修复Android NullPointerException

Question

我正在创建一个Android应用程序，但每次我尝试运行它时，我在第2行得到一个NullPointerException。可能导致这个？我肯定在build.xml文件中有微调器。当你调用这样的对象时它是不是被初始化了？

public void onStart(){

    spnLocale = (Spinner)findViewById(R.id.spinner1); //1

    spnLocale.setOnItemSelectedListener(new OnSelectListener()); //2

}
public static class MainSectionFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    public static final String ARG_SECTION_NUMBER = "section_number";

    public MainSectionFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {

        if(getArguments().getInt(ARG_SECTION_NUMBER)==1){
            View rootView = inflater.inflate(R.layout.browse_layout,
                container, false);
            Spinner spnLocale = (Spinner)findViewById(R.id.spinner1);


            spnLocale.setOnItemSelectedListener(new OnSelectListener());
            return rootView;

        }

        else {//if(getArguments().getInt(ARG_SECTION_NUMBER) == 3){
            View rootView = inflater.inflate(R.layout.fragment_main_dummy,
                    container, false);
            TextView dummyTextView = (TextView) rootView
                    .findViewById(R.id.section_label);
            dummyTextView.setText("help screen");
            return rootView;

        }

    }
}

这是我的xml：

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:orientation="horizontal" >

    <Spinner
        android:id="@+id/spinner1"
        android:layout_width="154dp"
        android:layout_height="wrap_content"
        android:entries="@array/state_array"
        android:prompt="@string/state_prompt" />

    <Spinner
        android:id="@+id/spinner2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" 
        />

</LinearLayout>

这是我的日志：

03-22 03:40:17.242: E/AndroidRuntime(2101): Caused by: java.lang.NullPointerException 
03-22 03:40:17.242: E/AndroidRuntime(2101): at com.XXXX.list.MainActivity.onStart(MainActivity.java:175)
03-22 03:40:17.242: E/AndroidRuntime(2101): at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1171)
03-22 03:40:17.242: E/AndroidRuntime(2101): at android.app.Activity.performStart(Activity.java:5241)
03-22 03:40:17.242: E/AndroidRuntime(2101): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2168) –  

Answer 1

如果您说NullPointerException @第2行即spnLocale.setOnItemSelectedListener(new OnSelectListener());，则spnLocale为空。

您可能引用了错误的布局。即，您为活动设置的布局没有带有ID的微调器。

编辑：

        View rootView = inflater.inflate(R.layout.build,
            container, false);
            spnLocale = (Spinner)rootView.findViewById(R.id.spinner1);
        return rootView;

Answer 2

您需要先将Adapter设置为spinner，然后才能使用onItemSelected Listener ...这可能会对您有所帮助

@Override
    protected void onStart() {
        // TODO Auto-generated method stub
        super.onStart();

String str[]={"Dummy","Dummy","Dummy"};
            spnLocale = (Spinner)findViewById(R.id.spinner1); //1

            ArrayAdapter<String> adapter=new ArrayAdapter<String>(MainActivity.this, android.R.layout.simple_spinner_item,str);
            adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

            spnLocale.setAdapter(adapter);
spnLocale.setOnItemSelectedListener(new OnItemSelectedListener() {

    @Override
    public void onItemSelected(AdapterView<?> arg0, View arg1, int arg2,
            long arg3) {
        // TODO Auto-generated method stub

    }

    @Override
    public void onNothingSelected(AdapterView<?> arg0) {
        // TODO Auto-generated method stub          
    }
});

    }

Answer 3

问题是我使用了滑动Tabs，所以我不得不使用rootView.findViewbyId（）而不仅仅是findviewbyID