java.lang.NoClassDefFoundError:

Make sure to do the following (from Support Library Setup):

1. Right-click the library project folder and select Build Path > Configure Build Path.
2. In the Order and Export tab, check the .jar files you just added to the build path, so they are available to projects that depend on this library project. For example, the appcompat project requires you to export both the android-support-v4.jar and android-support-v7-appcompat.jar files.
3. Uncheck Android Dependencies.

来源stackoverflow

翻译成中文就是：

1. 右键Build Path > Configure Build Path，Android，添加v7 library.
2. Order and Export —— Add jars ，选择该v7包下 libs —— v7.jar.

Unable to execute dex: Multiple dex files define Landroid/support/v7/app/ActionBar$LayoutParams;