Why java.lang.ClassNotFoundException when trying to use mediaplayer?

I was trying to run a program with a mediaplayer. Why am I getting this exception?

java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.android.myplayer/com.andr
oid.myplayer.MediaPlayerDemo_video}: java.lang.ClassNotFoundException: com.android.myplayer.MediaPlayerDemo_video in loader dal
vik.system.PathClassLoader@349c48d8
E/AndroidRuntime( 4102):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2417)
E/AndroidRuntime( 4102):        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
E/AndroidRuntime( 4102):        at android.app.ActivityThread.access$2200(ActivityThread.java:119)
E/AndroidRuntime( 4102):        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
E/AndroidRuntime( 4102):        at android.os.Handler.dispatchMessage(Handler.java:99)
E/AndroidRuntime( 4102):        at android.os.Looper.loop(Looper.java:123)
E/AndroidRuntime( 4102):        at android.app.ActivityThread.main(ActivityThread.java:4363)
E/AndroidRuntime( 4102):        at java.lang.reflect.Method.invokeNative(Native Method)
E/AndroidRuntime( 4102):        at java.lang.reflect.Method.invoke(Method.java:521)
E/AndroidRuntime( 4102):        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
E/AndroidRuntime( 4102):        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
E/AndroidRuntime( 4102):        at dalvik.system.NativeStart.main(Native Method)
E/AndroidRuntime( 4102): Caused by: java.lang.ClassNotFoundException: com.android.myplayer.MediaPlayerDemo_video in loader dalv
ik.system.PathClassLoader@349c48d8
E/AndroidRuntime( 4102):        at dalvik.system.PathClassLoader.findClass(PathClassLoader.java:243)
E/AndroidRuntime( 4102):        at java.lang.ClassLoader.loadClass(ClassLoader.java:573)
E/AndroidRuntime( 4102):        at java.lang.ClassLoader.loadClass(ClassLoader.java:532)
E/AndroidRuntime( 4102):        at android.app.Instrumentation.newActivi开发者_运维技巧ty(Instrumentation.java:1021)
E/AndroidRuntime( 4102):        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2409)
E/AndroidRuntime( 4102):        ... 11 more

That exception usually means a CLASSPATH problem: the class loader hasn't been told where to find that class when the JVM needs to load it. So you'll need to figure out which JAR contains that class and add it to the CLASSPATH.