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Well firstly, state you're design specification, what are you trying to accomplish?

Also is there a reason you are trying invoke a Boolean variable isUsed? i.e, its not a method.

Well yeah, but it has no reference within "extraClass", its an instance of extraClass.

&& (!Third.checkUsed))
Third.setUsed(Third);

You invoke methods on Third, which hasn't been declared anywhere in your 'extraClass' Class.
Not sure what you're trying to do with 'Third' in...