Recall: Accessing Location Info
private Location firstPoint;
public void onMousePress( Location pressPt ) {
new Text("Pressed", pressPt, canvas );
firstPoint = press;
}
public void onMouseRelease ( Location releasePt) {
new Text("Released", releasePt, canvas );
new Line( firstPoint, releasePt, canvas );
}
Being Unpredictable
• Formal parameters:
– allow us to refer to information that is
unknown when the program is written
• Are there other ways we access and use
“unknown” information?
The Highs and Lows
• What if we want to move an object only
vertically by dragging the mouse?
Pong:
public void onMouseDrag( Location mousePos ) {
paddle.moveTo( 10, mousePos.getY() );
}
Accessor Methods
• Methods that return information about an object
• Examples:
mousePos.getY();
canvas.getWidth();
Expressions vs Statements
• Statements:
– “Top of the morning to you.”
– paddle.moveTo( 10,mousePos.getY() );
Perform a useful action
• Expressions:
– “the morning”
– mousePos.getY()
Used in statements
Fun with Numbers
• initialPosition.getX() and -5 are both numbers
• Can add, subtract, divide, multiply numbers
–
–
–
–
initialPosition.getX() + 5
5 - initialPosition.getX()
initialPosition.getX() / 5
initialPosition.getY() * 3
Order of Arithmetic Operations
2 + (initialPosition.getX() – initialPosition.getY()) * 5
– Precedence Rules:
• Perform operations within parentheses first
• Divisions and multiplications before additions and
subtractions
• Operations of equal precedence performed left to
right
Instance Variables
• We can give names to numeric values as
we can name objects
private int brightness; //shade of gray
brightness = 50;
Using Variables
• First declare:
private int brightness;
private Color purple;
• Then initialize
brightness = 50;
purple = new Color( 255, 0, 255 );
• We can also reassign values to variables
brightness = 34;
Unless…
Using Constants
private static final int BRIGHTNESS = 255;
• The keyword final indicates we cannot
reassign a new value to brightness
• The Java convention for naming constants
is all caps
Displaying Numeric Information
• We can combine numbers and text for display
Text countDisplay;
int programNum = 3;
countDisplay = new Text("This is program #" +
programNum, …);
System.out.println
• Two ways to display textual information
– Text class
– System.out.println(…);
• System.out.println("Number " + 5 );
– prints Number 5 to the Java Console
Playing Dice with the Universe
• How can we simulate a roll of dice?
• Through random numbers!
Random Numbers
RandomIntGenerator die;
die = new RandomIntGenerator( 1, 6 );
//display a random integer between 1 and 6
System.out.println( die.nextValue() );
public class RollAnotherOne extends WindowController {
private static final int NUM_SIDES = 6;
private RandomIntGenerator die =
new RandomIntGenerator ( 1, NUM_SIDES );
public void begin() {
new Text("Click to make me roll the dice ",
TEXT_X, PROMPT_Y, canvas );
}
public void onMouseClick (Location point ){
roll1 = die.nextValue();
roll2 = die.nextValue();
System.out.println("You rolled a " + roll1 + "and a " + roll2 +
" for a total of " + ( roll1 + roll2 ) );
}
}
Review
• Numbers
• Expressions and statements
• Variables and Constants
• System.out.println(…)
• Generating random numbers