AN INTRODUCTION TO OBJECTS AND CLASSES CHAPTER 2 AN INTRODUCTION TO OBJECTS AND CLASSES

Chapter Goals To understand the concepts of classes and objects To realize the difference between objects and object references To become familiar with the process of implementing classes To be able to implement simple methods To understand the purpose and use of constructors To understand how to access instance fields and local variables To appreciate the importance of documentation comments

Objects and Classes Object: entity that you can manipulate in your programs (by invoking methods) Each object belongs to a class Class: Set of objects with the same behavior Class determines legal methods "Hello".println() // Error "Hello".length() // OK

Rectangle Class Construct a rectangle: new Rectangle(5, 10, 20, 30) new Rectangle() Use the constructed object System.out.println(new Rectangle(5, 10, 20, 30)); prints java.awt.Rectangle[x=5,y=10,w idth=20,height=30]

Rectangle Shapes

A Rectangle Object

Syntax 2.1: Object Construction new ClassName(parameters) Example: new Rectangle(5, 10, 20, 30) new Car("BMW 540ti", 2004) Purpose: To construct a new object, initialize it with the construction parameters, and return a reference to the constructed object.

Object Variables Declare and optionally initialize: Rectangle cerealBox = new Rectangle(5, 10, 20, 30); Rectangle crispyCrunchy; Apply methods: cerealBox.translate(15, 25); Share objects: r = cerealBox;

Uninitialized and Initialized Variables

Two Object Variables Referring to the Same Object

Syntax 2.2: Variable Definition TypeName variableName; TypeName variableName = expression; Example: Rectangle cerealBox; String name ="Dave"; Purpose: To define a new variable of a particular type and optionally supply an initial value

Writing a Test Program Invent a new class, say MoveTest Supply a main method Place instructions inside the main method Import library classes by specifying the package and class name: import java.awt.Rectangle; You don't need to import classes in the java.lang package such as String and System

Syntax 2.3 : Importing a Class from a Package importpackageName.ClassName ; Example: import java.awt.Rectangle; Purpose: To import a class from a package for use in a program

File MoveRect.java 1 import java.awt.Rectangle; 2 3 public class MoveTest 4 { 5 public static void main(String[] args) 6 { 7 Rectangle cerealBox = new Rectangle(5, 10, 20, 30); 8 // move the rectangle 9 cerealBox.translate(15, 25); 10 // print the moved rectangle 11 System.out.println(cerealBox); 12 }

A Simple Class public class Greeter { public String sayHello() { String message ="Hello,World!"; return message; } }

Method Definition access specifier (such as public) return type (such as String or void) method name (such as sayHello) list of parameters (empty for sayHello) method body in { }

Method Parameters public class Rectangle { . . . public void translate(int x, int y) { method body } . . . }

Syntax 2.4: Method Implementation public class ClassName { ... accessSpecifier returnType methodName(parameterType parameterName,...) { method body } ... } …Continue

…Continue Example: public class Greeter { public String sayHello() { String message ="Hello,World!"; return message; } } Purpose: To define the behavior of a method A method definition specifies the method name, parameters, and the statements for carrying out the method's actions

Syntax 2.5: The return Statement return expression; or return; Example: return message; Purpose: To specify the value that a method returns, and exit the method immediately. The return value becomes the value of the method call expression.

Testing a Class Test class: a class with a main method that contains statements to test another class. Typically carries out the following steps: Construct one or more objects of the class that is being tested. Invoke one or more methods. Print out one or more results

A Test Class for the Greeter Class public class GreeterTest { public static void main(String [] args)) { Greeter worldGreeter = new Greeter(); System.out.println(worldGreeter.sayHello()); }

Building a Test Program 1. Make a new subfolder for your program. 2. Make two files, one for each class. 3. Compile both files. 4. Run the test program.

Testing with the SDK Tools mkdir greeter cd greeter edit Greeter.java edit GreeterTest.java javac Greeter.java javac GreeterTest.java java GreeterTest

Testing with BlueJ

Instance Fields public class Greeter { ... private String name; } access specifier (such as private) type of variable (such as String) name of variable (such as name)

Instance Fields

Accessing Instance Fields The sayHello method of the Greeter class can access the private instance field: public String sayHello() { String message = "Hello, " + name + "!"; return message; }

Other methods cannot: public class GreeterTest { public static void main(String[] args) { . . . System.out.println(daveGreeter. name); // ERROR } } Encapsulation = Hiding data and providing access through methods

