findViewById返回null，求大神解决。

Question

发现在 findViewById(R.id.eat) 返回null，我不清楚哪里错了，返回了null

Answer 1

logcat06-22 11:30:27.610: E/AndroidRuntime(2013): FATAL EXCEPTION: main
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.app.ActivityThread.access$800(ActivityThread.java:135)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.os.Handler.dispatchMessage(Handler.java:102)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.os.Looper.loop(Looper.java:136)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.app.ActivityThread.main(ActivityThread.java:5017)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at java.lang.reflect.Method.invokeNative(Native Method)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at java.lang.reflect.Method.invoke(Method.java:515)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at dalvik.system.NativeStart.main(Native Method)
06-22 11:30:27.610: E/AndroidRuntime(2013): Caused by: java.lang.NullPointerException
06-22 11:30:27.610: E/AndroidRuntime(2013):  at com.hipad.checkboxtext.MainActivity.onCreate(MainActivity.java:38)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.app.Activity.performCreate(Activity.java:5231)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
06-22 11:30:27.610: E/AndroidRuntime(2013):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
06-22 11:30:27.610: E/AndroidRuntime(2013):  ... 11 more

Answer 2

你实例化控件错了地方
写在 /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
 
        public PlaceholderFragment() {
        }
 
        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container,
                    false);
        eat = (CheckBox)rootView .findViewById(R.id.eat);
        System.out.println(R.id.eat+"...");
        sleep = (CheckBox)rootView.findViewById(R.id.sleep);
        all = (CheckBox)rootView.findViewById(R.id.all);
            return rootView;
        }
    }中的onCreateViewl里面

Answer 3

你控件写在fragment_main.xml 加载页面却是activity_main.xml 当然报null啦

Answer 4

3#正解！注意PlaceholderFragment ，这是跟之前版本的不同！