! Two questions when we design an OO system : ! Our Initial goal: learn to design and implement simple objects.

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1 Designing Interactive Classes CS234 Chapter 3 Designing with objects! Two questions when we design an OO system :! what are the objects?! what features should these objects have?! Our Initial goal: learn to design and implement simple objects.! Objects are designed to support system functionality.! System specification are distributed as responsibilities to objects identified. 1 Object responsibilities! Think in terms of! what object must know.! what object must do.! Knowing responsibilities include:! knowing properties of the entity object is modeling;! knowing about other objects with which it needs to cooperate. Object responsibilities! Doing responsibilities include:! computing particular values;! performing actions that modify its state;! creating and initializing other objects;! controlling and coordinating the activities of other objects. 2 3

2 From responsibilities to class features! Knowing responsibilities! translate into data the object must maintain, or! translate into queries.! Doing responsibilities! translate into commands or! translate into queries.! To design a class:! determine an object s responsibilities,! classify them as knowing or doing responsibilities. Example: Design of Nim game! Game: Players take turns removing sticks from a pile. Each player in turn removes one, two, or three sticks. The player who removes the last stick loses. 4 5 Design of Nim game Designing Pile! Two objects for the picking:! Player! Pile a very simple object:! keeps track of how many sticks remain.! Pile of sticks! Pile and Player are part of the model of problem.! Model aspects of game independently of how is presented to a user or how a user interacts with it.! Pile responsibilities:! know:! number of sticks remaining! do:! reduce number of sticks (remove sticks) 6 7

3 Designing Player! Player takes a turn in the game to remove sticks from Pile.! Player responsibilities:! know:! this Player s name.! how many sticks this Player removed on his/her most recent turn.! do:! take a turn by removing sticks from the Pile From knowing responsibilities to queries! Class: Pile! queries: sticks the number of sticks remaining in this Pile, a non-negative integer! Class: Player! queries:! name : this Player s name, a String! stickstaken : number of sticks this Player removed on his/her most recent turn ( 1, 2, or 3). 8 9 From doing responsibilities to commands Interaction diagram:player takes turn! Class: Pile! commands: remove : reduce number of sticks by specified amount (number)! Class: Player! commands: taketurn remove 1, 2, or 3 sticks from the specified Pile (pile) 10 11

4 Pile, and Player constructors! How are the Player s name and the initial number of sticks in the Pile determined?! Set these values when objects are created: Constructors.! Constructors can have parameters:! Player s name parameter in Player s constructor.! Initial number of sticks parameter in Pile constructor. Pile specifications nimgame Class Pile public class Pile A pile of sticks for playing simple nim. Constructors public Pile (int sticks) Create a new Pile, with the specified number of sticks. sticks must be non-negative Pile specifications Player specifications Queries public int sticks () Number of sticks remaining in this Pile. Commands public void remove (int number) Reduce the number of sticks by the specified amount. Number must be non-negative and not greater than the number of sticks remaining. public class Player A player in the game simple nim. Constructors public Player (String name) Create a new Player with the specified name

5 Player specifications Queries Commands public String name () The name of this Player. public int stickstaken () The number of sticks this Player removed on this Player s most recent turn: 1, 2, or 3. Implementing the class Pile! Data maintained by Pile is number remaining sticks. private int sticksleft; // sticks left in the Pile! Instance variable sticksleft is initialized in constructor and value returned by query sticks: public Pile (int sticks) { sticksleft = sticks; public void taketurn (Pile pile) Remove 1, 2, or 3 sticks from Pile. public int sticks () { return sticksleft; Implementing the class Pile Dr Java P129! Command remove is specified with an int parameter number, indicating sticks to be removed: public void remove (int number) {! Executing command reduces instance variable sticksleft by value client supplies in number. public void remove (int number) sticksleft = sticksleft - number; 18 19

6 Implementing the class Player! Player needs to know name and number of sticks taken on most recent turn. private String name; // this Player s name private int stickstaken;! Variables should be initialized in the constructor. public Player (String name) { this.name = name; this.stickstaken = 0; // sticks taken on this // Player s most recent turn Implementing the class Player! Queries simply return the values of the instance variables: public String name () { return name; public int stickstaken () { return stickstaken; Invoking a method: acting as client Implementing Player s taketurn! General form for invoking a query: objectreference.queryname(arguments)! General form for invoking a command: objectreference.commandname(arguments);! taketurn method must:! determine how many sticks to take! Give Player strategy to take 1 stick.! remove it from the Pile pile.remove(1);! store number removed in instance variable stickstaken stickstaken = 1; /** * Remove 1, 2, or 3 sticks from * Pile. * The Pile must not be empty. */ public void taketurn (Pile pile) { pile.remove(1); stickstaken = 1; 22 23

7 Dr Java P136 Parameters v.s. arguments! Arguments are provided in a method invocation by expressions. Thus we could write something like pile.remove(stickstaken + 1); or even pile.remove(2*stickstaken+2); Parameters v.s. arguments! Arguments must match in number, type and order: public void move (int direction, double distance) {! An invocation of move must provide two arguments, an int and a double in that order. object.move(90, 2.5); Commands and queries! A command invocation is a form of statement.! A query, which produces a value, is an expression.! If mycounter is a Counter object and i an int, i = mycounter.currentcount(); i = mycounter.currentcount()+10; 26 27

8 Local variables in methods! local variable:! created as part of method execution! used to hold intermediate results needed during computation. Local variables in methods! Assume RetailItem includes instance variables: private double baseprice; private double discountrate; private double taxrate;! Want to implement RetailItem method netprice: public double netprice () The cost of this RetailItem after discount, and including tax Local variables in methods Local variables Instance variables public double netprice () { double discount; double subtotal; double tax; Local variables discount = baseprice * discountrate; subtotal = baseprice - discount; tax = subtotal * taxrate; return subtotal + tax; Defined inside a method. Exists while method executes. Must be initialized before used. otherwise, compiler error. Accessed only from the method. Meaningful only during method execution. Contains some intermediate value needed only during execution of method; value is not part of object s state. Defined outside any method. Exists as long as the object exists. Initialized in a constructor. Accessed from any class method. Has meaningful value during life of object, whether or not object is actively doing something. Represents an object s property; its value is part of object s state