Background Info

In this article you will find background information on how to work with GreenfootKara. This might be especially helpful if you are a teacher.

When used in schools, the entire GreenfootKara material will take about 16-20 lessons.

Topics Covered

  • 1 lesson: Installing Greenfoot, starting Greenfoot projects
  • 3 lessons: Getting to know Kara, creating objects and calling methods with the mouse, reading source code, writing the first program (chapter 1)
  • 3-4 lessons: flow diagram, control structures (conditions and loops), boolean and logic operators (chapter 2)
  • 3-4 lessons: Variables, data types and operators, the structure of a class, nested statements (chapter 3)
  • 3-4 lessons: Programming a game, keyboard input, comparing Strings, reading Javadoc, designing ASCII levels, publishing the program on the Greenfoot website (chapter 4)
  • 3 lessons: Writing own methods with parameters and return values, repetition (chapter 5)

How to Continue after GreenfootKara

  • Option 1: Develop a custom project with Greenfoot
  • Option 2: Start using a professional development environment like Eclipse, Netbeans, etc. (see GameGridKara)
  • Option 3: Start GUI-programming with JavaFX

The Kara Scenario

To use the Kara-Scenario Greenfoot must be installed first.

Run the setup program and follow the installation instructions. After installation, any Kara-Scenario may be opened using the project.greenfoot file in the scenario folder.

For each exercise there is a separate scenario (see folder scenarios-chapter-1, scenarios-schapter-1-solutions, etc.). In each scenario, the world of Kara (with trees, leafs, etc.), is already prepared for each exercise.

The possibilities of Kara remain the same for all exercises, except for chapter 4 and 5. In Chapter 4, Kara has a additional methods so that a Sokoban game can be programmed. In Chapter 5, Kara has a few additional methods to show messages and to ask the user for input.

The Classes Kara and MyKara

The most important classes are Kara and MyKara. The class Kara includes all the functionality of the ladybug Kara as seen above. But programming is always done in the class MyKara which, through inheritance, can accessed all the methods of Kara. Thus, the complexity of Kara's methods are hidden from the students at first.

Later, the students may choose to find out how the methods were implemented in Kara itself. For this step it is recommended that they first view the class in the documentation mode. This only shows the Javadoc comments inside the Greenfoot editor.

Starting with Mouse Only

Objects of classes can be instantiated through a right-click, new ….() and can then be placed in the world. (Tip: by pressing the shift-key you can place multiple instances in the world without using the context menu).

On first contact with GreenfootKara it is helpful to only use the mouse. If you right-click on a Kara-object, all available methods are shown and can be selected with the mouse. This way one can get accustomed with how Kara works.


The programming can be done inside the act()-method of MyKara. This method is executed when the Act-button is pressed. When the Run-button is pressed, the act()-method is called repeatedly.

Creating your own Scenarios

To create your own scenarios I recommend you copy an existing scenario and make changes to the WorldSetup.txt file.

Note: If you want to use a name other than WorldSetup.txt, adjust the constant WORLD_SETUP_FILE in the KaraWorld class.

A world setup file may contain multiple worlds. Each world must start with the following three lines:

World: [Your title]
X: [Width of the world]
Y: [Height of the world]
[Your actors]

Actors are represented as follows:

  • #: Tree
  • @: Kara
  • .: Leaf
  • $: Mushroom
  • *: Mushroom on a Leaf
  • +: Kara on a Leaf

Tip: Create the world inside the Greenfoot editor and use right-click on the world | saveWorldSetupToFile() or printWorldSetupToConsole() to save the created world.

Scenario Sources on GitHub

If you want to make adjustments to GreenfootKara or would like to report a bug, take a look at the GreenfootKara Repository on GitHub. This repository contains an Eclipse project with the source of GreenfootKara and all the scenarios.

General Tips

Greenfoot Editor Tips

  • Often the students have trouble with cleanly structuring the code. The editor helps you format by an autolayout feature found inside the Edit menu.
  • Ctrl-Space will open a pop-up for auto completion.
  • Top right in the editor you can switch the view from Source Code to Documentation.
  • In the menu Options | Preferences … the font size can be changed (e.g. for presentation with a projector).

Screen Output and User Input

There are several ways to interact with the user through Greenfoot over Input/Output:

  • System.out.println(...) will write something on the console.
  • With a Swing dialog (e.g. JOptionPane): This is used in KaraIO (Chapter 5).
  • Drawing labels: This is the most complex version but also elegant as the input and output appears directly on the world and not in a pop-up dialog. An example can be found in KaraSokoban (Chapter 4).

Sharing a Scenario with Others (Deployment)

With Greenfoot, scenarios can easily be exported and shared with others. There are three ways to this:

  • Upload the scenario on the Greenfoot Gallery. In the Greenfoot Gallery it can be directly started as an applet. Even the highscore of the Sokoban game should work.
  • Export the scenario as a runnable jar application.
  • Create a custom website with the scenario.

Known Bugs

The World Disappears

It is possible that Greenfoot blocks and the world of Kara is no longer drawn. Even recompiling or pressing reset does not help.

Reason: This happens if the program remains too long inside the act method and then the user tries to interrupt the program. This is a problem that is known for Greenfoot but it is difficult to solve. Indeed, it is hardly possible in Java to stop a running method from the outside.

Solution: Exit and restart Greenfoot. Alternatively, you can open the debugger and click on Terminate. This exits the scenario and automatically starts it again.

I recommend taking a look at the book by Michael Kölling Introduction to Programming with Greenfoot. It can either be used as inspiration for the teacher or as a textbook for the whole class.