You open this dialog box by right-clicking any and choosing Properties. The Sources page lets you define the
folders that contain the project's source packages and test packages. Use the
buttons on the right of the dialog box to add, remove, and change the order
of the folders.
You can have multiple source roots in a standard project, with the following
exceptions:
The source root cannot already exist in another IDE project.
The source root cannot already be added to another compilation unit of the
same project. For example, a source root that is registered under Test Packages
cannot be added to the list of Source Packages.
All of the source roots are packaged into the same JAR file and share the
same classpath.
If you have a source root that needs to be used by several projects, you
should create a separate project for the source root and set
up compilation dependencies between the projects.
You can use the Source Level combo box at the bottom of the Sources page to
set the Java platform version that the generated Java code should be compatible
with. This setting corresponds to the javac -source option.
Specifying the Source Level does not change the Java platform whose libraries
and executables are used to compile and run your project. To set the Java
platform that is used for compiling and running your project, click Libraries
and set the Java Platform setting accordingly. You cannot set the Source Level
of a project to a higher release than the project's Java platform.