I installed openjdk-6-jre from the package manager, then I installed Eclipse, and this was the default JRE.
Now I needed to install, in addition to 6, also Java 7, so I installed openjdk-7-jre from the package manager. However, in Eclipse I still see only JRE 6. Also from the command line I see only java 6:
$ java -version java version "1.6.0_24" OpenJDK Runtime Environment (IcedTea6 1.11.5) (6b24-1.11.5-0ubuntu1~12.04.1) OpenJDK 64-Bit Server VM (build 20.0-b12, mixed mode)
How can I use JRE 7 from Eclipse?