微调器在弹出窗口中不起作用-Android

Question

I have a popup window that asks you to enter some information and then suppose to select a state with a spinner . I can place the spinner in the xml and it shows up fine, but when I try to activate the spinner it crashes the app. Here is the code I am using to create the spinner ,

public class DashboardActivity extends ListActivity implements OnItemSelectedListener {
    private TextView state_selection;
    private static final String[] items = {
        "Select State of Driver",
            "Alabama",
            "Alaska",
            "Arizona",
            "Arkansas",
            "California",
            "Colorado",
            "Connecticut",
            "Delaware",
            "District of Columbia",
            "Florida",
            "Georgia",
            "Hawaii",
            "Idaho",
            "Illinois",
            "Indiana",
            "Iowa",
            "Kansas",
            "Kentucky",
            "Louisiana",
            "Maine",
            "Maryland",
            "Massachusetts",
            "Michigan",
            "Minnesota",
            "Mississippi",
            "Missouri",
            "Montana",
            "Nebraska",
            "Nevada",
            "New Hampshire",
            "New Jersey",
            "New Mexico",
            "New York",
            "North Carolina",
            "North Dakota",
            "Ohio",
            "Oklahoma",
            "Oregon",
            "Pennsylvania",
            "Puerto Rico",
            "Rhode Island",
            "South Carolina",
            "South Dakota",
            "Tennessee",
            "Texas",
            "Utah",
            "Vermont",
            "Virginia",
            "Washington",
            "West Virginia",
            "Wisconsin",
            "Wyoming"
    };

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.xmlfile);

        state_selection = (TextView) findViewById(R.id.state_selection);
        Spinner my_spin = (Spinner) findViewById(R.id.state_spinner);
        my_spin.setOnItemSelectedListener(this);
        ArrayAdapter aa = new ArrayAdapter(this, android.R.layout.simple_spinner_item, items);
        aa.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        my_spin.setAdapter(aa);
    }

    @Override
    public void onItemSelected(AdapterView arg0, View arg1, int pos, long arg3) {
        state_selection.setText(items[pos]);
    }

    @Override
    public void onNothingSelected(AdapterView arg0) {
        // TODO Auto-generated method stub
        state_selection.setText("");
    }
}

Here is my Logcat

07-20 13:28:25.535: E/AndroidRuntime(1378): FATAL EXCEPTION: main
07-20 13:28:25.535: E/AndroidRuntime(1378): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example/com.example.DashboardActivity}: java.lang.NullPointerException
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2194)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2229)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.app.ActivityThread.access$600(ActivityThread.java:139)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1261)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.os.Handler.dispatchMessage(Handler.java:99)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.os.Looper.loop(Looper.java:154)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.app.ActivityThread.main(ActivityThread.java:4945)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at java.lang.reflect.Method.invokeNative(Native Method)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at java.lang.reflect.Method.invoke(Method.java:511)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at dalvik.system.NativeStart.main(Native Method)
07-20 13:28:25.535: E/AndroidRuntime(1378): Caused by: java.lang.NullPointerException
07-20 13:28:25.535: E/AndroidRuntime(1378):     at com.example.DashboardActivity.onCreate(DashboardActivity.java:160)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.app.Activity.performCreate(Activity.java:4531)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1071)
07-20 13:28:25.535: E/AndroidRuntime(1378):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2150)

Answer 1

07-20 13:28:25.535: E/AndroidRuntime(1378): Caused by: java.lang.NullPointerException

you forgot to add

setContentView(R.layout.layout_name); after super.onCreate(savedInstanceState); .

Answer 2

check your

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

}