Android - Where should I place my listeners?

Question

I'm having problems trying to work out where to put my listeners (I have two listeners I'm trying to setup but will only reference one here and hopefully will be able to work the second one out for myself!).

I have a spinner and want to update some TextViews when the spinner item changes so think I want an onItemSelected listener. I have tried putting it in the onCreate method and onStart method after reading someone else's question on SO but the app will just FC when it loads.

The app uses Tabs/Fragments for navigation, could this be something to do with the problem (Fragments not inflated when trying to set the listener)?

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    // Create the adapter that will return a fragment for each of the three
    // primary sections of the app.
    mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

    // Set up the ViewPager with the sections adapter.
    mViewPager = (ViewPager) findViewById(R.id.pager);
    mViewPager.setAdapter(mSectionsPagerAdapter);

    spinner = (Spinner)findViewById(R.id.spinner);
    spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
        @Override
        public void onItemSelected(AdapterView<?> parentView, View selectedItemView, int position, long id) {
            Toast.makeText(getApplicationContext(), "hello", Toast.LENGTH_LONG).show();
        }

        @Override
        public void onNothingSelected(AdapterView<?> parentView) {
            // your code here
        }

    });


}

And the logcat;

08-24 17:19:16.256  13803-13803/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
    java.lang.RuntimeException: Unable to start activity     ComponentInfo{com.example.myapplication/com.example.myapplication.MainActivity}: java.lang.NullPointerException
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2211)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2261)
    at android.app.ActivityThread.access$600(ActivityThread.java:141)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1256)
    at android.os.Handler.dispatchMessage(Handler.java:99)
    at android.os.Looper.loop(Looper.java:137)
    at android.app.ActivityThread.main(ActivityThread.java:5103)
    at java.lang.reflect.Method.invokeNative(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:525)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
    at dalvik.system.NativeStart.main(Native Method)
    Caused by: java.lang.NullPointerException
    at com.example.myapplication.MainActivity.onCreate(MainActivity.java:57)
    at android.app.Activity.performCreate(Activity.java:5133)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2175)
    ... 11 more

Answer 1

When you get the NullPointerException at spinner.setXXX() then your spinner variable contains null. That means that the findViewById() in the pine before did not find the spinner in the current context. The context is set with your line setContentView() .

Bottom line: It looks like your R.layout.activity_main does not contain R.id.spinner .