Sharing Java Classes from Another Eclipse Project
In Chapter: Getting Started
Author: Ian Darwin ('idarwin')
You want to use a class from another project, but don't want to copy-and-paste
Add the project as a "referenced project", and Eclipse (and DEX) will do the work.
You often need to re-use classes from another project. In JPSTrack GPS tracking program, the Android version borrows classes like the file I/O module from the Java SE version. You surely do not want to copy and paste classes willy-nilly from one project into another, because this makes maintenance improbable.
In the simplest case, when the library project contains the source
of the classes you want to import, all you have to do is declare the project
containing the needed classes (the Java SE version in this case) as a referenced project on the build path.
Select Project->Properties->Build Path, select Project, and click "Add".
In the screenshot I am adding the SE project "jpstrack" as a dependency on the Android version project "jpstrack.android".
A newer method that is often more reliable, and is now the
offically-recommended way, but is only useful
if both projects are Android projects, is to declare the library one
as a library project, under "Project->Properties>Android>Library Tab",
and use the Add button on the other project on the same screen to list
the library project as a dependency on the main project.
This is now the preferred method.
For the command-line fans, the first method involves editing
the .classpath file, while the second method simply creates
entries in the project.properties file, for example:
# Project target
Since you are probably keeping both projects under source control (if these are programs you ever ship, you should!), remember to tag both projects when you release the Android project - one of the points in favor of source control is that you need to be able to re-create exactly what you shipped.
See the official documentation on Library Projects.