code reading skills level game. common scenario: students raise hand. point to code, say they...

Code reading skills

LEVEL GAME

Common scenario: Students raise hand. Point to code, say they don’t understand

why it’s not working. public void someFn() { if (some condition) do some action; }

assumption: method is called assumption: if condition is true

Test your assumptions! How? println OR use debugger GUI program… println may be better repeated code… println may be better otherwise… debugger may be better

DEBUGGING AS SCIENTIFIC METHOD

Eclipse will take you directly to the line of code generating the error

obj.var OR obj.method()

What if: obj.method(obj2.getter().method()); May need to break apart to see which is null

In general, null pointer exceptions are fairly easy to diagnose.1. Figure out which object is null2. Figure out where you think memory should have been allocated3. Figure out why it wasn’t (often because method that allocates space

is not actually called, OR you allocate space for a local variable) Remember, you need to allocate space for the collection (array,

ArrayList, etc.) AND allocate space for each element in the collection.

NULL POINTER EXCEPTION

UML

for (Moveable piece : movingPieces) { piece.move(pieces, playerLocation);}

for ( InteractingPiece piece : interactingPieces) { Interact ionResult result = piece. interact(pieces, playerLocation); / / use result to determine how to update player status}

for (? piece : pieces) { if this piece moves move it…. NOTE: all pieces must now have a move method (may be blank) if this piece interacts get the result …. NOTE: all pieces must now have interact method update

INTERFACES CAN SIMPLIFY CODE

Don’t make your interfaces “fat”no client should be forced to depend on methods it

does not use. ISP splits interfaces which are very large into smaller

and more specific ones so that clients will only have to know about the methods that are of interest to them.

INTERFACE SEGREGATION PRINCIPLE

Wikipedia

Single Responsibility Principle every class should have responsibility over a single part of the

functionality provided by the software, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility.

Martin defines a responsibility as a reason to change, and concludes that a class or module should have one, and only one, reason to change.

Why would LevelEngine change? Ultimately, it would change if confi g fi les that defi ne levels change.

Why would GameEngine change? If rules of game play change.

What if we don’t expect level defi nition format to change? May not need two classes.

DO WE NEED LEVEL ENGINE?

WITHOUT LEVEL ENGINE

Purpose of UML is to communicate Agile manifesto: value working software over

comprehensive documentation Does NOT mean no documentation ever!

UML is used especially to show relationships between classes

Do we need all the GameEngine details?

In fact, do all the GameEngine methods need to be public? Would any other class call these methods?

UML COMPLEXITY

WITH PRIVATE METHODS

Use abstract class if there are common attributes Use abstract class if you want to provide some (but not all)

method implementations

THIS EXERCISE No attributes that are really needed for ALL pieces BUT, it’s a simple enough game that a valid alternative might be to

use an abstract Piece class

In a more complex game, might want to stick with interfaces Enemy: weapon, strength, attack() Friend: goodWill, assist() Treasure: pointValue Obstacle: secretWord, disable() All can be drawn, some move and some don’t

INTERFACE VS ABSTRACT CLASS

ABSTRACT CLASS