java.lang.NoClassDefFoundError: Activity

====================================
08-30 13:06:43.736: E/AndroidRuntime(31225): FATAL EXCEPTION: main
08-30 13:06:43.736: E/AndroidRuntime(31225): Process: com.example.test, PID: 31225
08-30 13:06:43.736: E/AndroidRuntime(31225): java.lang.NoClassDefFoundError: Failed resolution of: Lcom/example/test/PlayActivity;
08-30 13:06:43.736: E/AndroidRuntime(31225): at com.example.test.MainActivity.onClick(MainActivity.java:167)
08-30 13:06:43.736: E/AndroidRuntime(31225): at android.view.View.performClick(View.java:4909)
08-30 13:06:43.736: E/AndroidRuntime(31225): at android.view.View$PerformClick.run(View.java:20390)
08-30 13:06:43.736: E/AndroidRuntime(31225): at android.os.Handler.handleCallback(Handler.java:815)
08-30 13:06:43.736: E/AndroidRuntime(31225): at android.os.Handler.dispatchMessage(Handler.java:104)
08-30 13:06:43.736: E/AndroidRuntime(31225): at android.os.Looper.loop(Looper.java:194)
08-30 13:06:43.736: E/AndroidRuntime(31225): at android.app.ActivityThread.main(ActivityThread.java:5815)
08-30 13:06:43.736: E/AndroidRuntime(31225): at java.lang.reflect.Method.invoke(Native Method)
08-30 13:06:43.736: E/AndroidRuntime(31225): at java.lang.reflect.Method.invoke(Method.java:372)
08-30 13:06:43.736: E/AndroidRuntime(31225): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1009)
08-30 13:06:43.736: E/AndroidRuntime(31225): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:804)
08-30 13:06:43.736: E/AndroidRuntime(31225): Caused by: java.lang.ClassNotFoundException: Didn't find class "com.example.test.PlayActivity" on path: DexPathList[[zip file "/data/app/com.example.test-2/base.apk"],nativeLibraryDirectories=[/data/app/com.example.test-2/lib/arm64, /vendor/lib64, /system/lib64]]
08-30 13:06:43.736: E/AndroidRuntime(31225): at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:56)
08-30 13:06:43.736: E/AndroidRuntime(31225): at java.lang.ClassLoader.loadClass(ClassLoader.java:511)
08-30 13:06:43.736: E/AndroidRuntime(31225): at java.lang.ClassLoader.loadClass(ClassLoader.java:469)
08-30 13:06:43.736: E/AndroidRuntime(31225): ... 11 more
08-30 13:06:43.736: E/AndroidRuntime(31225): Suppressed: java.lang.NoClassDefFoundError: com.example.test.PlayActivity
08-30 13:06:43.736: E/AndroidRuntime(31225): at dalvik.system.DexFile.defineClassNative(Native Method)
08-30 13:06:43.736: E/AndroidRuntime(31225): at dalvik.system.DexFile.defineClass(DexFile.java:226)
08-30 13:06:43.736: E/AndroidRuntime(31225): at dalvik.system.DexFile.loadClassBinaryName(DexFile.java:219)
08-30 13:06:43.736: E/AndroidRuntime(31225): at dalvik.system.DexPathList.findClass(DexPathList.java:322)
08-30 13:06:43.736: E/AndroidRuntime(31225): at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:54)
08-30 13:06:43.736: E/AndroidRuntime(31225): ... 13 more
08-30 13:06:43.736: E/AndroidRuntime(31225): Suppressed: java.lang.ClassNotFoundException: com.example.test.PlayActivity
08-30 13:06:43.736: E/AndroidRuntime(31225): at java.lang.Class.classForName(Native Method)
08-30 13:06:43.736: E/AndroidRuntime(31225): at java.lang.BootClassLoader.findClass(ClassLoader.java:781)
08-30 13:06:43.736: E/AndroidRuntime(31225): at java.lang.BootClassLoader.loadClass(ClassLoader.java:841)
08-30 13:06:43.736: E/AndroidRuntime(31225): at java.lang.ClassLoader.loadClass(ClassLoader.java:504)
08-30 13:06:43.736: E/AndroidRuntime(31225): ... 12 more
08-30 13:06:43.736: E/AndroidRuntime(31225): Caused by: java.lang.NoClassDefFoundError: Class not found using the boot class loader; no stack available
个人按下面顺序排查：
1.
在Manifest.xml中查看Activity的名字是否书写正确。
2.
在Manifest.xml确认Activity标签中的android:name属性的格式，例如我的activity的名字是helloworld，确认android:name=”.helloworld”（其中的点很重要）。
3.
如果有添加外部包的情况下，在工程根目录下建libs目录，然后将需要导入的外部包放入此目录内再add jars。
4.
如果上面都确保OK的情况下仍发生问题的话，clean工程->关闭eclipse->在命令行中重启adb（adb kill-server adbstart-server）->打开eclipse->编译启动。
 
上面4步基本能解决问题。