Difference between view.findViewById(R.id....) and just findViewById(R.id...)

Question

I'm trying to implement a Custom Array Adapter ,

Problem is my code crashes when I use

 ImageView imageView=(ImageView)findViewById(R.id.imageView);

rather than

ImageView imageView=(ImageView)rowView.findViewById(R.id.imageView);

where rowView is the layout file for the ListView that i have infalted.

Why does this occur , I thought rowView.findViewById(R.id..) will just make it faster to find the element by id, but without it the app crashes, Can you please explain

This is my Activity code

public class MainActivity extends AppCompatActivity {

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

    //Now for the list view

    ListView listView=(ListView)findViewById(R.id.listView);

    String[] values =new String[]{"iOS","android","firefoxOs","Ubuntu"};

   MySimpleAdapter simpleAdapter=new MySimpleAdapter(this,values);

   listView.setAdapter(simpleAdapter);
    //

    }

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();

    //noinspection SimplifiableIfStatement
    if (id == R.id.action_settings) {
        return true;
    }

    return super.onOptionsItemSelected(item);
}

class MySimpleAdapter extends ArrayAdapter<String>{

    private Context context;
    private String[] values;

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
    //listView calls this on its adapter for each row
        Log.v("list","getView");
        LayoutInflater inflater = (LayoutInflater) context
                .getSystemService(Context.LAYOUT_INFLATER_SERVICE);

        View rowView=inflater.inflate(R.layout.custom_row,parent,false);

        TextView textView1= (TextView) rowView.findViewById(R.id.textView1);

        textView1.setText(values[position]);
        //for the android row alone set an image , others leave it as blank
        if(values[position].startsWith("and"))
        {
            ImageView imageView=(ImageView)rowView.findViewById(R.id.imageView);
            imageView.setImageResource(R.drawable.ic_launcher);//Ok R.drawale....is also an id
        }


        return rowView;
    }

    public MySimpleAdapter(Context context,String[] values) {
        super(context, R.layout.custom_row,values);

        this.context=context;
        this.values=values;
        Log.v("list","constructor");
    }
}

}

Answer 1

findViewById always requires a context to find the view.

If you are calling this from a class which is extends the Activity class, you could just use findViewById, because Activity is the context.

When you are calling it from a class that extends fragment, you have to use getActivity().findViewById

In your case, since you are calling it in an adapter, you need to find that view within listview row. so you use view.findViewById

Answer 2

findViewById() is used to find a view in the layout of your Activity. This is not possible in a Fragment for example. view.findViewById() is used to find a view inside a specific other view. For example to find a view inside your ListView row layout. Your app crashes without it because the view you're searching for is inside rowView. By using the normal findViewById() you won't find the view.