CS51 - Fall 2009 - Lecture 5

  • What is an expression? A statement? What is the difference?
       - An expression is a phrase that describes an object or a value
          - literals
             - 17
             - true
          - variables
             - counter
             - carryingBall
             - display
             - hoop
          - object constructors
             - new Text(...)
          - some method invocations (e.g. accessor methods)
             - point.getX()
             - box.contains(point)
       - A statement instructs Java to perform some action
          - some method invocations (e.g. mutator methods)
             - rect.move();
             - display.setText(...);
          - variable assignment
             - counter = 17;
          - control statements
             - if( ... ){}
          - object constructors
             - new Text(...)

  • Where can we use statements and expressions?
       - our code consists of a series of standalone statements (think commands)
          - it doesn't make sense to have an expression by itself
             - 17;
             - carryingBall;
       - expressions
          - assigning a value to a variable
             - rect = new Text(...);
          - as parameters to method invocations
             - rect = new FramedRect(X_OFFSET, Y_OFFSET, ...)
          - if and if-else statements
             - if( rect.contains(point) )
             - if( true )
             - if( carryingBall )

  • Are all expressions the same? Can we use them interchangeably?
       - No. Java is what is called a "strongly" typed language
       - each expression has a "type" associated with it
       - For the expressions above, what are the types?
       - For the following phrases, what are the types we can use:
          - new FramedRect(?)
          - Text display;
             - display = ?
          - int count;
             - count = ?
          - if( ? )
          - ? && ?
          - ? + ?
       - For all of these we can substitute ANY expression that represents that has the expected type
          - boolean
             - if( true )
             - if( rect.contains(point) )
             - if( carryingBall )
          - int: "private int count;"
             - count = 17;
             - count = count + 1;
             - count = count * 20;
             - count = count + 2 * count;
             - count = rect.getWidth();

  • What types have we seen so far?
       - objects
          - FramedRect (FilledRect)
          - FramedOval (FilledOval)
          - Location
          - Line
          - Text
          - Color
       - primitive types
          - int
          - boolean

       - What are the differences between object variables and built-in?
          - rect = new FramedRect(...) vs. counter = 16
          - no methods associated with primitive types (counter.?)

  • 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

  • Better3PointBasketball demo
       - what do we need to add?
          - variables
             - lines
                - length
                - positions
             - keep track of if we've crossed either of the 3 point lines (what type of variable?)
          - begin
             - add lines
          - onMousePress
             - set crossed 3 point line to not crossed
          - onMouseDrag
             - check if we've crossed the 3 point lines
             - if so, set the boolean to true
          - onMouseRelease
             - 2 cases for scoring points
             - if-else statement

  • Better3PointBasketball code
       - variables: we can define constants with respect to other variables
       - onMouseDrag: > and <
          - what other types of math inequalities might we ask? >, <, <=, >=, ==, !=
       - onMouseRelease
          - if-else statement
       - += operator
       - ++, --

  • ColorScribbler code
       - what does this do?
       - another shortcut: we can define a variable and set it to an object in one statement!
          - private RandomIntGenerator colorPicker = new RandomIntGenerator(0, 255);

  • ColorScribbler demo

  • time permitting, "What is computer science?"
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