Chapter 8Objects & Classes

Definition of Object-Oriented Programming (OOP)Object-Oriented Programming (OOP) uses the analogy of real objects as a template for creating computer programs involving those objects.

For example, we all know what a circle is. A circle can be defined by its radius and the location of its center. A circle has an area that is strictly a function of its radius.

We can create a class that defines a circle. For now we will not concern ourselves with position.

It is important to note that a class is not a program. There is no main method. A class is used by an executable program.

class Circle{ double radius; Circle(double rad) { radius = rad; } double getArea() { return radius*radius*Math.PI; }}

this is called a constructor ofthe Circle class. It has the samename as the class and it must define all properties of the class

this is a property or attributeof the Circle class

this is an action or method ofthe Circle class

Constructors

class Circle{ double radius; Circle(double rad) { radius = rad; } double getArea() { return radius*radius*Math.PI; }}

Constructors are a special kind of method that initialize objects. Constructors have the following characteristics:

A constructor must have the same name as its class. Constructors do not have a return type, or void. Constructors are invoked using the new operator.

Reference Variables and Reference Types

Circle myCircle;

myCircle = new Circle();

Circle myCircle = new Circle();

A class is similar to a user-defined type

myCircle is declared as a Circle type. At this point myCircle is a reference to objects of type Circle.

We can also create an instance of the Circle class by using new.

Accessing an Object’s Data and Methods

After an object is created, its data can be accessed and its methods invoked using the dot operator (.), also known as the object member access operator.

myCircle.radius

myCircle.getArea()

returns the radius of myCircle as a double

computes and returns the area of myCircle

public class objtest1{ public static void main(String[] args) { Weasel myWeasel = null; aWeasel = new Weasel(1,2,10.4);

if(myWeasel == null) System.out.println("Yes"); else System.out.println("No"); }}

class Weasel{ int color; int age; double weight; Weasel(int col, int a, double w) { color = col; age = a; weight = w; }}

Pointing to Nowhere (null)

myWeasel null

Weasel

color: int = 1age: int = 2 weight: double = 10.4

aWeasel

Weasel myWeasel = null;

Weasel myWeasel = new Weasel(1,2,10.4);

class test{ public static void main(String[] args) { Student a_student = new Student(); System.out.println("name = " + a_student.name); System.out.println("age = " + a_student.age); System.out.println("gender = " + a_student.gender); }}

class Student{ String name; int age; boolean isScienceMajor; char gender;}

Reference Types can Point to Null

name = nullage = 0gender = 00

Unified Modeling Language (UML) Diagrams

The illustration of class templates and objects in Figure 8.2 can be standardized using UML (Unified Modeling Language) notations. This notation is called a UML class diagram, or simply a class diagram.

In the class diagram, the data field is denoted as dataFieldName: dataFieldType

The constructor is denoted as ClassName(parameterName: parameterType)

The method is denoted as methodName(parameterName: parameterType): returnType

Instance vs Static Classes

We have used several of the methods provided in the Math class in our programs. These methods are invoked by calling the class name followed by the method using dot notation. For example:

Y = Math.sqrt(X);rnd = Math.random();maxval = Math.max(Math.max(a,b),Math.max(c,d));

More recently we have been creating instances of classes and then accessing the attributes and methods of the object class using the same dot notation. For example:

Circle circ = new Circle(10.0);System.out.println("A circle of radius " + circ.radius +

" has an area = " + circ.getArea());

It is important to note that in the first case we cannot create an instance of the Math class and in the second we access the methods and attributes only through an instance of the class.

The methods in the Math class are referred to as static methods while those in an instance class are called instance methods.

public class StaticInstanceDemo{ public static void main(String[] args) { Thing thing1 = new Thing(1); Thing thing2 = new Thing(2); System.out.println("thing1.x = " + thing1.x); System.out.println("thing2.x = " + thing2.x); System.out.println("thing1.y = " + thing1.y); System.out.println("thing2.y = " + thing2.y); thing1.changex(42); thing1.changey(99); System.out.println("thing1.x = " + thing1.x); System.out.println("thing2.x = " + thing2.x); System.out.println("thing1.y = " + thing1.y); System.out.println("thing2.y = " + thing2.y); }}

class Thing{ public static int x = 0; int y; Thing(int a) { y = a; } public void changey(int newy) { y = newy; } public void changex(int newx) { x = newx; } int getx() { return x; } int gety() { return y; }}

1thing1.x = 0thing2.x = 0thing1.y = 1thing2.y = 2

thing1.x = 42thing2.x = 42thing1.y = 99thing2.y = 2

Static vs Instance Revealed

An Example: The Card Deck

Problem: Build a model of a deck of playing cards that can be used in a Java program.

We can define a card object in an instance class as part of a model for a deck of cards. While many of the details of the Card class will depend on what we want to do with the card deck, there are a few important attributes common to most card-playing applications.

suit - Hearts, Clubs, Diamonds, Spades

rank - Ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, King

value - 1, 2, 3, 4, 5, 6, 7, 8, 9, 10

public class Card{ String suit; String rank; int value; Card(String s, String r, int v) { suit = s; rank = r; value = v; }}

Card constructor

To create instances of the 52 cards in a deck of playing cards using the Card class, we need to specify the suit and the rank of each card. We could do this by simply writing 52 declarations, each one explicitly giving the string names for suit and rank.

This would be a lot of work and we would have 52 variables that would be difficult to use in a program. Alternatively we could generate an indexed list of Card type objects.

