Lecture 7: More on Defining Classes

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Lecture 7 More on Defining Classes
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Another example

• Lets look at another custom graphics object
• The T-shirt
• This could be used to soup up your laundry
  program
• You could have shirts that look like shirts!
• Currently, all your clothes are square
• Not exactly a fashion statement, or at least not
  the one you wanted to make

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T-Shirts

• TShirt is kind of like BBall
• It draws an object composed of six subpieces
• Its constructor creates all the subpieces and
  associates names with them
• Its move method moves them all
• Its moveTo method depends on its move method.

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T-Shirts

• There are also a few differences
• TShirts contains method can not just call the
  contains method of one other object
• It has some extra methods (setColor,
  sendToFront).
• You could pretty easily put this in your laundry
  program instead of a FilledRect
• Maybe just change the type

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Custom Classes

• Youve seen move() and moveTo() on rectangles and
  other shapes
• Hopefully this makes it easy to understand the
  idea of defining your own methods in a new class
• You shouldnt think that you can only define
  methods like the existing ones
• You should design methods that are appropriate
  for the role of the object in your program

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Back to the Robot

• Robot might be one object
• made up of FilledRects for the arms, legs, body,
  and head
• Or, he might have custom Leg, Arm, Body, and Head
  objects!
• Each could have methods
• leg.move()
• arm.grab()

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Custom Classes

• You might also want a totally new method on BBall
• In our program, the ball always has to reset to
  the starting point
• How about a reset() method?
• This would put the ball back in its initial
  position

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Adding reset() to BBall

• How would you add reset() to BBall?
• What instance variables would you need?
• How would you change the constructor?
• What would the reset() method look like?

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Adding reset() to BBall

• How would you add reset() to BBall?
• What instance variables would you need?
• How would you change the constructor?
• What would the reset() method look like?

double initX, initY
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Adding reset() to BBall

• How would you add reset() to BBall?
• What instance variables would you need?
• How would you change the constructor?
• What would the reset() method look like?

double initX, initY
initX top initY left
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Adding reset() to BBall

• How would you add reset() to BBall?
• What instance variables would you need?
• How would you change the constructor?
• What would the reset() mutator method look like?

double initX, initY
initX top initY left
public void reset() this.moveTo(initX,
initY)
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Adding reset() to BBall

• Now you can just change onMouseRelease() to call
  reset(), in the class that extends
  WindowController
• Before
• After

public void onMouseRelease(Location
point) if ( hoop.contains(point)
ball.contains(lastMouse) )
score score 2
display.setText("Your score is " score )
ball.moveTo(BALLX,BALLY)

public void onMouseRelease(Location
point) if ( hoop.contains(point)
ball.contains(lastMouse) )
score score 2
display.setText("Your score is " score )
ball.reset()
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Variables vs Parameters

• reset() is a good example of the difference
  between instance variables and parameters
• In the original code, we construct BBall with its
  initial position
• And we have to pass the same information to BBall
  when we call moveTo().

public void begin() ball new BBall(
BALLX, BALLY, BALLSIZE, canvas)
public void onMouseRelease(Location point)
if ( hoop.contains(point)
ball.contains(lastMouse) )
score score 2
display.setText("Your score is " score )
ball.moveTo(BALLX,BALLY)
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Variables vs Parameters

• Now look at the new version
• We still tell the ball the initial position when
  we construct it
• But it remembers the position in initX and initY,
  so we dont have to tell it again when we call
  reset().

public void begin() ball new BBall(
BALLX, BALLY, BALLSIZE, canvas)
public void onMouseRelease(Location point)
if ( hoop.contains(point)
ball.contains(lastMouse) )
score score 2
display.setText("Your score is " score )
ball.reset()
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Variables vs Parameters
public void begin() ball new BBall(
BALLX, BALLY, BALLSIZE, canvas)

• Parameters are used when one method needs to
  transmit information to another method that it is
  calling.
• Instance variables are used when a method needs
  access to information that was available at some
  earlier point in a method call that is now
  complete.

ball.moveTo(BALLX, BALLY)
ball.reset()
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Use assignments

• In reset(), we used instance variables that we
  assigned values to in the constructor.
• Because we remembered initX and initY, we can use
  them later
• This is how BBall remembers where it is

public BBall(double top, double left, double
size, DrawingCanvas canvas) . . . initX
top initY left
public void reset() this.moveTo(initX,
initY)
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Why?
public BBall(double top, double left, double
size, DrawingCanvas canvas) . . . initX
top initY left

• Names used for formal parameters are only
  available in their method.
• When we send an object a message (a method call),
  the object receives the information via the
  parameters
• If you want to use them outside that method, you
  have to save them somewhere else
• Instance variables like initX and initY stick
  with your object, even after the method is
  finished executing

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This should be familiar
public void onMousePress(Location point)
dragging ball.contains(point) lastMouse
point

• Weve already done this before
• We had to remember lastMouse in our basketball
  program
• We needed this to compute the distance to move
  the basketball when we dragged it

public void onMouseDrag(Location point) if
(dragging) ball.move(point.getX() -
lastMouse.getX(), point.getY() -
lastMouse.getY()) lastMouse point
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Local variables

• What if you want something like an instance
  variable, but you only want to use it in one
  method?
• You can use local variables
• Your random numbers really should have been
  local, but you didnt know how to do that yet.
• Lets look at something similar
• Well make a 3-color version of Spirograph

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Local variables

• Well modify Spirograph to make its lines one of
  three random colors.
• Before, wed declare an extra instance variable
  outside the methods, like this

int randomInt
public void onMousePress(Location point)
nextLineStarts point // pick the color
randomInt colorGenerator.nextValue()
if ( randomInt 0 ) lineColor
Color.white else if ( randomInt 1 )
lineColor Color.blue else
lineColor Color.red
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Local variables

• The entire object doesnt need randomInt
• Its only used in onMousePress()
• You can eliminate the instance variable and just
  declare the random int inside the method

public void onMousePress(Location point)
int randomInt nextLineStarts point
// pick the color randomInt
colorGenerator.nextValue() if ( randomInt
0 ) lineColor Color.white
else if ( randomInt 1 ) lineColor
Color.blue else lineColor
Color.red
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Local variables

• Note that local variables dont need to be
  labeled as public or private
• They are only visible inside their own method
• Other objects can never access them
• Only the method uses them

public void onMousePress(Location point)
nextLineStarts point // pick the color
int randomInt colorGenerator.nextValue()
if ( randomInt 0 ) lineColor
Color.white else if ( randomInt 1 )
lineColor Color.blue else
lineColor Color.red