1. Based on Java Software Development, 5th Ed. By Lewis &Loftus
4. Writing Classes
Based on Java Software Development, 5th Ed.
By Lewis &Loftus

2. Topics Anatomy of a Class Encapsulation Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields

3. User-defined Classes Java program consists of a set of classes.
One class must contain a method namd main()—which becomes the starting point of the program.
We have been using classes from the Java Standard Library (API).
You will now write your own classes.

4. Classes & Objects Class has E.g., to represent dice in a game program:
Name
State (attributes)
Behavior (methods)
E.g., to represent dice in a game program:
Name: Die (singular of dice)
Attributes: MAXVALUE, faceValue
Methods: constructor, roll(), toString(), getFacevalueValue()

5. Classes A class can contain data declarations and method declarations
MAXVALUE, faceValue
Data declarations
Method declarations

6. Classes & Objects Necessary methods RollingDice.java Die.java
Constructor – to create an object of Die class
toString – returns the String value that represents the object in some way
getFaceValue – to return the current face value of a die
RollingDice.java
Die.java

7. Constructor Method
Constructor method(s) is used to instantiate objects.
It can set initial values for objects.
It has the same name as the class name.
It has no return type.
E.g., Die(){…} Student (String name, int age){…}

8. toString() Method
Every class should include a toString() method, which returns some String value representing an object of the class.
It is called automatically when an object is passed to the System.out.println() mthod.
E.g., String toString() { return “Die object”; }

9. Scope of Data Scope of data
Area in the program where data can be referenced (can be used)
Data declared at the class level can be reference in all methods of the class (global visibility).
Data declared in a method can be referenced only in that method (local visibility).

10. Instance Data
In class Die, faceValue is called instance data, because each instance of Die maintains its own memory for faceValue with a value.
Each instance (object) of class Die shares its methods, but maintains its own data space.
Every time class Die is instantiated, a new memory for faceValue is allocated.

11. UML Diagram
UML (Unified Modeling Language) diagrams show relationships among classes and objects.
RollingDice
main (args : String[]) : void
Die
faceValue : int
Die() roll() : int
setFaceValue (int value) : void
getFaceValue() : int
toString() : String

12. Topics Anatomy of a Class Encapsulation Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields

13. Encapsulation Two views of a class Interface
Internal (all variables and code visible)
External (only public elements are visible)
Interface
Method names with parameters—no internal details of method body
Methods and instance variables are encapsulated – combined into a single entity, for the purpose of
Information hiding
Data and behavior abstraction

14. Visibility Modifiers
Modifier specifies some characteristic of method or data—e.g., final.
Visibility Modifiers
public
Can be referenced from anywhere
protected
Can be referenced from subclasses (derived from a class)
private
Visible only from within the class
default
Visible to all classes within the same package

15. Visibility Modifiers Instance variables Methods Constants
Should not be declared public
Should not be accessible directly
Should be accessed or modified via public methods
Methods
that provide services should be public
that provide support for methods should not be public
Constants
Can be public, because they cannot be modified

16. Accessors and Mutators
Given: class Fraction
Accessors
Methods which return the value of an instance variable
E.g., getNumerator(), getDenominator()
Mutators
Mehtods which change the value of an instance variable
E.g., setNumerator(), setDenominator()
Required for each instance variable

17. Topics Anatomy of a Class Encapsulation Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields

18. Method Header A method declaration begins with a method header
boolean isEven (int num)
return
type
method
name
parameter list
The parameter list specifies the type
and name of each parameter
The name of a parameter in the method
declaration is called a formal parameter

19. Method Body The method header is followed by the method body
Boolean isEven (int num)
result is a local variable.
It is created each time the method is called, and is destroyed when it finishes executing
{ boolean result;
if (num % 2 == 0) result = true; else result = false; return result; }
The return expression must be consistent with the return type

20. Invoking a Method If (isEven(a * b - c)){ … } else{ … }
Actual parameter
boolean isEven (int num)
Formal parameter
{ boolean result;
if (num % 2 == 0) result = true; else result = false; return result; }
The value of actual parameter is copied to the formal parameter

21. Example Programs
Transactions.java
Account.java

22. Topics Anatomy of a Class Encapsulation Anatomy of a Method
Graphical Objects
Graphical User Interfaces
Buttons and Text Fields