Sun JRE is available in the non-free repository. You need to update your repositories configuration file (/etc/apt/sources.list).

Add the following line:

deb http://ftp.us.debian.org/debian/ etch main contrib non-free

Update apt with apt-get update command and you’re ready to install the JRE.

$ apt-get install sun-java5-jre

Check your java binary:

$ java -version
java version "1.5.0_14"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_14-b03)
Java HotSpot(TM) Client VM (build 1.5.0_14-b03, mixed mode, sharing)