Programs and Classes

1. 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.

2. 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.

3. 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

4. The class name appears on top of the icon. The object’s name appears on top of the icon. An icon for a class is the rectangle. An icon for an object is the rounded rectangle. The class name is placed inside the object icon. Graphical Representation Account SV129 Account

5. Employee Before you can create instances of a class, the class must be defined. The dotted line shows the instance-of relationship. Steve Bill Andy The class name can be omitted since it is clear which class these objects belong to . Employee Employee Employee Instance-of Relationship

6. 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.

7. In general, be private (military demotion) • Make class components private whenever you can • This supports the notion of encapsulation, which makes for more robust software development

8. Class and Instance Data Values • A class data value (indicated by the static modifier) is used to maintain information shared by all instances or aggregate information about the instances. • An instance data value is used to maintain information specific to individual instances. • Make instance data values private always

9. minimum balance There is one copy of minimum balance for the whole class and shared by all instances. SV506 SV129 SV008 100.00 Account Account Account current balance current balance current balance 1304.98 908.55 354.00 Sample Data Values Account All three Account objects possess the same instance data value current balance.

10. 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

11. 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.

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

13. Message deposit with the argument 250.00 is sent to chk-008. chk-008 Account deposit 250.00 deposit Message name is usually omitted in the diagram. 250.00 deposit Sending a Message

14. This message has no argument. chk-008 Account getMonthlyFee monthly fee The method returns the value monthly fee back to the message sender. Getting an Answer

15. Account getAverageBalance average balance The average balance of all accounts is returned. Calling a Class Method

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

17. 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

18. 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.