Classes

1
Classes

• Variables That Are Not of a Built-in Type Are
  Objects
• A Class Is an Object Type
• A Class Represents a Thing (e.g., employee,
  wrench, list, store, shopping cart, etc.)
• Service Class Class used by Other Programs
• Must Be Created in ClassName.java File

2
Generic Object Declaration

• class useObject
• public static void main(String args)
• Object varName new Object()//instance
• varName.objectMethod() //invoke method
• Instance of Object Type Is Called Invoking Object
  of Class Methods
• varName is an instance of Object
• varName is invoking object of objectMethod()

3
Creating a Class

• Programmers Define Classes with
• Data Members (or Fields, Attributes)
• Used to store information about each object
• Each member is a variable of some other type
• Methods
• Used to give programmers access to object

4
Designing a Class

• Decide How It Will Be Used
• Decide on Interface (i.e., public representation,
  public methods)
• Decide on Implementation (i.e., private data and
  data manipulation)
• Example String Methods (interface) includes
  length(), toUpper(), toLower(), charAt(), etc.
• Example String Implementation includes
  (probably) char variables to hold characters,
  integer to hold length (we dont actually need to
  know)

5
Constructor

• Class Method of Same Name
• Example class Employee Method Employee()
• Called When Variable Declared (instantiated) of
  Class Type
• Initializes Instantiated Object
• May Have Multiple Constructors Each with
  Different Parameters

6
Access Modifiers

• Used to Identify What May Use Methods
• public any method may use
• private only methods in same class may use
• protected only methods in same class or related
  classes may use.

7
Example Class Employee
Employee emp1
String firstName
String lastName
double salary
First and last names and salaries are data
members, or fields or attributes.
8

• //Employee.java file
• class Employee
• private String firstName
• private String lastName
• private double salary
• private final double MAXSALARY 2000000
• public Employee()
• // access to invoking object attributes
• firstName "NoFirstName"
• lastName "NoLastName"
• salary 0.0

9
Employee Declaration
Employee emp1 new Employee()
Main Memory (allocated by new, initialized by
Employee() ) emp1
String firstName
"NoFirstName"
String lastName
"NoLasttName"
double salary
0.0
10
Client Program

• A Program that Uses a Class Is a Client of that
  Class
• Class Objects are Declared and Instantiated Using
  the new keyword.
• new ClassName(parameters) Calls Constructor for
  Class
• Class Public Methods Called Using Dot (e.g.,
  variable.GetName())

11
Client Test Program

• class useEmployee
• public static void main(String args)
• Employee emp1 new Employee()

12
Accessor Methods

• Public Methods for Getting Attributes of Class

13

• class Employee
• private String firstName
• private String lastName
• private double salary
• private final double MAXSALARY 2000000
• public Employee()
• firstName "NoFirstName"
• lastName "NoLastName"
• salary 0.0
• public String GetFirstName()
• return firstName
• public String GetLastName()
• return lastName

14

• class useEmployee
• public static void main(String args)
• Employee emp1 new Employee()
• System.out.println("Name is"
  emp1.GetFirstName() " " emp1.GetLastName())

15
Mutator Methods

• Class Methods Used to Modify a Class Object are
  Called Mutator Methods
• Allows Restricted Manipulation of Class Object
  Data

16

• public boolean SetSalary(double passedSalary)
• if (passedsalary lt MAXSALARY)
• salary passedSalary
• return true
• else
• return false

17

• class useEmployee
• public static void main(String args)
• Employee emp1 new Employee()
• System.out.println("Name is"
  emp1.GetFirstName() " " emp1.GetLastName())
• if (emp1.SetSalary(55000))
• System.out.println("Salary set to 55000")
• else
• System.out.println("Salary not set")

18
Exam 2

• EXAM 2 Monday in Lecture!!!
• 20 of Final Grade
• Know
• loops, switch/case
• Files
• Input failure (e.g. scan.hasNextInt())
• Methods
• Classes
• Everything Through Lecture Wednesday