Greenfoot LIVE

In order to provide more ways to support teachers who are using Greenfoot (or thinking about using Greenfoot), as well as anyone else interested in Greenfoot, we are starting a series of live chat sessions:

Greenfoot Live
Programming Education Chat With The Greenfoot Team

Greenfoot Live will be a regular chat event where members of the Greenfoot team will be live online and talk about Greenfoot, among ourselves and with you – the users of Greenfoot. The event is aimed mostly at teachers who use Greenfoot in their classes, but also at general Greenfoot users.

We will discuss educational aspects, as well as general programming topics, related to Greenfoot.

You will have a chance to listen to us talk, as well as ask questions.

The first Greenfoot Live event will be held on

Monday, 8 May 2017, at 5pm UK time

That is

  • 5pm in the UK
  • 4pm GMT
  • 9am in San Francisco
  • 12noon in New York
  • 18:00 in Germany, Scandinavia, Italy, France, …
  • 2am in Sydney (sorry…)

You can join us here.

After this first event on 8 May, we will host Greenfoot Live every two weeks. Recordings of these events will be accessible afterwards for viewing retrospectively.

Don’t forget to mark this date in your diary with a big red pen and join us in a couple of weeks!

Stride: With a little help from my editor

In the third part of our series introducing Stride and frame-based editing, we look at the various ways in which the editor can provide you with help in writing your program. If you consider using Stride, it will come handy to know these sources of information.

Stride is implemented in the Greenfoot environment, available from here.

What’s in a symbol? Assignment revisited

I was at a Dagstuhl seminar recently, about programming education, and one of the things we discussed were common misconceptions of novice programmers. One of these, reported by several seasoned programming teachers, was the difficulty in interpreting the assignment symbol in C-style languages:

number = 42

There are several things that can be misinterpreted here. Firstly, the equality operator is non-directional. Some students have problems remembering which way it operates. Consider this statement:

a = b

Which way does it assign? To all of us who have programmed for some time this may seem a silly question – we get used to the right-to-left semantics so thoroughly that we can hardly imagine it to be different. But for beginners? Not so clear. (And, of course, the direction is arbitrary; language designers could just as well have decided the other way.)

The second possible misconception is related to the previously learned meaning of the equals symbol: to express equality. This has been used in maths for centuries, and taught to most pupils before they ever encounter programming, and is quite different from assignment. Consider this code fragment:

a = 0;
b = a;
a = 7;

What, now, is the value of b? For learners holding the equality misconception, it will be 7. This is not entirely unreasonable; it is indeed an internally consistent mental model. The misconception is that variables, once expressed as equal, remain linked. So the equals symbol is interpreted as equality (“b is the same as a“), which might then remain true for the future.

Many people have commented for a long time that the C-style equality-as-assignment is not ideal. Many languages do better. Algol and Pascal, for example, already used a different symbol in the 1960s, quite explicitly to express the directionality more clearly, and to distinguish from equality:

number := 42

I have always like this much better than the single equals and have always been mildly annoyed by the C-style syntax.

So when we designed our language Stride recently, the question surfaced again. What should we choose?

We had two competing goals: On the one hand, we wanted the syntax to be clear and expressive, but on the other hand we want Stride to be a pathway into Java, so there is also a strong incentive to be consistent with Java syntax. (Java, of course, uses the C-style syntax, so these two goals are in conflict.)

At first, we prioritised the Java-compatibility argument and chose the equality symbol. The first release of Stride had this as the assignment operator. But then the Dagstuhl discussion happened, and I started to think again.

In Stride we have an advantage: Because the representation of the program is not directly typed in, but produced by the system in reaction to command keys, we are not restricted to characters that can easily be typed on a standard keyboard. So initially I considered going back to a syntax that Smalltalk used (in early versions, before it fell back onto the := variant), a left arrow:



I liked it for its clarity of expression, but it has one disadvantage in a teaching language that is intended to lead to Java and similar languages: it does not directly prepare learners for the equal symbol so widely used for assignment in other languages. After we discussed this in one of our team meetings, Neil Brown, one of our team members, came up with what I think is a brilliant compromise:


This is what assignment now looks like in Stride. The double arrow still has a visual link to an equals sign, but is clearly distinct from it; it is also clearly directional. We still use the equals key as a command key to enter an assignment, further reinforcing the link for the future.

Here’s what our assignment looks like in context:



I am very happy with it. I think it reads well and is the closest we can get to reaching two competing goals.

Stride: Creating a game in 7 minutes

This video is mostly for those who want to get a first impression of Greenfoot. We write a simple game in seven minutes (realtime; no cuts!), using Stride. For those who already know Greenfoot with Java: You will see that writing in Stride is not very different to writing in Java. For those new to Greenfoot: This is a quick introduction to Greenfoot and Stride at the same time.

As always, Greenfoot is available for free download.

Stride – A new programming language for beginners

At the Greenfoot headquarters, we – that is: Neil Brown, Amjad Altadmri and myself – have recently worked on creating a new language within the Greenfoot environment: Stride.

The interesting thing about Stride is not so much the language design itself, but its interaction design: editing programs involves different interactions than existing editors.

The goal is that Stride sits halfway between block-based systems (such as Scratch, AppInventor, PencilCode, Alice, etc.) and text-based editors (such as Java or Python), maintaining advantages of both.

I will start a series of posts (text and/or video) here over the next few weeks trying to tell you what Stride is and why you should care. In the meantime, you can get Greenfoot, install it, and have Stride ready to go when the introduction here starts. Stride is built into Greenfoot from version 3 onwards.