Syntax 2.6 : Instance Field Declaration accessSpecifier class ClassName { ... accessSpecifier fieldType fieldName; ... }

Example: public class Greeter { ... private String name; ... } Purpose: To define a field that is present in every object of a class

Constructors A constructor initializes the instance variables Constructor name = class name public class Greeter() { public Greeter(String aName) { name = aName; } . . . } Invoked in new expression new Greeter("Dave")

Syntax 2.7 : Constructor Implementation accessSpecifier class ClassName { ... accessSpecifier ClassName(parameterType parameterName ...) { constructor implementation } ... }

Example: public class Greeter { ... public Greeter(String aName) { name = aName; } ... } Purpose: To define the behavior of a constructor, which is used to initialize the instance fields of newly created objects

File Greeter.java 1 public class Greeter 2 { 3 public Greeter(String aName) 4 { 5 name = aName; 6 } 7 8 public String sayHello() 9 {

10 String message = "Hello, " + name + "!"; 11 return message; 12 } 13 14 private String name; 15 }

File GreeterTest.java 1 public class GreeterTest 2 { 3 public static void main(String[] args) 4 { 5 Greeter worldGreeter = new Greeter("World"); 6System.out.println(worldGreeter. sayHello()); 7

8 Greeter daveGreeter = new Greeter("Dave"); 9System.out.println(daveGreeter.s ayHello()); 10 } 11 }

Designing the Public Interface Behavior of bank account: deposit money withdraw money get balance Methods of BankAccount class: deposit withdraw getBalance

BankAccount Public Interface public BankAccount() public BankAccount(double initialBalance) public void deposit(double amount) public void withdraw(double amount) public double getBalance()

Using the Public Interface Transfer balance double amt = 500; momsSavings.withdraw(amt); harrysChecking.deposit(amt); Add interest double rate = 5; // 5% double amt = acct.getBalance() * rate / 100; acct.deposit(amt);

Commenting the Public Interface /** Withdraws money from the bank account. @param the amount to withdraw */ public void withdraw(double amount) { implementation filled in later }

/. Gets the current balance of the bank account /** Gets the current balance of the bank account. @return the current balance */ public double getBalance() { implementation filled in later }

Class Comment /** A bank account has a balance that can be changed by deposits and withdrawals. */ public class BankAccount { ... }

Javadoc Method Summary

Javadoc Method Detail

BankAccount Class Implementation Determine instance variables to hold object state private double balance Implement methods and constructors

File BankAccount.java 1 /** 2 A bank account has a balance that can be changed by 3 deposits and withdrawals. 4 */ 5 public class BankAccount 6 {

7 /** 8 Constructs a bank account with a zero balance 9 */ 10 public BankAccount() 11 { 12 balance = 0; 13 } 14

15 /** 16 Constructs a bank account with a given balance 17 @param initialBalance the initial balance 18 */ 19 public BankAccount(double initialBalance) 20 { 21 balance = initialBalance; 22 }

23 24 /** 25 Deposits money into the bank account. 26 @param amount the amount to deposit 27 */ 28 public void deposit(double amount) 29 { 30 double newBalance = balance + amount; 31 balance = newBalance; 32 } 33

34 /** 35 Withdraws money from the bank account. 36 @param amount the amount to withdraw 37 */ 38 public void withdraw(double amount) 39 { 40 double newBalance = balance - amount; 41 balance = newBalance; 42 } 43

44 /** 45 Gets the current balance of the bank account. 46 @return the current balance 47 */ 48 public double getBalance() 49 { 50 return balance; 51 } 52 53 private double balance; 54 }

File BankAccountTest.java 1 /** 2 A class to test the BankAccount class. 3 */ 4 public class BankAccountTest 5 { 6 /** 7 Tests the methods of the BankAccount class. 8 @param args not used 9 */

10 public static void main(String[] args) 11 { 12 BankAccount harrysChecking = new BankAccount(); 13 harrysChecking.deposit(2000) 14 harrysChecking.withdraw(500) 15 System.out.println(harrysChecking.getBalance()); 16 } 17 }

Calling a Method in BlueJ

The Method Return Value in BlueJ

Variable Types Instance fields (balance in BankAccount) Local variables (newBalance in deposit method) Parameter variables (amount in deposit method)

Explicit and Implicit Parameters public void withdraw(double amount) { double newBalance = balance - amount; balance = newBalance; } balance is the balance of the object to the left of the dot: momsSavings.withdraw(500) means double newBalance = momsSavings.balance - amount; momsSavings.balance = newBalance;