At this point in your arsenal of Java programming knowledge, you can create variables of differing types and manipulate them. For more complex operations you can modify them conditionally or derive values through iterative means.
Assuming you completed the exercises in the previous chapter, you have a good set of code that computes prime numbers, but what happens when you need to compute prime numbers again in another application? It would be nice to package that piece of code you wrote to determine whether a number is prime in such a way that you can reuse it.
