Inheritance

1
Inheritance
2
Inheritance

• Child class inherits data and methods from the
  parent, but not constructors
• Child class can override parent methods it wants
  to change
• Child class can have additional data and methods
• In Java, classes that dont explicitly inherit
  from another class inherit from class Object
• You can use a child object wherever a parent
  object is expected (Not the other way around!)

3
Example class Box

• Some members
• position (x, y)
• (upper-left hand corner of the Box)
• height
• width
• isVisible
• Could be
• private
• public
• protected

4

• public class Box
• protected int x, y
• protected int width, height
• public boolean isVisible
•  
• public Box( int startX, int startY, int w, int
  h )
• x startX
• y startY
• width w
• height h
•  
• ...

5

• public class Box
• ...
• public void draw()
• // Some code that draws the box
• public void hide()
• // Fill in rectangle with the background
  color
• public void resize ( int newW, int newH )
• hide()
• width newW
• height newH
• draw()
• ... 

6

• public class Box
• ...
• public void move ( int deltaX, int deltaY )
• hide()
• x deltaX
• y deltaY
• draw()
•  
• public int area()
• return width height
• // more methods perimeter, getLocation,
  getSize, ....

7
Subclass ColoredBox

• Box with a color
• Will inherit everything from Box but the
  constructor
• Will want to write its own version of draw
• Leave other methods alone
• Can access x, y, height and width, since
  protected
• Cant access x, y, height and width if private
• Would still have x, y, height and width even
  private

8
Some Rules on Child Class

• The first line of the constructor must invoke a
  parent constructor via super (superclass)
• Don't call any methods in the parameters of the
  constructor
• If you override a method and want to get at the
  parents version super.
• For example super.draw()
• Protected
• Allow the children access to its data
• Allow classes in the same package to
  access the data

9

• public class ColoredBox extends Box
• private Color color
•  
• public ColoredBox( int startX, int startY, int
  w, int h,
• Color c )
• super(startX, startY, w, h)
• color c
•  
• public void draw()
• super.draw() // method draw is
  overridden
• ...
• public Color getColor()
• return color

10
Example

• Box b new Box( 50, 40, 20, 30)
• ColoredBox cb new ColoredBox( 80, 60, 10, 20,
  Color.red )
• b.draw()
• cb.draw()
• cb.move( 3, 4 )
• b cb
• b.resize( 10, 15 )
• b.draw()

11
Prog3
12
Testing
You must test all classes THOROUGHLY!
But I dont know how to test it!
13
Testing

• Method equals
• 12.3 12 12.3 12
• 12.3 14 12.6 14
• 12.6 14 12.3 14
• 12.3 12 12.3 14
• 12.3 14 12.3 12

13
14
Prog4

• Lab on Tuesday
• Work plan and Test document due
• 11 PM Thursday, Oct 23
• Program due
• 11 PM Tuesday, Oct 28
• Grace Date
• 11 PM Friday, Oct 31

14
15
CS/SE 2430
You must come to the class every day, including
labs!
Yes, sir!
15