File list error on load Android ListActivity

Question

I got this error, and i dont know who is the problem. In some devices work de app but in other make this error.

//THIS IS THE CODE

package com.neoadn.takerenamephoto;

import java.io.File;
import java.util.ArrayList;
import java.util.List;
import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.app.AlertDialog;
import android.app.ListActivity;
import android.content.DialogInterface;
import android.content.Intent;
import android.view.View;
import android.view.Window;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemLongClickListener;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;

public class ListaFotos extends ListActivity {

private File file, temp_file;
private List<String> myList;
String root_sd;
File imagen;
String direccionFile;

public void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);
    requestWindowFeature(Window.FEATURE_NO_TITLE);
    getListView().setBackgroundResource(R.drawable.fondo_p);

    myList = new ArrayList<String>();   

    root_sd = Environment.getExternalStorageDirectory().toString();
    file = new File( root_sd + "/Pictures/TRPHOTO" ) ;       
    File list[] = file.listFiles();

    for( int i=0; i< list.length; i++)
    {
            myList.add( list[i].getName() );
    }

    setListAdapter(new ArrayAdapter<String>(this,
            android.R.layout.simple_list_item_1, myList ));

The line 41 is:

    for( int i=0; i< list.length; i++)

This shows a listactivity of files and folders into some folders.

UPDATED:

Finally i used this to fix the problem: ( if list is not null, then load. If is null not load)

    if(list != null) {
    for( int i=0; i< list.length; i++) {
        // do your for loop
    }
}

Answer 1

It happens when the following statement returns null:

File list[] = file.listFiles();

That can happen when your directory "file" doens't contain any files. Because your list list is therefore null, the following statement

for( int i=0; i< list.length; i++)

crashes with a NullPointerException. I'd recommend you to check if your list variable is null before entering the for loop like this:

if(list != null) {
    for( int i=0; i< list.length; i++) {
        // do your for loop
    }
}