The Android 4.2 Branch
The tree shown in this figure has an Android 4.2 branch, but it isn’t a directory on the computer’s file system. In the Package Explorer view, the Android 4.2 branch (or Android 3.0 branch or Android whatever branch) reminds you that the project includes prewritten Android code (the Android API).
A .jar file is a compressed archive containing a useful bunch of Java classes. In fact, a .jar file is a .zip archive. You can open any .jar file by using WinZip or StuffIt Expander or the operating system’s built-in unzipping utility. (You may or may not have to change the filename from whatever.jar to whatever.zip.)
Anyway, an android.jar file contains prewritten Android code (the Android API) for a particular version of Android. In the figure, a Package Explorer branch reminds you that your project contains a reference to another location on the hard drive (to one containing the .jar file for Android 4.2).
The android.jar file contains code grouped into Java packages, and each package contains Java classes. These figures show you the tip of the android.jar iceberg. The android.jar file contains classes specific to Android and classes that simply help Java do its job. This figure shows some Android-specific packages in android.jar.
This figure displays some general-purpose Java packages in the android.jar file.