How do I programmatically determine operating system in Java?

As indicated in other answers, System.getProperty provides the raw data. However, the Apache Commons Lang component provides a wrapper for java.lang.System with handy properties like SystemUtils.IS_OS_WINDOWS, much like the aforementioned Swingx OS util.

You can use:

System.getProperty("os.name")

P.S. You may find this code useful:

class ShowProperties {
    public static void main(String[] args) {
        System.getProperties().list(System.out);
    }
}

All it does is print out all the properties provided by your Java implementations. It'll give you an idea of what you can find out about your Java environment via properties. :-)