DON’T PANIC!!

1. DON’T PANIC!! • Lots of new notions coming in these slides • Don’t worry if not all of it makes perfect sense • We’ll meet most of this stuff again in detail later • Do worry if none of it makes any sense • You should get the general picture now • Now brace yourself … stop me if you get confused … ask questions … throw money …

2. Programs and Classes • A program is made up from classes • Classes may be grouped into packages • A class has two parts • static parts exist independently • Non-static parts define what objects in the class look like. • Every class is automatically in existence when the program runs.

3. Classes and Objects • An object is an instance of a class, and is created using the new operator. • The non-static part of the class defines what each object looks like. • Many instances (objects) can be created from a class … no limit except reality • An object contains information and functionality of a “thing”, e.g., Account, Vehicle, Employee, etc.

4. Classes’ and Objects’ Components • Classes (and thus also objects) are composed of methods and data values • Data values store information • Methods do things, and also have their own local data

5. Visibility Modifiers: public and private • The modifiers public and private designate the accessibility of objects’ and class’ data values and methods • If a component is declared private, nothing outside the class can access it. • If a component is declared public, anything outside the class can access it. • Make things private whenever you can • That helps with encapsulation

6. Class and Instance Data Values • Class data (indicated by the static modifier) is used to maintain information shared by all instances or aggregate information about the instances. • Make class data private whenever you can • Instance data is used to maintain information specific to individual instances. • Make instance data private always

7. Data Type reference primitive long String byte short Applet MessageBox double char InputBox int HiLo boolean etc. float Primitive and Reference Data Values • primitive variables contain values • Reference variables point at objects

8. minimum balance account prefix 100.00 6427 Account SV129 Account current balance opening balance 908.55 100.00 Variable and Constant Data Values There are two types of data values: A variable whose value can change over time. A constant whose value must remain fixed over time. • Constants are indicated by the final modifier • Non-final public static data can give you warts

9. Methods • Methods have code (to do stuff) and data • A method defined for a class is called a class method (indicated by the static modifier) and a method defined for an object is called an instance method. • Every program has one static method called main. That’s where the program starts.

10. Method Data = Local Variables • A local variable is a variable that is declared within a method. • Local variables are accessible only in the method in which they are declared.

11. Calling Methods • To instruct a class or an object to do something, we call one of its methods • Values passed to a method are called arguments or parameters of the message. • The (formal) parameters of a method are local variables that receive the parameters • Methods can return one data value

12. Call deposit with the argument 250.00. chk-008 deposit 250.00 deposit Calling a Method

13. This call has no argument. chk-008 getMonthlyFee monthly fee The method returns the value monthly fee. Getting an Answer

14. Program Components • A Java file is composed of • comments, • import statements, and • class declarations.

15. Comment Program Component: Comment // Program MyFirstApplication /* This program displays a window on the screen. The window is positioned at the center of the screen, and the size of the window is almost as big as the screen. */ import javabook.*; public class MyFirstApplication { public static void main(String[ ] args) { MainWindow mainWindow; mainWindow = new MainWindow(); mainWindow.setVisible( true ); } }

16. Import Statement Program Component: Import Statement // Program MyFirstApplication /* This program displays a window on the screen. The window is positioned at the center of the screen, and the size of the window is almost as big as the screen. */ import javabook.*; public class MyFirstApplication { public static void main(String[ ] args) { MainWindow mainWindow; mainWindow = new MainWindow(); mainWindow.setVisible( true ); } }

17. Class Declaration Program Component: Class Declaration // Program MyFirstApplication /* This program displays a window on the screen. The window is positioned at the center of the screen, and the size of the window is almost as big as the screen. */ import javabook.*; public class MyFirstApplication { public static void main(String[ ] args) { MainWindow mainWindow; mainWindow = new MainWindow(); mainWindow.setVisible( true ); } }

18. Method Declaration Program Component: Method Declaration // Program MyFirstApplication /* This program displays a window on the screen. The window is positioned at the center of the screen, and the size of the window is almost as big as the screen. */ import javabook.*; public class MyFirstApplication { public static void main(String[ ] args) { MainWindow mainWindow; mainWindow = new MainWindow(); mainWindow.setVisible( true ); } }

19. Modifier Modifier Return Type Method Name Parameter Method Body Method Declaration Elements public static void main (String[ ] args){ MainWindow mainWindow; mainWindow = new MainWindow(); mainWindow.setVisible( true ); }

20. Statements • Method bodies contain statements • Simple statements end with a ; • Compound statements are enclosed in {}s • public static void main (String[] args) { • int someData = 0; • if (someData == 27) { • System.out.println(“Cosmic rays!”); • someData = 0; • } • }

21. Parameter Local Variables Sample Method public double fromDollar( double dollar ) { double amount, fee; fee = exchangeRate - feeRate; amount = dollar * fee; return amount; }

22. Files and Classes • A Java program file ends with .java • There must be one public class per file • It must have the same name as the file • One public class (i.e., one file) must have the main method

23. Simple Java Programs • Simple Java programs can be written in just the one file, containing • One public class (with the main method) • Other class methods and final data values as required • Such programs do not create any objects, but simply run class methods (starting with the main method) and use primitive data.

24. center of the screen, and the size of the window is almost as big as the screen. */ import javabook.*; public class MyFirstApplication { public static void main(String[ ] args) { MainWindow mainWindow; mainWindow = new MainWindow(); mainWindow.setVisible( true ); } } Comment Import Statements Class Name Method Body Template for Simple Java Applications

25. DON’T PANIC!! • We’ll write some programs without creating objects, i.e., you’ll have to think about only classes (and their methods and data values) • We’ll write some programs that create objects from pre-existing classes, i.e., you’ll use objects before you have write code to define them. • Then we’ll write programs that define and create their own objects