in the class.

Some comments on the code:

return books.size() == 5;
Always use the >= test. Or have other code that makes sure the size never goes past 5


for(int i = 0; i < 0; i++)
What will that loop...