ACE 1050 Design Computing

1
ACE 1050 Design Computing

• Tutorial 4
• 8thFebruary, 2007

2
Overview

• Course homepage
• http//www.acae.cuhk.edu.hk/ace1050
• Course newsgroup
• news//news.erg.cuhk.edu.hk/cuhk.acae.ace1050
• Java Class Structure
• instance variable
• constructor
• mutator
• accessor
• method

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Custom Class Java Programming

• Object-oriented programming
• Create the default object with different
  variables
• Manipulate objects with attributes and behaviors
• Advantages
• Simple to control and access specific behaviors
• Can be reused for different situations

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Class Structure

• instance variable, method (attribute, behavior)
• constructor, mutator, accessor

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Demo SnakeSnake Class

• demonstrate the java program to interactive with
  the Class (SnakeSnake)

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Class structure (step 1)

• declare the SnakeSnake Class
• declare the instance variables

import javax.swing. import java.awt. public
class SnakeSnake // instance variables to
describe object attributes private int width //
rectangle width private int height //
rectangle height private int x // rectangle
x-coordinate position private int y //
rectangle y-coordinate position private JFrame
window // window displaying rectangle private
Color color // color of rectangle //
constructors, mutators, accessors, methods
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Class structure (step 2)

• declare the default constructor (only execute
  once at the beginning)
• prepare the default variables
• constructor name must be as the class name

public class SnakeSnake //
snakesnake() default constructor public
SnakeSnake() window new JFrame("snake
snake") window.setSize(500, 500) width
20 height 20 x 230 y 230 color
Color.BLUE window.setVisible(true) // other
constructors, mutators, accessors, methods
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Class structure (step 3)

• declare the mutators
• change specific variable individually

public class SnakeSnake // setX() x
mutator public void setX(int ulx) x
ulx // setY() y mutator public void setY(int
uly) y uly //other constructors,
accessors, methods
No return function
Instance Variables
New Input
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Class structure (step 4)

• declare the accessors
• return instance variable individually

public class SnakeSnake // getX() x
accessor public int getX() return x //
setY() y accessor public int getY() return
y // getColor() color accessor public Color
getColor() return color //other
constructors, accessors, methods
accessor
Return instance variable
Return variable type
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Class structure (step 5)

• declare the other constructor - public classname
  (int, char, String,)
• update the all class variables when requested
• constructor name must be as the class name

public class SnakeSnake //
snakesnake() specific constructor public
SnakeSnake(int w, int h, int ulx, int uly, JFrame
f, Color c) setWidth(w) setHeight(h) setX(u
lx) setY(uly) setWindow(f) setColor(c) //
methods
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Class structure (step 6)

• declare the method
• Perform specific task (just like simple normal
  function)

public class SnakeSnake // method
paint() public void paint() Graphics g
window.getGraphics() g.setColor(color) g.fillR
ect(x, y, width, height)
Method name
Display the rectangle with the instance variable
of the class
A complete/full class should combine with
instance variables, constructors, mutators,
accessors, methods.
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Class Usage

• use the custom class (SnakeSnake)
• SnakeSnake r new SnakeSnake()
• declare the class
• use the class constructor
• SnakeSnake(int w, int h, int ulx, int uly, JFrame
  f, Color c)
• update all instance variables
• use the class mutator
• r.setX(int ulx)
• change individual instance variable
• use the class accessor
• obtainY r.gety()
• return individual instance variables to the
  external program
• use the class method
• r.paint()
• perform some tasks

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Class Usage(e.g. By another Java program)

• Java program demo Obtain user input and
  interactive with the class (SnakeSnake)

public class MovingSnake while( UserInputC
! '5' ) // check if exit the
program UserInputC System.in.read() if
(UserInputC '6') // move right r.update(
r.getX() 5 , r.getY() ) System.out.println("Cur
rent position at " r.getX() " , "
r.getY()) else if (UserInputC '4') //
move left r.update( r.getX() - 5, r.getY()
) System.out.println("Current position at "
r.getX() " , " r.getY()) else // nothing
to do with other buttons r.paint() System.
out.println("\nProgram closed!")
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Class structure(custom new method)

• declare the method - update(int newx, int newy)
• label the old path and highlight the head

public class SnakeSnake public void
update(int newx, int newy) Graphics
gwindow.getGraphics() // label the original
position as path by changing the color to
red setColor(Color.RED) g.setColor(color) g.fill
Rect(x,y,width,height) // update the new
position setX(newx) setY(newy) // change back
to color(blue) and label the new position on the
screen. setColor(Color.BLUE) g.setColor(color) g
.fillRect(x,y,width,height)
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Summary

• Java class declaration
• A complete/full class combines instance
  variables, constructors, mutators, accessors and
  methods.
• External program to use the class
• declare the object to the class - e.g. newobject
  IamNew new newobject()
• use the class - e.g. IamNew.SomeMethod()

• Exercise
• Try to download the demo. program in course
  homepage
• Make the rectangle box to be moved to 4 different
  direction by the key 2, 4, 6, 8
• hint add the if cases in the MovingSnake.java
  for other input, or by appending else if block
• Make a class method to scale the rectangle box
• hint define a function in SnakeSnake.java to
  interact with scaling mutators
• Make a class method to change the color rectangle
  box randomly
• hint 1 define a function in SnakeSnake.java to
  access the setColor mutator
• hint 2 modify the updating mutators if want to
  keep the generated color after movement scaling