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CSE 142

Lecture Notes

Week 3

Key Words

  • scope: the part of the program where a variable exists
  • A variable declared in a for loop exists only in that loop
  • A variable declared in a method exists only in that method
  • primitive types: int, double, boolean, char
  • object types: name of class (.e.g., String, DrawingPanel)
  • parameter: sends information in from the caller of a method
  • return: sends information out from a method to its caller

Drawing Panel

Not built into Java, something written specifically for this class. Will need to place DrawingPanel.java in the same folder as your code and compile it in order for it to work properly

  • In computer graphics, the upper-left hand corner is (0,0)
  • In order to use Color, you will need to add import java.awt.*; to the top of your code
  • Methods to consider using
Method Name Description
dp.setBackground(Color); sets the background color of the panel
dp.clear(); clears the background of the panel
dp.pause(miliseconds); pause the program for the number of miliseconds passed
g.drawLine(x1, y1, x2, y2); draws a line between the points (x1, y1) and (x2, y2)
g.drawOval(x, y, width, height); draws the outline of an oval that fits exactly into a box of size width x height with the top-left corner of the box at (x, y)
g.drawRect(x, y, width, height); draws the outline of a rectangle of size width x height with the top-left corner at (x, y)
g.drawString(text, x, y) draws text with a bottom-left corner of (x, y)
g.fillOval(x, y, width, height) draws a filled in oval that fits exactly into a box of size width x height with a top-left corner at (x, y)
g.fillRect(x, y, width, height) draws a filled in rectangle of size width x height with a top-left corner at (x, y)
g.setColor(Color); sets Graphics to paint any following shapes in the given color
  • Basic program setup
 // needed in order to use Color
import java.awt.*;

public class BasicDrawing {
   public static void main(String [] args) {
      // creates a new Drawing Panel object of width 400, height 200
      DrawingPanel dp = new DrawingPanel(400, 200);

      // sets the background color of the panel
      dp.setBackground(Color.GREEN);

      // creates a new Graphics object
      Graphics g = dp.getGraphics();

      g.setColor(Color.RED);
      g.fillRect(100, 100, 50, 70);
      g.setColor(Color.BLUE);
      g.drawRect(100, 100, 50, 70);
   }
}

Math class

  • The Math class has a number of useful methods including Math.sqrt(), Math.abs(), and Math.pow(); check out more details in the Java API

return

  • A return statement sends information out from a method to its caller
  • There cannot be statements executed after a return statement in a method
  • The return type should be provided as the third keyword in your method header
// This method has a return type of void
public static void hello() {
   System.out.println("Hello");
}

// This method has a return type of int
public static int three() {
   return 3;
}

Casting

  • Changes the type of a piece of information
int x = 2;
// a will contain 2.0
double a = (double) x;

double y = 2.5;
// b will contain 2
int b = (int) y;
  • Updated precedence list with casting
Priority Operation
1 parens
2 unary operations, casting
3 multiplication, division, mod
4 addition, subtraction, string concatenation
5 less than, less than or equal to, greater than, greater than or equal to
6 equal to, not equal to