CS 40 - Lecture 13

Cay S. Horstmann

Lecture 12 Recap

• everything is a number: characters, pixel positions, colors, processor instructions
• a few simple instructions: move data, arithmetic, jump
• executed very quickly: computers are lightning fast but dumb

Java

• A “real” language; suitable for almost any programming task
• Instead of dragging “speak” tile to the toaster object, type

  System.out.println("Hello");

• Typical uses: file processing, user interfaces, web applications
• Google calendar, fedex.com, etc. etc. were programmed Java
• Alice was written in Java

Greenfoot

• Developed for simulations and games
• We use it for image manipulation
• Also suitable for light programming
• Student-friendly (but not as friendly as Alice)

Installing Greenfoot

On the lab machines, skip steps 1 and 2. Repeat step 3 at the start of each lecture.

On a Mac, skip step 1.

• Step 1: Install the Java Development Kit

  The first download on http://java.sun.com/javase/downloads

• Step 2: Install Greenfoot

  http://greenfoot.org/

• Step 3: Unzip the starter project PictureWorld.zip onto the desktop

Greenfoot Basics

Lab 1: Starting Greenfoot

• Launch Greenfoot
• First time only: Click on Scenarion -> Open
• Navigate to Desktop
• Pick PictureWorld

  If it doesn't show up, check that you downloaded and unzipped it in step 1

• Right-click on MyPicture1.
• Pick new MyPicture1()
• Drag icon to big white rectangle at left
• Continued...

Lab 1: Starting Greenfoot

• Right-click on the icon in the big white rectangle
• Find the pick method. (It is in one of the parent classes.)
• Which class is it in? ActiveLecture.org
• Select the method.
• What happens? ActiveLecture.org
• Pick images/The Road.jpg.
• Right-click the image and pick the stripeRows method.
• What happens? ActiveLecture.org

Programming in Greenfoot

• Make new class
• Right-click on parent Picture, pick New subclass
• Name it MyPicture2, MyPicture3, etc. etc.
• Right-click on new class
• Pick Open editor
• Add code for new methods to the end
• Click on Compile when you are done editing
• Fix your syntax errors
• When the program compiles cleanly, right-click on your new class and pick the new Classname() option
• Then proceed as before

Pictures

• Parent class Picture defines useful methods
• pick: Pick an image on your computer
• getPixels: Get all pixels of a picture

  for (Pixel pix : getPixels())
  {
     // do something with pix
  }

• Pixel class defines methods getRed, setRed, getGreen, setGreen, getBlue, setBlue

  double green = pix.getGreen();
  green = green / 2;
  pix.setGreen(green);

Basic Java

• Every statement ends in a semicolon
• Numbers are called double (for a stupid, erm, historical reason...)
• Declare variable as

  VariableType variableName = initialValue;

• Method calls

  object.methodName()
  object.methodName(parameterValue)

• “for each” loop

  for (Pixel pix : getPixels())
  {
     // do something with pix
  }

• Method definition

  public void methodName()
  {
     // put method code here
  }

Don't Panic

Lab 2: Working with Pictures

• Make a subclass MyPicture2 of Picture
• Open the editor
• Add the following method:

  public void decreaseGreen()
  {
     for (Pixel pix : getPixels())
     {
        double green = pix.getGreen();
        green = green / 2;
        pix.setGreen(green)
     }
  }

• Compile
• The code above contans an error. What complaint did the compiler have? ActiveLecture.org
• How did you fix it? ActiveLecture.org
• Now run the method. What happens? ActiveLecture.org
• Try calling the method more than once on the same image. What happens?

Lab 3: Working with Pictures

• Add a new method decreaseRed that decreases the red values instead of the green values.
• Compile, run.
• When it works, paste your method code into ActiveLecture.org
• Try it with a picture other than The Road
• What happens when you invoke both decreaseRed and decreaseGreen?

Reminders

• Homework is due Tuesday after Spring break
• Project teams are due today. Email me the names of all team members
• With your team, develop
• • a proposal for a programming project
  • a proposal for a non-programming project
• Project proposals due Tuesday after Spring break (title + 2-3 sentences)