Chessproblems is built on Java, in the sense that the problem solving interface is a Java Applet. Java was chosen to build this site because it offers the power, to run an application as complicated as Chessproblems (several thousand lines of code) in a cross-platform, consistent manner.
The version of Java required for Chessproblems is 1.4 or greater. This allows the smooth 2-D graphics, Swing UI, and other useful tools only introduced in the more recent version of Java. There may be some users who do not have this version installed, and for them I recommend an upgrade.