Chapter 5 Classes and Methods II

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Chapter 5Classes and Methods II

• Lecture Slides to Accompany
• An Introduction to Computer Science Using Java
  (2nd Edition)
• by
• S.N. Kamin, D. Mickunas, E. Reingold

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Chapter Preview

• In this chapter we will
• formally introduce the class construct as it is
  used in the Java language
• discuss the use of instance variables to
  facilitate method communication
• demonstrate the use of classes to improve program
  structure

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Building Classes with Multiple Methods

• Computer programs can be thought of using phases
• Input
• Computation
• Output
• Using separate methods for each phase can improve
  the maintainability of a class or program

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Building Class Definitions

• public class classname
• // Author, data, explanation
• declarations of instance variables
• public void methodName1 (parameter)
• declarations of local variables
• executable statements with comments
• public void methodName2 (parameter)
• declarations of local variables
• executable statements with comments

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Instance Variables

• Local variables
• variables declared inside methods
• not accessible to any other method
• cannot be used for communication
• Instance variables
• declared outside the methods, but declared inside
  the class
• all class methods have access to the class
  instance variables
• can be used for communication inside class

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Initialization of Instance Variables

• Instance variable declarations can contain
  initializers just like local variables
• Unlike local variables, instance variables will
  be initialized to default values if no
  initializers are found
• integers and doubles are initialized to 0
• characters are initialized to the null character
  (ASCII code 0)
• booleans are initialized to false
• object-type variables are initialized to the
  reference value null

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Hose Class Methods

• void getData()
• // Reads and stores the height and
• // weight data.
• void compute()
• // Computes and stores hose size.
• void display()
• // Displays the results of the
• // computation.

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UML Diagram for Hose Class
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Variable Scope Rules

1. The scope of an instance variable is the entire
   class body unless another identifier is found
   with the same name.
2. The scope of a formal argument in a method header
   is the entire method body.
3. The scope of a local variable in a method is from
   the point of declaration to the end of the method
   body.
4. It is not legal to declare a variable within a
   method using the same name as variable in the
   enclosing block in that method. You cannot
   declare two instance variables using the same
   name.

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Scope Example
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Bad Variable Declarations
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Class Constructors with Arguments

• A constructor is a special method that is called
  when an object is allocated.
• We can write
• OutputBox out new OutputBox(A Title)
• Instead of
• OutputBox out new OutputBox()
• Out.setTitle(A Title)
• Writing constructors for programmer defined
  classes will be discussed in Chapter 7.

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Return Types

• It is possible to have methods that have return
  types other than void
• Example
• public class Clock (
• int hour
• public int getHour ()
• return hour

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return Statement

• The return statement
• allows a method to return a value to the caller
• can appear any where in the method body
• can be conditionally executed
• results in immediate exit from a method when
  executed
• Form
• return expression

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Clock Class Methods
Example Explanation
void setup() Initializes the clock
void getData() Reads and stores the hour and minute data
String toString() Returns string version of time suitable for printing
void setHour(int h) Sets hour to h
void setMinute(int m) Sets minute to m
int getHour() Returns value of hour
int getMinute() Returns value of minute
boolean priorTo(Clock c) Returns true is receiver lt c
void display (DrawingBox d, int x, int y, int r) Draws the clock with center at (x,y) and radius r, in the DrawingBox referred to by d
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Clock Class Outline
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Geometry of Clock Drawing
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Theta Calculations

• For the hour hand
• theta
• 2Math.PIminute/60.0
• For the minute hand
• theta
• 2Math.PI(hour minute)/60.0/12.0

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Drawing the Clock Hands

• Assuming (x, y) is the bottom vertex and
  recalling that computer graphics coordinates are
  upside down
• x1 x (int) (rMath.sin(theta))
• y1 y - (int) (rMath.cos(theta))
• d.drawLine(x, y, x1, y1)
• For the hour hand use r.8 in place of r

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TwoClocks Client

• public class TwoClocksClient
• public static void main (String args)
• TwoClock twins new TwoClocks()
• twins.drawClocks()
• twins.compareClocks()

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Output from TwoClocks Client
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UML Class Diagram for Clock-DrawingBox
Composition