Chapter 2 Classes and Methods I

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Chapter 2Classes and Methods I

• 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
• describe structure of Java programs
• present techniques for creating and using class
  instances
• introduce two classes for doing output
• OutputBox for text output
• DrawingBox for graphical output

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Running Java Programs

• Enter the program source code in a data file
  called Hitwall.java using an editor
• Compile the source program by typing
• javac Hitwall.java
• Execute the compiled program by typing
• java Hitwall

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Program Elements Part 1

• white space
• blank lines and spaces includes in program
  source listings to make things more readable
• comments
• lines beginning with two slashes //
• single or multiple lines enclosed by / /
• that allow the programmer to insert notes to help
  other people understand the program
• documentation
• program comments and data files describing a
  programs structure and behavior

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Program Elements Part 2

• import directive
• tells the Java compiler which packages the
  program makes use of
• packages
• predefined collections of programs providing
  services to many programmers (e.g. CSLib.
  package used throughout the text)
• class heading
• needs to be included at the beginning of every
  program
• class name must match root of file name
• each class must be stored in a file of its own

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Program Elements Part 3

• main method
• the chief computational unit of a Java
  application
• executed first when application is run
• functions
• also known as methods
• define operations that may be applied to a Java
  data object (class instance)
• body
• Java statements that contain the implementations
  of classes and their methods

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Program Elements Part 4

• variable declaration
• statement giving the name and data type of a data
  location used by the program
• executable statement
• statement which manipulates variables or
  determines program control during execution
• atomic statements
• single Java expressions terminated by a
• variable assignment statement
• executable statement which copies a particular
  value to a data location

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Identifiers

• Java uses identifiers to name
• variables
• methods
• classes
• packages
• Syntax rules
• Must begin with a letter (upper- or lower-case)
• May be followed by any number (including 0) of
  letters and digits
• The characters and _ are considered letters
• Java identifier names are case sensitive
• May not duplicate a Java keyword (e.g. class or
  main)

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Building a Simple Class

• import CSLib.
• public class WarningMouse
• // Exterminate a rodent.
• // Author C. Mickunas 11/21/00
• public void shout()
• TrickMouse alert
• alert new TrickMouse()
• alert.setTitle(WARNING)
• alert.speak(Look out!)

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Method shout( )

• public method
• Means that methods in other classes (or clients)
  may use it
• void return type
• Means it does not return a value to the caller
  (client)

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OutputBox Class

• int literals
• 0, 1, -1, 2, -2, 3, -3,
• double literals
• 3.45, -48.2, 33.0,
• print
• Display text representation of argument and leave
  output cursor on the same line
• println
• Display text representation of argument and
  advance output cursor to the next line

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Using OutputBox

• import CSLib.
• public class Forecast
• // Give the weather forecast.
• // Author E. Reingold 11/12/00
• public void predict()
• OutputBox out
• out new OutputBox()
• out.print(The temperature will be )
• out.print(-10)
• out.println( degrees.)
• out.println(Thats cold, folks!)

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DrawingBox Window

• A DrawingBox window is divided into a rectangular
  grid of picture elements (pixels)
• The size of a pixel depends on the resolution
  (the number of pixels in the grid) of your
  workstation monitor
• A typical screen resolution might be 1028 by 768
  pixels.
• Pixels may have two colors (black or white) or
  many colors (depending on the resolution of the
  monitor)

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Using DrawingBox

• import CSLib.
• public class Concentric
• // Draw concentric circles.
• // Author A. Baranowicz 12/2/00
• public void drawThem()
• DrawingBox g
• g new DrawingBox()
• g.setDrawableSize(300, 300)
• g.drawOval(110, 110, 80, 80)
• g.drawOval(95, 95, 110, 110)
• g.drawOval(80, 80, 140, 140)

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Concentric Client Program

• import CSLib.
• public class ConcentricClient
• public static void main(String args)
• Concentric circles
• circles new Concentric()
• circles.drawThem()

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Color

• DrawingBox defines several symbolic constants to
  simplify selecting drawing colors
• Color.white, Color.black,
• Color.red, Color.blue, etc.
• You may select the color to draw with by calling
  setColor before drawing
• g.SetColor(Color.blue)
• g.fill Rectangle(0, 0, 50, 25)