You can install as many different Java Runtime Environments (JREs) as you like. JREs are used to run and debug java programs. Your JREs are managed on the Installed JREs preference page.
A JRE definition consists of:
You can switch the default JRE for the workbench. The default JRE is used by default when building, running, and debugging applications. Alternatively, projects may specify a specific JRE that they should be built and run with.
Java development tools (JDT)
Adding a new JRE definition
Deleting a JRE definition
Choosing a default JRE
Choosing a JRE for launching a project
Installed JREs preference page