The default JRE is used for compiling and launching Java programs in all projects unless you specifically override the default JRE on a project's build path or on a launch configuration.
Here is how you can change the default JRE:
Java > Installed JREs preference page.
General >
Workspace preference page).
Working with JREs
Adding a new JRE definition
Deleting a JRE definition
Choosing a JRE for launching a project