Install Java

Java Virtual Machine (JVM) is the part of the Java software platform that executes Java code. It is a program which executes certain other programs, namely those containing Java bytecode instructions. To compile Java code, you need to have some version of JVM installed. You'll also require the JDK (Java Development Kit).

Java is most likely preinstalled on your computer, unless you are using Mac OS X 10.8 (Mountain Lion). It is likely, however, that you will not have the JDK installed.

Mac

Open a Terminal window (click on Applications -> Utilities -> Terminal from the dock). Then type "java -version" at the prompt. If it return with something like "java version "1.6.0_35"", then Java is already installed and you don't have to do anything else. Otherwise, it should pop up a window to install Java. Click the "Install" button to install Java.

Windows/PC

Go to http://www.oracle.com/technetwork/java/javase/downloads/index.html. Click the "Download" button under the box labeled "Java" (the first of the three; the other two are JavaFX and Netbeans).

Scroll to the section labeled "Java SE Development Kit 7u40". Click the radio button to "accept license agreement". There are two versions of the JDK for Windows. "Windows x86" is for 32-bit machines; "Windows 64" is for 64-bit machines. Choose the appropriate link for your machine. (See the "Install Eclipse" page for how to tell which one you are.)

Once the executable is downloaded, you can run it. Just accept the defaults for all choices.

Linux/Ubuntu

You can install Java two ways: either installing OpenJDK Java 7 Runtime from the Ubuntu Software Center, or running the following command in Terminal:

    $ sudo apt-get install openjdk-7-jre