I'm not sure. With Dapper, it's the first time that Sun Java is in Ubuntu's repositories. Additionally, 1.4.2 is not the current Java version (1.5 is current). - I tend to think there's a bug within the packages in the repositories: When you choose the 1.4.2 Mozilla plugin, the matching 1.4.2 Java Virtual Machine should be marked as selected, automatically.
I also believe there's no actual need for the browser plugin (at least not with Sun Java 1.5), so try to just install the 1.5 JRE.
BTW,
Here's a Java version checker for the browser.
At last, pjlvaledez' method should work, at any rate.