Designing Classes

1
Designing Classes

• Chapter 2, Part II

2
Java Classes

• Design a class or a blueprint for each of the
  object in a system
• Example 1 Customer in the DirectClothing Inc.
• Example 2 Student, Faculty, and Course in
  a university scheduling system
• Class is described by a noun.
• Class name begins with a capital letter, and
  capitalize each subsequent word if the name is
  made up of more than one word.
• A class has attributes that characterize the
  object
• Example customer has first name, last name,
  customer ID, status, and total purchase.
• An attribute name starts with a lower case, and
  capitalize each subsequent word if the name is
  made up of more than one word.
• A class can have operations called methods
• Example customer can have methods such as add a
  new customer, update the customer status, and
  calculate the total purchases
• The name of a method begins with a lower case,
  and capitalize each subsequent word if the name
  is made up of more than one word.

3
Modeling a Class

• In HTM 484D, we will use the Unified Modeling
  Language (UML) to model an object
• The first box shows the name of the object, which
  always starts with a capital letter
• The second box shows the attributes of the class.
• The third box shows the method(s) of the class
• The sign means an attribute is public
  (accessible to any objects)

4
Modeling a Class

• After the colon (), indicates the data type
• customerID is an integer
• status is a character
• totalPurchases is a double floating point
• The key word , void, means the displayCustomerInfo
  () method does not return any information to the
  calling object

5
Modeling classes in Java

• Customer.java
• // Designing the first Java class named Customer
• public class Customer
• public int customerID
• public char status
• public double totalPurchases
• // This method displays the values for an
  customer
• public void displayCustomerInfo()
• System.out.println("Customer ID "
  customerID)
• System.out.println("Status "
  status)
• System.out.println("Purchases "
  totalPurchases)
• // end of display method
• // end of class

6
Attribute Variables

• Java uses variables for attributes
• Variables defined outside a method are called
  member, class member, or instance variables
• The syntax is
• modifier type identifier value
• where means optional
• For example
• public int customerID
• public char status
• public int customerID 0 // This initializes
  the CustomerID to a value of 0
• public int I, j, k // declare several variables
  at once
• Modifier can be public, private, and protected
  (we will use only public for the time being)

7
Basic Attribute Types (I)

• Integer
• byte (8 bits), short (16 bits) int (32 bits), and
  long (64 bits)
• In HTM 484D, we will mostly use the int integer
  type
• public int customerID
• public int customerID 1 // initializing during
  decalaration
• customerID 1 // assign 1 to customerID inside
  a method
• Floating point
• float (32 bits) and double (64 bits)
• For example
• public float short_non_integer 34.67f
• public double long_non_integer 34.67d
• public double long_non_integer 34.67 // you
  can ignore the d when initializing a dlouble
  floating point variable
• public double totalPurchases
• totalPurchases 100.0 // assign 100.0 inside a
  method

8
Basic Attribute Types (II)

• Character type
• char (16 bits)
• Example public char status U
• status N // Assign N to
  status inside a method
• public char ch a
• Java use unicode to represent characters
• To assign a value to a char variable, the
  character must be enclosed in single quotes
• Boolean (true or false)
• boolean (I bit)
• Example public boolean end true
• public boolean redLight false
• redLight true // set redLight
  to true
• A boolean variable can have a value of either
  true or false (true and false are key words)

9
Basic Attribute Types (II)

• Sequence of characters
• String
• Example public String firstName Jack
• public String lastName Leu
• Note String must starts with an upper case
  because it is a class
• String is a special class that we do not need to
  use the key word, new, to create an object
• String can be combined by using the operation
• Example System.out.println(Customer ID
  customerID)
• This will print Customer ID followed by a number
  on your screen

10
Testing the Customer Class

• CustomerTest.java
• // Test Customer.class
• public class CustomerTest
• // Java execution starts here
• public static void main (String args)
• Customer myCustomer new Customer()
• myCustomer.customerID 1 //assign 1 to
  customerID
• myCustomer.status N // for a
  new customer
• myCustomer.totalPurchases 100.0
• myCustomer.displayCustomerInfo() //
  call the method

11
Lab 1

• Create and compile Customer.java
• Create and compile CustomerTest.java
• Note if you compile CustomerTest.java first, the
  Java compiler will compile Customer.java and
  CustomerTest.Java at the same time
• Run CustomerTest.class

12
Initializations with a Constructor

• Constructors are method-like structures that are
  invoked automatically when you instantiate an
  object.
• Constructors are usually used to initialize
  values in an object.
• Syntax of a constructor
• modifiers class ClassName
• modifier ConstructorName(arguments)
• code_block

13
Classes with Constructors

• Customer.java
• // Designing the first Java class named Customer
• public class Customer
• public int customerID
• public char status
• public double totalPurchases
• // The following is a constructor
• public Customer ()
• status U
• totalPurchases 0.0
• // This method displays the values for an
  customer
• public void displayCustomerInfo()
• // the statements for the method are
  the same as before

14
Lab 2

• Modify Customer.java to include a constructor
• Compile modified Customer.java
• Modify CustomerTest.java to include the
  following
• Customer firstCustomer new Customer()
• Customer secondCustomer new Customer()
• firstCustomer.displayCustomerInfo()
• secondCustomer.customerID 1 //assign 1 to
  customerID
• secondCustomer.status N // for a new
  customer
• secondCustomer.totalPurchases 100.0
• secondCustomer.displayCustomerInfo()
• Compile revised CustomerTest.java
• Run CustomerTest