Classes and Objects What is Design The parts

Classes and Objects

What is “Design”? • The parts of the software including – what information each part holds – what things each part can do – how the various parts interact

Why do We Care About Design? • As system grows larger – helps us understand the software we are about to build – helps teams develop components that will work together – helps us find where we need to work to • add new features • fix defects

Object Oriented Design • Design of the system is structured around the objects in a system • Objects are the “things” the system manages or uses to accomplish its goals • Object == Instance

Objects • An object is made of two types of things: – What it knows • stored in “instance variables” – What it does • coded as methods

How do we Build Objects? • A “class” is template for objects of a particular type • So a class must contain: – instance variables • variables that are not inside a method • each instance of the class gets its own copy of those instance variables to that it can hold the information it needs – methods • encoding the operations the objects must be able to do

The Over Used Word “Class” • We use the word “class” to mean many things: – the java code that is a template for building objects of a particular type – a non-primitive type that we are defining – the set of objects of the same type

What a Class Looks Like public class Account { private double balance; Instance Variables /** * Deposit a certain amount into the account * @param deposit. Amount the amount to deposit */ public void deposit(double deposit. Amount) { balance = balance + deposit. Amount; } /** * Get the current balance of the account * @return the current balance */ public double get. Balance() { return balance; } } Method to deposit money into the account Method to see how much money is in the account

Creating and Using Objects • Show this in Eclipse: public class Runner { public static void main(String[] args) { Account savings; Account ira; savings = new Account(); savings. deposit(32. 13); ira = new Account(); ira. deposit(2324. 23); ira. deposit(333. 22); System. out. println("Savings balance: " + savings. get. Balance()); System. out. println("IRA balance: " + ira. get. Balance()); } }

Classes in Our Lab • We want to build the start of a card playing game (up through shuffling the cards) • The objects we need: – 52 card objects – 1 deck object

Card Class • A card needs to know two things: – its face value and its suit – we’ll store both as ints • face value: 0 = Ace, 1 = 2, . . . 9 = 10, 10 = Jack, 11 = Queen, 12 = King • suit: 0 = spades, 1 = hearts, 2 = diamonds, 3 = clubs • A card needs three operations – one to retrieve its face value – one to retrieve its suit – one to build a description of this card • Find these things in the code in Eclipse

Creating a Card • It doesn’t make sense to have a card with no face value or suit • If we don’t initialize them, primitive instance variables default to zero, so every card would start out as the Ace of Spades • We’d like a way to initialize the card when we create it

Constructors • Special methods that are used only for the creation of objects • We know a method is a constructor if – it has no return value AND – its name matches the name of the class

Our Constructor and Its Use /** * Create a new * * @param v the * @param s the */ public Card(int { face. Value = suit = s; } card with a given suit and value face value of the card (0 - 12) suit of the card (0 - 3) v, int s) v; Card c 2; c 2 = new Card(12, 3); What card will this create?

Card. Runner – What’s that main method? • Look at Card. Runner class • method with this declaration: – public static void main(String[] args) • This is the method that the Java Virtual Machine runs when you run your program • We’ve seen this method in all of our previous labs – we just didn’t explain what it is!

Types of Classes • We now have two types of classes – Runnable: has a main() method and can be run by the Java Virtual Machine – Instantiable: is a template for creating objects • Has a constructor • if we don’t declare a constructor, the compiler will give us the default (no parameters) constructor that allocates the space, but doesn’t initialize the instance variables – they default to zeros for the primitive types we’ve studies and nulls (no pointer) for reference types)

to. String() • We often convert objects to Strings – for example, when we concatenate them into output statements • Java gives every class a method to do this – provides default conversion • <class name> @<heap address> • Card@E 0352 • We can make our own to replace the default – must be declared as: • public String to. String()

Converting our int to a Suit • We’d like the description of a card to have “Spades” instead of “ 0” • Make an array that encodes that conversion: SUIT_DESCRIPTION: String “Spades” String “Hearts” String “Diamonds ” String “Clubs”

In the Code private static final String[] SUIT_DESCRIPTION = { "Spades", "Hearts", "Diamonds", "Clubs" }; SUIT_DESCRIPTION[suit] What will this evaluate to if suit has a value of 0? If suit has a value of 3?

The Deck Class • Instance of deck need to know: – what cards they have and what order they are in • an array of type Card • Instance of deck need to do: – output the cards in order (to. String()) – shuffle

Deck Constructor /** * Create the deck by filling it with the appropriate cards */ public Deck() { cards = new Card[NUMBER_OF_CARDS]; int position = 0; for (int suit = 0; suit < NUMBER_OF_SUITS; suit++) { for (int face. Value = 0; face. Value < NUMBER_OF_CARDS/NUMBER_OF_SUITS; face. Value++) { cards[position] = new Card(face. Value, suit); position++; } } }