First, you came to ask a question and used punctuation and capitalization. You're my hero.
Second, GNU Java (gij) doesn't work with JavaOp. I don't know about Debian, but Ubuntu (a derivative) provides sun-java in it's universe repositories for APT. Try asking on Debian's forums, or using a repository search feature, to see if you can find a compatible package.
Java is typically backwards compatible. For a class compiled against 1.4 but run on 1.5, the JRE will fall back and run it against 1.4's class files, causing it to perform exactly the same, ideally. Sometimes, I believe, it doesn't work out that way, but JavaOp should work right.
However, if you can replace GNU Java 1.4.2 with Sun Java 1.4.2, you should be good to go.