Java. lang. NoClassDefFoundError occurs during Android development using Eclipse.

I have encountered this problem before, but now I have solved it. But I forgot how to solve it. After a long time, I finally found the solution, record it now to prevent further attacks. This error occurs in one of my activities, but I have repeatedly confirmed that the package name and class name have not been written incorrectly, and the compilation is fine, but this exception is thrown only at runtime, so I suddenly thought that my Activity inherits FragmentActivity, and FragmentActivity exists in the support package. It may be because the jar package of the support package is not added to the APK during compilation, this problem occurs. OK, the problem is found, now let's talk about how to solve, first open Eclipse Java Build Path, check the android-support-v4.jar package in which Library, my is in Android Private Libraries, click the fourth tab above, that is, the Order And Export tab, And check it in front of Android Private Libraries. OK, Clean the project again, run it, And solve the problem!