This creates a list of 52 references to Card types but it doesn't create instances of cards. We will still have to declare each of them separately. However, this time we can use a for-loop to save ourselves a lot of typing.

Card card_1 = new Card("Heart","Ace",1);Card card_2 = new Card("Heart","2",2); :Card card_51 = new Card("Spade","Queen",10);Card card_52 = new Card("Spade","King",10);

Card[] card = new Card[52];

Generating the Cards

A Bit of Computer ScienceIn order to avoid generating 52 separate declarations, we need a way to specify the suit and rank of each card in a for-loop. The loop index will range from 0 to 51. We can use this value to generate a suit number (0-3) and a rank number (1-13).

We can generate all the cards of the same suit first or all cards of the same rank first. In each case we can use integer division and modulo to convert the loop index into a suit number and a rank number.

int suitnum, ranknum;

for(int i = 0; i<52; i++){ suitnum = i/13; ranknum = i%13 + 1;}

int suitnum, ranknum;

for(int i = 0; i<52; i++){ ranknum = i/4 + 1; suitnum = i%4;}

i suitnum ranknum 0 0 1 1 0 2 2 0 3 :50 3 1251 3 13

i suitnum ranknum 0 0 1 1 1 1 2 2 1 :50 2 1351 3 13

Converting Numbers to Strings

The constructor for the Card class needs strings for the name of the suit and the rank. We can use conditional statements that choose the strings based on the values of the suit number and the rank number.

switch (suitnum){ case 0: suit = "Club"; break; case 1: suit = "Spade"; break; case 2: suit = "Heart"; break; case 3: suit = "Diamond"; break;}

if(ranknum==1) rank = "Ace";if(ranknum>1 & ranknum<10) rank = Character.toString((char)(ranknum + 48));if(ranknum == 10) rank = "10";if(ranknum == 11) rank = "Jack";if(ranknum == 12) rank = "Queen";if(ranknum == 13) rank = "King";

value = 10;if(ranknum<=9) value = ranknum;

deck[i] = new Card(suit,rank,value);

public class Deck{ String suit = ""; String rank = ""; int suitnum, ranknum, value; Card[] card = new Card[52]; int topcard; Deck() { topcard = 0; for(int i = 0;i<card.length;i++) { suitnum = i%4; ranknum = i/4 + 1; value = 10; if(ranknum<=9) value = ranknum; switch (suitnum) { case 0: suit = "Club"; break; case 1: suit = "Spade"; break; case 2: suit = "Heart"; break; case 3: suit = "Diamond"; break; }

if(ranknum==1) rank = "Ace"; if(ranknum>1 & ranknum<10) rank = Character.toString((char)(ranknum + 48)); if(ranknum == 10) rank = "10"; if(ranknum == 11) rank = "Jack"; if(ranknum == 12) rank = "Queen"; if(ranknum == 13) rank = "King";

card[i] = new Card(suit,rank,value); } } public void shuffle() { Card tmpcard; topcard = 0; for(int i=0;i<1000;i++) { int k,m; k = (int)(Math.random()*52.0); m = (int)(Math.random()*52.0); tmpcard = card[k]; card[k] = card[m]; card[m] = tmpcard; } } public Card dealCard() { Card the_card; the_card = card[topcard]; if(topcard<51) topcard += 1; return the_card; }}

We have written several programs using classes, most of which leave the data value types as public.

There are times which we do not want to allow direct access to the data values inside a class. In these cases we can simply declare them a private.

If we wish to permit limited access to them we can include accessors in the class.

If we wish to allow the user of the class to see the value we create a get accessor. If we also want to allow the class user to change the value we create a set accessor.

public class Circle3 { private double radius = 1; private static int numberOfObjects = 0;

public Circle3() { numberOfObjects++; } public Circle3(double newRadius) { radius = newRadius; numberOfObjects++; }

public double getRadius(double newRadius) { return radius; } public void setRadius(double newRadius) { if(radius>=0) radius = newRadius; else radius = 0.0; }

public static int getNumberOfObjects() { return numberOfObjects; } public double getArea() { return radius * radius * Math.PI; }}

Data Encapsulation

public class TestCircle3 { public static void main(String[] args) { Circle3 Circ = new Circle3(5.0); System.out.println("The area of the circle of radius " + Circ.getRadius() + " is " + Circ.getArea());

Circ.setRadius(myCircle.getRadius() * 1.1); System.out.println("The area of the circle of radius " + Circ.getRadius() + " is " + Circ.getArea());

System.out.println("The number of objects created is " + Circle3.getNumberOfObjects()); }}

Testing the Circle3 Class

The data field radius is declared private. Private data can be accessed only within their defining class. You cannot use Circ.radius in the client program. A compile error would occur if you attempted to access private data from a client.

Since numberOfObjects is private, it cannot be modified. This prevents tampering. For example, the user cannot set numberOfObjects to 100. The only way to make it 100 is to create 100 objects of the Circle3 class.

public class Deck{ String suit = ""; String rank = ""; int suitnum, ranknum, value;

Card[] card = new Card[52];

int topcard; : : Deck() { topcard = 0; for(int i = 0;i<card.length;i++) { suitnum = i%4; ranknum = i/4 + 1; : : card[i] = new Card(suit,rank,value);}

Array of Objects

Declaration of an array of objects

Initialization of the array of objects

Sometimes it is convenient to create an array of objects. The declaration of the array creates an array of reference types but does not instantiate any of the objects. This is done in a separate statement calling the class constructor.