java.lang.RuntimeException: Unable to start activity ComponentInfo{myActivityPathName}: java.lang.NullPointerException

Question

I know there are similar questions around, but all the answers were specific to those applications. Please help. This is my code for the activity which is having the problem

 public class MainActivity extends Activity implements OnClickListener
{
private static final int SWIPE_MIN_DISTANCE = 120;
private static final int SWIPE_MAX_OFF_PATH = 250;
private static final int SWIPE_THRESHOLD_VELOCITY = 200;
ViewFlipper flipper;
private GestureDetector gestureDetector;
static View passedView;
static String folderName = "myOffice";
static String folderPath = Environment.getExternalStorageDirectory() + "/" + folderName;

@SuppressWarnings("deprecation")
@Override
public void onCreate(Bundle savedInstanceState) 
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    flipper = (ViewFlipper)findViewById(R.id.viewFlipper1);
    flipper.setOnClickListener(this);
    Button button1_1 = (Button)findViewById(R.id.button1_1);
    button1_1.setOnClickListener(this);
    Button button1_2 = (Button)findViewById(R.id.button1_2);
    button1_2.setOnClickListener(this);
    Button button3 = (Button)findViewById(R.id.button3);
    button3.setOnClickListener(this);
    Button newDoc = (Button)findViewById(R.id.newDoc);
    newDoc.setOnClickListener(this);
    listTest();
    gestureDetector = new GestureDetector(new MyGestureDetector());
    View mainview = (View) findViewById(R.id.viewFlipper1);

    // Set the touch listener for the main view to be our custom gesture listener
    mainview.setOnTouchListener(new View.OnTouchListener() {
        public boolean onTouch(View v, MotionEvent event) 
        {
            if (gestureDetector.onTouchEvent(event)) 
            {
                return true;
            }
            return false;
        }
    });
}

public static void checkForDirectory() 
{
    File folder = new File(folderPath);
    if (folder.exists() == false) 
    {
        folder.mkdir();
    }       
}

public String[] getFiles() 
{
    String[] fileNames;
    File f = new File(folderPath);
    File[] files = f.listFiles();
    if (files.length == 0)
    {
        return null;
    }
    else 
    {
        fileNames = new String[files.length];
        for (int i=0; i < files.length; i++) 
        {
            fileNames[i] = files[i].getName();
        }
        return fileNames;
    }       
}

public void listTest()
{
    final ListView listView;
    String[] filesInFolder = getFiles();
    listView = (ListView)findViewById(R.id.listView1);
    listView.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, filesInFolder));
    listView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() 
    {
        @Override
        public boolean onItemLongClick(AdapterView<?> arg0, View v, int position, long id) 
        {
            TextView tv = (TextView)v;
            String fName = (String) tv.getText();
            File file = new File(folderPath, fName);
            showDeletedToast();
            return file.delete();               
        }

        private void showDeletedToast() 
        {
            Toast toast = Toast.makeText(getApplicationContext(), "File deleted, Will be removed from the list.", Toast.LENGTH_LONG);
            toast.show();
        }
    });

    listView.setOnItemClickListener(new AdapterView.OnItemClickListener()
    {
        public void onItemClick(AdapterView<?> parent, View v, int position, long id) 
        {
            Intent intent = new Intent(getApplicationContext(), TextActivity.class);
            startActivity(intent);
            passedView = TextActivity.getView(v);
        }               
    }); 
    listView.setAdapter(new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, filesInFolder));

}   

@Override
public boolean onCreateOptionsMenu(Menu menu) 
{
    getMenuInflater().inflate(R.menu.activity_main, menu);
    return true;
}

@Override
public void onClick(View v)
{   
    if (v.getId()==  R.id.button1_1) 
    {
        flipper.showNext();
    }
    else if(v.getId()==  R.id.button1_2)
    {
        flipper.showNext();
        flipper.showNext();
    }
    else if(v.getId()== R.id.newDoc)
    {
        Intent intent = new Intent(this, TextActivity.class);
        startActivity(intent);
    }
}

class MyGestureDetector extends SimpleOnGestureListener 
{
    @Override
    public boolean onFling(MotionEvent e1, MotionEvent e2, float velocityX, float velocityY) 
    {
        if (Math.abs(e1.getY() - e2.getY()) > SWIPE_MAX_OFF_PATH)
        {
            return false;
        }

        // right to left swipe
        if(e1.getX() - e2.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY)
        {
            flipper.showNext();
            MainActivity.this.overridePendingTransition(
                    R.anim.slide_in_right,
                    R.anim.slide_out_left
                    );
            // left to right swipe
        } 
        else if (e2.getX() - e1.getX() > SWIPE_MIN_DISTANCE && Math.abs(velocityX) > SWIPE_THRESHOLD_VELOCITY)
        {
            flipper.showPrevious();
            MainActivity.this.overridePendingTransition(
                    R.anim.slide_in_left, 
                    R.anim.slide_out_right
                    );
        }

        return false;
    }

    // It is necessary to return true from onDown for the onFling event to register
    @Override
    public boolean onDown(MotionEvent e) {
        return true;
    }
}

}

Why does this happen and how does one fix this? LogCat output:

11-19 22:55:58.730: E/AndroidRuntime(10964): FATAL EXCEPTION: main
11-19 22:55:58.730: E/AndroidRuntime(10964): java.lang.RuntimeException: Unable to start                                                        activity ComponentInfo{com.illuminatiRN.myOffice/com.illuminatiRN.myOffice.MainActivity}:   java.lang.NullPointerException
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1818)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1834)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.app.ActivityThread.access$500(ActivityThread.java:122)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1027)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.os.Handler.dispatchMessage(Handler.java:99)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.os.Looper.loop(Looper.java:132)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.app.ActivityThread.main(ActivityThread.java:4126)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at java.lang.reflect.Method.invokeNative(Native Method)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at java.lang.reflect.Method.invoke(Method.java:491)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:844)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:602)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at dalvik.system.NativeStart.main(Native Method)
11-19 22:55:58.730: E/AndroidRuntime(10964): Caused by: java.lang.NullPointerException
11-19 22:55:58.730: E/AndroidRuntime(10964):    at java.util.Arrays$ArrayList.<init>(Arrays.java:47)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at java.util.Arrays.asList(Arrays.java:163)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.widget.ArrayAdapter.<init>(ArrayAdapter.java:128)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at com.illuminatiRN.myOffice.MainActivity.listTest(MainActivity.java:100)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at com.illuminatiRN.myOffice.MainActivity.onCreate(MainActivity.java:49)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1050)
11-19 22:55:58.730: E/AndroidRuntime(10964):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1782)

Answer 1

The filesInFolder String array that you pass to the ArrayAdapter is null and that will throw the NullPointerException you see in the stacktrace. Make sure that this array(filesInFolder) is not null. As you return that array from the getFiles() method, modify it so you return an empty array if files.length is 0.