NullPointerException from spinner.setAdapter()

Question

I want to create a spinner to show the list m. But the following code gives NullPointerException at spinner.setAdapter(spinner_adapter). Why does this happen?

public class Request extends ListActivity {
    private static final String[] m={"A","B","O","AB","others"};   
    private TextView view;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        SimpleAdapter adapter = new SimpleAdapter(this, getData(),
            R.layout.listitem, new String[] { "name", "quant" },
            new int[] {R.id.name, R.id.quant });
        setListAdapter(adapter);

        view = (TextView) findViewById(R.id.spinnerText);  
        Spinner spinner = (Spinner) findViewById(R.id.spinner01);  
        ArrayAdapter<String> spinner_adapter = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_item,m);  
        spinner_adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);  
        spinner.setAdapter(spinner_adapter);              
        spinner.setOnItemSelectedListener(new SpinnerSelectedListener());  
        spinner.setVisibility(View.VISIBLE);  
    }

    class SpinnerSelectedListener implements OnItemSelectedListener{              
        public void onItemSelected(AdapterView<?> arg0, View arg1, int arg2,  
            long arg3) {  
            view.setText(""+m[arg2]);  
        }  
        public void onNothingSelected(AdapterView<?> arg0) {       
        }  
    }
}

Answer 1

Your line

    Spinner spinner = (Spinner) findViewById(R.id.spinner01);

ends up the spinner being null, since no content view has been assigned to your activity.

Thus, when trying to interact with your spinner, a NPE is thrown.

Add the setContentView(R.layout.your_layout_content); to the onCreate method, before initializing its views.

Answer 2

Perhaps, you are trying to use IDs from layout and you missed to call setContentView. Call it just below super.onCreate(savedInstanceState);

setContentView(<YourLayout>);

Answer 3

You are accessing those views in your onCreate i.e

 Spinner spinner = (Spinner) findViewById(R.id.spinner01); 

The id is present in R.java file, cause you have proper layout , But for accessing those will fails causing nullpointer at runtime to all of your views

You Need to add

setContentView(R.layout.main);

inside onCreate where main is your layout i.e main.xml which contains spinner01

Answer 4

Since you use ListView Default layout both TextView and Spinner will be null cause there are no R.id.spinnerText nor R.id.spinner01

So you should implement your custom layout

setContentView(...);

in this layout there should be

ListView with an id of @android:id/list

<ListView
    android:id="@android:id/list"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

Spinner with an id of @+id/spinner01

<Spinner
    android:id="@+id/spinner01"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />

TextView with an id of @+id/spinnerText

<TextView
    android:id="@+id/spinnerText"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />