A Java update issued by Apple through Software Update last week contained a bug that could stop some Java applications working. Apple fixed the bug and released a new update, but if you did the software update soon after it was made available, you may have the old version installed.
Try Software Update first, then use the following terminal command to check which version you have installed:
/usr/libexec/java_home -v 1.6 -exec java -version
If you see xM4509, you’re good. If you see xM4508, and Software Update hasn’t sorted it, then you can manually download and install the corrected version from the Apple support site.
More details here.(Thanks, Walter.)