The Java Programming Language

1. The Java Programming Language • Simple – but abstract • Safe • Platform-independent ("write once, run anywhere") • Has a Rich growing library • Designed for the internet (applets and java scripts ) Primary slides from Horstmann

2. Becoming Familiar with your Computer • Login • Locate the Java compiler - javac • Understand files and folders • Write a simple program (later) • Save your work

3. An Integrated Development Environment

4. File Hello.javasource code 1 public class Hello 2 { 3 public static void main(String[] args) // implies // application class 4 { 5 // display a greeting in the console window 6 System.out.println("Hello, World!"); 7 } 8 }

5. A simple program • public class ClassName • public static void main(String[] args) • // comment • Method call object.methodName(parameters) • System class • System.out object • println method

6. Syntax : Method Call object.methodName(parameters ) • Example: • System.out.println("Hello”); Purpose: • To invoke a method of an object and supply any additional parameters

7. Compiling and Running • Type program into text editor • Save • Open command shell • Compile into byte codesjavac Hello.java • Execute byte codesjava Hello

8. From Source Code to Running Program

9. The Edit-Compile-Test Loop

10. Objects and Classes • Object: entity that you can manipulate in your programs (by invoking methods) • Each instant object belongs to a class • Object Class: Set of objects with the same behavior – each object of the class is called an instance object

11. Rectangle Class • Construct a rectangle: an instance of the Rectangle classnew Rectangle(5, 10, 20, 30)new Rectangle() • Use the constructed objectSystem.out.println(new Rectangle(5, 10, 20, 30));prints java.awt.Rectangle[x=5,y=10,width=20,height=30]

12. Rectangle Shapes

13. A Rectangle Object

14. Syntax : 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.

15. 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;

16. Uninitialized and Initialized Variables Uninitialized Initialized

17. Two Object Variables Referring to the Same Object

18. Syntax : 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

19. Syntax : Importing a Class from a Package • Import packageName.ClassName ; • Example: • import java.awt.Rectangle; • Purpose: • To import a class from a package for use in a program

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

21. 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{ }

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

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

24. …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

25. 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

26. 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()); } }

27. 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.

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

29. 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)

30. 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;}

31. 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

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

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

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

35. Syntax : Constructor Implementation • accessSpecifier classClassName{ ... accessSpecifier ClassName(parameterTypeparameterName ...) { constructor implementation } ... }

36. 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

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

38. 10 String message = "Hello, " + name + "!"; 11 return message; 12 } 13 14 private String name; 15 }

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

40. 8 Greeter daveGreeter = new Greeter("Dave"); 9System.out.println(daveGreeter.sayHello()); 10 } 11 }

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

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

43. 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);

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

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

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

47. Javadoc Method Summary

48. Javadoc Method Detail

49. BankAccount Class Implementation • Determine instance variables to hold object stateprivate double balance • Implement methods and constructors

50. 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 {