CS51 - Spring 2010 - Lecture 3

  • http://xkcd.com/292/

  • Exercise 2.7.7
       - Look at the two pieces of code. Do they produce different outcomes? If the outcomes are different, explain the difference.
          a. public void onMouseClick(Location point){
             new Text("Hello.", point, canvas);

          public void onMouseClick(Location clickPoint){
             new Text("Hello.", clickPoint, canvas);

          b. Assume clickPoint is an instance variable of type Location that has been initialized in a begin method
          public void onMouseClick(Location point){
             point = clickPoint;
             new Text("Hi", clickPoint, canvas);

          public void onMouseClick(Location point){
             clickPoint = point;
             new Text("Hi", clickPoint, canvas);

  • Notes on style
       - use indenting
          - if you select all of your code and then type CMD+i (CMD = the apple key), Eclipse automatically indents for you!
       - use whitespace
       - put comments inside your code

  • show SimpleBallMove code
       - what will this do?
       - move method: move(int x_amount, y_amount);
          - moves an object relative to current position
          - mutator, accessor or constuctor method?
       - other moving method: moveTo
          - moveTo(int x, int y)
          - moveTo(Location point)

  • show SimpleBallMove demo

  • show BallDrag demo
       - how can we do this (hint: think about scribble)?
          - onMousePress
             - keep track of where we pressed the mouse
          - onMouseDrag
             - if where we pressed the mouse was inside the ball
                - move it by the difference between where it was and where we are now

  • look at BallDrag code
       - why couldn't we just use "ball.moveTo(point)" in onMouseDrag?

  • show BetterBasketball demo
       - we can combine BasketBall and BallMove. What do we need to change?
          - onMouseRelease
             - check if we're carrying the ball
             - check if we're inside the hoop
             - if both true
                - update score
                - update display
             - move the position of the ball back to the starting point

  • BetterBasketball is pretty good, but I find "if( ball.contains(lastMouse))" to be a bit confusing. What are we really asking here?
       - Are we carrying the ball or not?
       - We can save this with a "boolean" variable. Another built-in data type (like int or double). It has two values it can take:
          - true
          - false
       - How can we change our code to use a boolean instance variable?
          - add the boolean instance variable
          - in onMousePress, see if we clicked inside the ball. If so, then we're carrying the ball. Set the boolean instance variable accordingly.
          - in onMouseDrag, just have to check if we're carrying the ball, using the boolean instance variable.

  • show EvenBetterBasketball code
       - functionality doesn't change, but makes it a bit easier to read
       - notice that ball.contains(points) returns/gives us a boolean
          - we can just assign the value of carryingBall
       - nested if statements
          - AND ('&&') and OR ('||') can be used to combine boolean expressions (they should feel natural)
             - show truth tables
  • show Color4Scribbler demo
       - what is the difference between this version and scribbler?
          - onMousePress
             - pick a random color from 4 options         
             - keep track of which color we're current using with an instance variable (what will be the type?)
          - onMouseDrag
             - set the color of the line to the current color we're using

  • show Color4Scribbler code
       - RandomIntGenerator class
          - specify range inclusive in constructor
          - nextValue() method gives you a random integer in range
       - a shortcut: we can define a variable and set it to an object in one statement!
          - private RandomIntGenerator colorPicker = new RandomIntGenerator(1, 4);
       - if-else statement?
          - just like an if statement except we add a block of code to execute if the condition is NOT true (that is, false)
          - notice that we can chain them together
          - when will the last "else" block get executed?
             - only if all of the previous if statements are false
       - == checks for equality
       - another shortcut: notice that we can create the line and then call a method on the object. Why?
          - the constructor returns an object
          - notice that we can't chain them, i.e. "new Line(...).setColor(currentColor).moveTo(...)"
             - setColor does NOT return a Line object

  • black ball is kind of boring: how can we change the color?
       - in begin: ball.setColor(Color.ORANGE)
       - the orange is a little off and what about arbitrary colors?
          - how can we specify a color? RGB
             - specify the amount of color between 0 and 255
             - anyone guess why 0 to 255?
          - ball.setColor(new Color(250,115,10))
             - new Color object
             - change code

  • look at EvenBetterColorBasketball code

  • show ColorScribbler demo
       - why only 4 colors... why not many, many more :)
       - how can we do this?
          - RandomIntGenerator(0, 255)
          - create a new Color with 3 random values

  • show ColorScribbler code
       - actually simplifies the code

  • Discuss lab 2
       - you'll need to bring a design to the lab before hand (but only for part 1)
       - program designs
          - classes
          - instance variables
             - constants
          - methods
             - including constructors
             - comments about what the method should do
          - NO CODE in the methods
       - be careful with your if/if-else statements
       - should be able to leverage code from the examples we've seen this week
       - identifying the correct basket
          - it's ok to have two variables reference the same object
             - just like you may have multiple ways that people my refer to you... first name, nickname, lastname, pet name, etc.