28-Oct-15 Information Hiding A very short talk

Modularization Whenever a program is broken into two parts, there comes into being an interface between them: The parameter list Any global or common More subtle ways of information transmission This interface should be kept as narrow and as explicit as possible. How to make it narrow? Have each method do only one thing Be able to describe what it does without saying “but” or “except” Keep parameter lists short Avoid global variables How to make it explicit? Avoid global variables Avoid side effects

Information hiding The principle of information hiding is, informally, Every method should mind its own business To use a method, you need to know: What information do you need to give it? What does it tell you (or do for you) in return? You do not need to know: How it does its job The method should not need to know: How you do your job

What’s wrong with these? int max (int size, int a[]) { sort (a, size); return a[0]; int max (int size, int a[]) { int i; temp = a[0]; for (i = 1; i temp) temp = a[i]; return temp; } int max (int size, int a[]) { int i, temp; temp = 0; for (i = 0; i temp) temp = a[i]; return temp; }

A reasonably good program void play() { setup(); player = who_goes_first(); do { if (player == HUMAN) { do { move = get_humans_move(); } while !legal(move); game_over = make_move(HUMAN, move); player = COMPUTER; } else { /* player == COMPUTER */ move = choose_computers_move(); game_over = make_move(COMPUTER, move); player = HUMAN; } } while(!game_over); }

A not-so-good program What routine or routines update player ? Do they do it in such a way that the main program works? If the human's move is not ok, has player already been updated? How does make() know whose move to make? Who sets ok ? When does the game end? Who's responsible for deciding this? Is movecounter being initialized properly? Computed properly? What is it anyway, and who uses it? void play() { setup(); player = who_goes_first(); do { if (player == HUMAN) { do { move = get_humans_move(); check_if_legal(move); movecounter++; } while(!ok); make_move(move); } else { /* player == COMPUTER */ move = choose_computers_move(); make_move(move); } } while (!game_over); }

Test-Driven Development (TDD) It is difficult to add JUnit tests to an existing program The program probably wasn’t written with testing in mind It’s actually better to write the tests before writing the code you want to test When tests are written first, you have a clearer idea what to do when you write the methods Because the tests are written first, the methods are necessarily written to be testable Writing tests first encourages you to write simpler, single-purpose methods Because the methods will be called from more than one environment (the “real” one, plus your test class), they tend to be more independent of the environment

The End