One, the Java working set:
Eclipse has a small function of creating a Java working Set. Its role is to solve the problem of congestion when creating many projects in the Package Explorer pane.
You can add the original Java project when you create it (the New dialog box). Once created, select Select Working Set in the upper-right drop-down menu in the left Package Explorer pane and select selected working sets to open the created working set. After that, you can select it directly from the drop-down menu (which automatically appears in the drop-down menu).
If you want to add a new project to the working set, you can choose (at the bottom) at the end of the wizard, or choose Edit Active Working set from the drop-down menu in the top right corner of the package explorer.
See the Eclipse website for more information:
Http://help.eclipse.org/indigo/index.jsp?topic=%2Forg.eclipse.platform.doc.user%2Fconcepts%2Fcworkset.htm
Second, multi-project construction path:
Eclipse supports referencing another project as an additional classpath. The method is to right-click->properties->java Build path->projects. Press the Add button to add another project.
The Java working set and the multi-project build path for Eclipse