在xml 文件里设置了 onclick ，然后又在mainactivity 里面  findViewById 找了按钮，点击后出现

闪退， 后面试了一下又可以

08-17 15:25:50.686: E/AndroidRuntime(2203): FATAL EXCEPTION: main

08-17 15:25:50.686: E/AndroidRuntime(2203): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ex00_example/com.example.ex00_example.SecondActivity}: java.lang.ClassCastException: android.widget.RadioGroup cannot be cast to android.widget.Button

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2059)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.app.ActivityThread.access$600(ActivityThread.java:130)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.os.Handler.dispatchMessage(Handler.java:99)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.os.Looper.loop(Looper.java:137)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.app.ActivityThread.main(ActivityThread.java:4745)

08-17 15:25:50.686: E/AndroidRuntime(2203): at java.lang.reflect.Method.invokeNative(Native Method)

08-17 15:25:50.686: E/AndroidRuntime(2203): at java.lang.reflect.Method.invoke(Method.java:511)

08-17 15:25:50.686: E/AndroidRuntime(2203): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)

08-17 15:25:50.686: E/AndroidRuntime(2203): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)

08-17 15:25:50.686: E/AndroidRuntime(2203): at dalvik.system.NativeStart.main(Native Method)

08-17 15:25:50.686: E/AndroidRuntime(2203): Caused by: java.lang.ClassCastException: android.widget.RadioGroup cannot be cast to android.widget.Button

08-17 15:25:50.686: E/AndroidRuntime(2203): at com.example.ex00_example.SecondActivity.onCreate(SecondActivity.java:25)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.app.Activity.performCreate(Activity.java:5008)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)

08-17 15:25:50.686: E/AndroidRuntime(2203): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)

08-17 15:25:50.686: E/AndroidRuntime(2203): ... 11 more