Check back soon for the first overview video.

Java 7 for introductory programming — does it make a difference?

The recently released BlueJ 3.0.5 brought full support for Java 7. While it is always good to be up-to-date with the latest versions — what does that actually mean for introductory programming teaching?

Java 7 brought a considerable list of new features to the Java system. Most of them deal with fairly advanced or specialised topics, and will have no influence on introductory teaching with Java. Some, however, will become visible even close to the beginning of programming. Here, I give a short overview of what’s new, and what you — as a teacher of introductory programming — should be aware of.

Java 7 – What’s new

Most of the new features in Java 7 are under the hood or in specialised libraries. These are very unlikely to affect an introductory programming course.

Improvements to the internals of the JDK include changes to the VM to support dynamically typed languages and improvements to the class loader architecture. On the library side, there are new frameworks for concurrency, cryptography and new versions of JDBC and Unicode. All these will not be relevant for most introductory courses.

The two areas that are relevant are “Project Coin”, the code name given to a project defining several small language enhancements, and NIO.2, a new library for I/O. We’ll discuss these in more detail in a moment.

Equally important, what’s not included in this release includes support for closures (known as “Project Lambda”). This would have been the most significant change to Java since the introduction of generics in Java 5 or, arguably, since the original definition of the Java language in 1995. Project Lambda has, however, been deferred to JDK 8 in what’s known as Plan B. JDK 8 is currently scheduled for release in late 2012.

So, which of the new Java 7 features are actually relevant for our intro programming course?


The I/O classes in Java have been improved several times over the years. The release of JDK 1.4 in 2002 brought some significant improvements in the somewhat short-sightedly named NIO library. The naming is rather less than ideal, since it stands for “New I/O”, and the passing of time dictates that everything new will eventually get old.

Which brings us to Java 7 and the New New I/O. So what will this be called now — NNIO? Well, the designers settled on NIO.2.

NIO.2 brings a number of new classes and interface, some with very useful methods. If you are doing any programming that accesses the file system, it’s worth familiarising yourself with these. Particularly interesting are the Files and Paths classes, and the Path interface, which you can find in the java.nio.file package.

All other relevant new features are part of Project Coin. They are diamond notation, strings-in-switch and improved exception handling.

Project Coin

Diamond notation

Generic type details on the right hand side of an assignment can now be inferred. For example, where in Java 6 we had to write

   HashSet<String, Monster> monsters = new HashSet<String, Monster>();

to declare a HashSet variable and initialise it with a fresh HashSet object, in Java 7 we can write

   HashSet<String, Monster> monsters = new HashSet<>();

In other words: We do not have to repeat the generic types of the HashSet on the right hand side, and can instead just write <>. (This syntax is the reason that this construct is known as the “diamond notation”). The effect of this is exactly as the Java 6 version above. It is a purely syntactic shortcut. The compiler will fill in the generic types by copying them from the variable declaration on the left, and all behaves just as before. Of course, the old notation is still allowed.

Strings in switch statements

The variable used in switch statements can now be of type String. For example

   switch (command) {
       case "go":
       case "help":

Before Java 7, strings could not be used in switch statements.

Improved exception handling 1: multi-catch

It is now possible to catch multiple exceptions in a single exception handler. For example:

   try {
      file = new File("readme.txt");
   catch (FileNotFoundException | UnsupportedEncodingException ex) {

As you can see in this example, the catch clause can list multiple types of exception in its header, separated with an OR symbol, to catch any of these types of exception.

Improved exception handling 2: try-with-resources

The second new feature relating to exceptions is called try-with-resources. It solves a hard problem: It was previously surprisingly hard to correctly guarantee that resources (such as files or network connections) were correctly closed in the case of an exception.

The new construct solves this. Look at this example:

   try (FileWriter writer = new FileWriter(filename)) {
   catch (IOException e) {

Before Java 7, the standard way to close a resource (a FileWriter in this example) would have been in a finally block, which followed the catch block. In Java 7, the FileWriter is opened in round brackets following the try keyword, marking it as a resource to be auto-closed. Once the try/catch block is completed, the resource will automatically be closed by the Java runtime system.

For classes to be used with the auto-close mechanism, they must implement the new AutoCloseable interface. All the relevant classes in the Java library have been retrofitted to implement this interface.

That already sums up the relevant Java 7 changes. As we can see — not much to worry about here. (This will be different next year, when JDK 8 will bring us closures. I expect we will have to have a lively discussion then about how to treat these in a first-year course. But we can leave that for a while.)

To find out more about the new constructs in Java 7, see this summary. The next version of the Object First book (5th edition, to be released in late October) includes discussion of these new features.

Note: This post was first published in the Blueroom.

Use the Microsoft Kinect with Greenfoot

Those of you who know Greenfoot know that one of its aims is to make programming for beginners exciting and engaging. (Those of my readers who don’t know it should have a look here.)

The most recent addition to Greenfoot is a library that allows programmers to easily use the Microsoft Kinect module with their Greenfoot scenarios. This means that you can now write simple Greenfoot games that are controlled by players body movements.

Probably the easiest way to show what I mean, is to show you what I mean. Here’s a short video:

Programming the Kinect with Greenfoot is probably the easiest way to write programs with the Kinect module. Neil Brown, one of our developers on the Greenfoot team, has adapted open source server software that communicates with the Kinect and designed and implemented a Greenfoot library that makes access surprisingly simple.

If you are interested to try it yourself — here are the detailed instructions. But beware: you might stand in the middle of your room waving your arms around for the next few days! Some people might look at you strangely, but it’s great fun.