To run the scenarios on this website, you must have the Java Runtime Environment (JRE) installed. The JRE includes a web browser plugin that works with major browsers (Firefox and Internet Explorer; for Chrome, see this note).
If you want to run Greenfoot itself, you need the Java Development Kit (JDK), which includes the JRE and browser plugin. You can download the JDK or the JRE from here:
Make sure to get the correct download for your operating system.
If you are using Microsoft Windows, you need to determine whether you have a 32-bit or 64-bit operating system.
If you have a 32-bit version of Windows, you should download the 32-bit Java environment which is labelled as Windows x86 and named something like jre-7u51-windows-i586.exe or jdk-7u51-windows-i586.exe.
If you have a 64-bit version of Windows, you can theoretically run either the 32-bit Java environment (labelled as Windows x86) or the 64-bit Java environment (labelled as Windows x64). However, you should choose depending on whether your browser is 32- or 64-bit. See this webpage for guidance.
If you are using a Linux distribution, you may be able to install the Oracle JDK via your distribution's package management system - check the distribution documentation. You can however still install the JDK manually be downloading it from the Oracle website (link above). You may also need to read the installation instructions.
If you see a message saying "Application Blocked by Security Settings", you need to configure the Java plugin to allow running applets on the greenfoot.org website. To do this:
Go to the "Security" tab.
Under the "Exception Site List" heading, click the "Edit Site List..." button.
Click "Add"; a dialog appears. Enter
http://www.greenfoot.org/, and click "Ok".
A warning appears saying that "Including an HTTP Location on the Exception Site List is considered a security risk." Click continue.
Click "Ok" to close the Java Control Panel.
You may now need to restart your web browser.
If you get the "Application Blocked" dialog when you try to run a scenario, with a complete error message reading:
Your security settiings have blocked an applicaiton from running with an out-of-date or expired version of Java.
... then your Java version if most likely out-of-date. You should install the latest available version.
Be aware that some scenarios may have bugs or problems which prevent them from running. You should try several scenarios before concluding that you are unable to run any scenario.
If you do have any other problems getting scenarios to run, try posting in the discussion forum.