This component provides the Java JDK (based on OpenJDK).
Multiple Java versions may be installed at any point in time. However, you may need to select which version should be active as java in your PATH.
You can find out which ones are available by running eselect java-vm list:
bob@vm00 ~ $ eselect java-vm list Available Java Virtual Machines:  icedtea-bin-6 system-vm  icedtea-bin-7
To choose a specific version for your service user you can do so by running eselect java-vm set user <slot>. The slot number is the one that is shown to you in the output of the list command above.
This setting then becomes permanent for all future sessions of this user.