Android app and open source projects

To use an open source project code, one needs to check the license first. GPL is a cancer.  Once you touch a project with GPL, the app has to comply with the strict rules of GPL no matter how insiginificant the open source project is.  In other words, the app must have its souce code open. LGPL is OK.

The next step is either using the binary jar files, or the source code of the project.  The latter is better for debugging and tweaking.  Many open source projects use either SVN or Git to manage the source code.  A copy of the source code can be easily retieved using a GUI app or command lines.  

To use the source code instead of jar files, a Java project needs to be created. For example, put open source project projectopen's code in a folder projectopen at location C:\Eclipse\Projects, create a java project named projectopen in that location.  Eclipse will take care of the rest of the work. This new project needs to be added to the build path of the app project referencing the open source project.