Define and use a simple class

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Define and use a simple class

• First version of GradeBook.java (Fig. 3.1, p. 75)
• public class GradeBook
• public void displayMessage()
  System.out.println ( Welcome )
• First GradeBookTest.java (Fig. 3.2, p. 77)
• public class GradeBookTest
• public static void main( String args )
• GradeBook myGradeBook new GradeBook()
• myGradeBook.displayMessage()
• Notice all GradeBook objects are exactly the same

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Instance variables

• Each object is an instance of its class, and each
  instance can have different attributes
• e.g., course name for GradeBook object
• private String courseName
• Related set and get methods
• public void setCourseName(String name)
• courseName name
• public String getCourseName()
• return courseName
• See enhanced GradeBook.java (Fig. 3.7, p. 83) and
  new GradeBookTest.java (Fig. 3.8)
• Notice name is null before set method is used
• Numeric values default to 0 boolean values to
  false

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Constructors

• Definition looks like a method, but always has
  same name as the class, and no return type
• e.g., alternate constructor for GradeBook
• public GradeBook(String name)
• courseName name
• Initialize course name as object constructed
• GradeBook myBook
• new GradeBook (CS 5JA)
• No need to set later, and never equals null
• See another GradeBook.java (Fig. 3.10) and
  another new GradeBookTest.java (Fig. 3.11)

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Syntax for defining methods

• Method has two parts a header and a body
• type name (parameter declarations) // header
• local declarations and statements // body
• Parentheses in header and brackets around body
  are required
• type refers to the result of the method
• May be any primitive type, or any class
• Or may be void means it does not return any
  results
• If not void, statements in the method body must
  include a return statement

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Java has 8 primitive data types(everything else
is an object)

• 7 are number types
• 5 of the number types are integral types
• int most fundamental 4, -123, 9587123 are int
• long for longer integers (gt2,147,483,647)
• short, byte save space for shorter integers
• char to represent characters A, a, \n
• Other 2 number types are floating point types
• double most fundamental 0.4, -123.3, 95.
• float save space for less precision
• 8th type is boolean to represent true or false

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About floating point types

• Rounding errors occur when an exact conversion
  between numbers is not possible
• double f 4.35
• System.out.println(100 f) // prints
  434.99999999999994
• Illegal to assign a floating-point expression to
  an integer
• double balance 13.75
• int dollars balance // Error
• Casts used to convert a value to a different
  typeint dollars (int) balance // OK
• Cast discards fractional part truncates
• Math.round converts floating-point to nearest
  integer
• long rounded Math.round(balance)
• If balance is 13.75, then rounded is set to 14

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Another example class from Deitel

• Account.java (Fig. 3.13, p. 92)
• Instance variable, balance, is type double
• Method credit adds amount (also a double)
• Also getBalance, but no setBalance
• UML
• Static class diagram
• AccountTest.java uses 2 Account objects

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Summary Objects classes

• An object represents a thing (e.g., window,
  spaceship) or a concept (e.g., power, trajectory)
• A software entity with data and methods
• i.e., more complex than just a number
• Technically, an instance of a class
• A class defines a type of object
• e.g., class String defines what a String knows
  (data), and what a String can do (methods)
• Definition includes a public interface (what we
  use), and a private implementation (unimportant
  to user)

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Syntax for invoking methods

• Essentially methodName(list of arguments)
• Effect transfers control to the method named
  may pass data via the list of arguments
• When method completes control returns to the
  point in the program where the method was called
• Also returns a result if not a void method
• Need more if method defined in a different class
• Full syntax is objectReference.methodName()
• Or just ClassName.name() if method is static

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Aside using dialog boxes

• Simplest type of GUI (Graphical User Interface)
• Import javax.swing.JOptionPane
• Message dialogs show user something
• e.g., a String (and other types of objects)
• Input dialogs get a String from the user
• Must parse string to convert to numbers/other
• See NameDialog.java (Fig 3.18, p. 97)
  - and try GUI/Graphics Case Study Exercise 3.1
  (p. 98)

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