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This chapter is from the book

This chapter is from the book


One of the primary means of expressing logic in Java is the message. Procedural languages use procedure calls as an information hiding mechanism:

compute() {

says, “For purposes of understanding this computation all you need to know is that it consists of these three steps, the details of which are not important at the moment.” One of the beauties of programming with objects is that the same procedure also expresses something richer. For every method, there is potentially a whole set of similarly structured computations whose details differ. And, as an extra added bonus, you don’t have to nail down the details of all those future variations when you write the invariant part.

Using messages as the fundamental control flow mechanism acknowledges that change is the base state of programs. Every message is a potential place where the receiver of the message can be changed without changing the sender. Rather than saying “There is something out there the details of which aren’t important,” the message-based version of the procedure says, “At this point in the story something interesting happens around the idea of input. The details may vary.” Using this flexibility wisely, making clear and direct expressions of logic where possible and deferring details appropriately, is an important skill if you want to write programs that communicate effectively.

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