unfortunately the application has stopped

Question

I am trying to list the files in the sdcard and sort them according to last modified date.

I dont get any errors when I build it but when I start it in emulator logcat shows an error and the emulator show " unfortunately the application has stopped ". Please help me understand the error message and how to solve this.

Here is the code I use:

main activity.java public class MainActivity extends Activity {

@SuppressWarnings("rawtypes")
public final class ComparatorImplementation implements Comparator
{
    public int compare(Object f1, Object f2)
    {
      if (((File) f1).lastModified() < ((File) f2).lastModified())
      {
        return -1;
      }
      else if (((File) f1).lastModified() > ((File) f2).lastModified())
      {
        return 1;
      }
      else
      {
        return 0;
      }
    }
}

@SuppressWarnings("unchecked")
@Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);

    setContentView(R.layout.activity_main);

    TextView info = (TextView)findViewById(R.id.info);


    File dir = new File("/storage/sdcard/");

    if(dir.isDirectory())
    {
      // Fetching the list from the directory
      File[] files = dir.listFiles();

      // Sort files by date.
     Arrays.sort(files, new ComparatorImplementation());

      //Prints files in order by last modified date
      for(File file:files)
      {
          info.setText( "list of files according to last modified date \n " + file.getName());
      }
    }       
    }
}

This is the logcat output

10-14 01:58:10.470: E/AndroidRuntime(3399): FATAL EXCEPTION: main
10-14 01:58:10.470: E/AndroidRuntime(3399): Process: com.listfiles, PID: 3399
10-14 01:58:10.470: E/AndroidRuntime(3399): java.lang.RuntimeException: Unable to start activity          ComponentInfo{com.listfiles/com.listfiles.MainActivity}: java.lang.NullPointerException
10-14 01:58:10.470: E/AndroidRuntime(3399):     at    android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at android.app.ActivityThread.access$800(ActivityThread.java:135)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at android.os.Handler.dispatchMessage(Handler.java:102)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at android.os.Looper.loop(Looper.java:136)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at android.app.ActivityThread.main(ActivityThread.java:5017)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at java.lang.reflect.Method.invokeNative(Native Method)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at  java.lang.reflect.Method.invoke(Method.java:515)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at dalvik.system.NativeStart.main(Native Method)
10-14 01:58:10.470: E/AndroidRuntime(3399): Caused by: java.lang.NullPointerException
10-14 01:58:10.470: E/AndroidRuntime(3399):     at java.util.TimSort.sort(TimSort.java:169)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at java.util.Arrays.sort(Arrays.java:2023)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at com.listfiles.MainActivity.onCreate(MainActivity.java:55)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at android.app.Activity.performCreate(Activity.java:5231)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
10-14 01:58:10.470: E/AndroidRuntime(3399):     at                       android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
10-14 01:58:10.470: E/AndroidRuntime(3399):     ... 11 more

Answer 1

You are getting NullPointerException . This means object is not initialized properly.

You can see that there is one line at java.util.Arrays.sort(Arrays.java:2023) , so I guess problem is in Arrays.sort() function.

I think there is some problem with following code

File[] files = dir.listFiles();

// Sort files by date.
Arrays.sort(files, new ComparatorImplementation());

listFiles() can return null if the directory is invalid.

Try following code to prevent exception:

File[] files = dir.listFiles();

// Sort files by date.
if(files!=null){
    Arrays.sort(files, new ComparatorImplementation());

    //your rest of the code based on "files" object. If object is `null`, you can
    //not proceed
}

You should also use the method Environment.getExternalStorageDirectory() instead of specifying it manually, as some devices has different paths to the sd-card.

Answer 2

In your code,

File[] files = dir.listFiles();

files variable is set to null so it is crashing. Check before sorting if files is null or not.

Answer 3

that's possible there were no file there. you should check if there is any one with something like:

if (Dir.exists()) {
    // type your code here!
}

Answer 4

arun you are not handling the file

File[] files = dir.listFiles();
Arrays.sort(files, new ComparatorImplementation());

case:- suppose if dir path does exist, case:- suppose if dir doesn;t contain the files then,files object is null,and it obviously throws null pointer exception, and what exactly happening here

solution: handle files null pointer condition by:

if(files==null){
Toast.makeText(getApplicationContext(),"directory doesnt contain files",0).show();}
else{
for(File file:files)
  {
      info.setText( "list of files according to last modified date \n " + file.getName());
  }}

Answer 5

You are using listFiles() which can be null if this file is not a directory.

I can see here:

File dir = new File("/storage/sdcard/");

And then,

dir.listFiles();

public File[] listFiles()
Added in API level 1

Returns an array of files contained in the directory represented by this file. The result is null if this file is not a directory. The paths of the files in the array are absolute if the path of this file is absolute, they are relative otherwise.

Returns an array of files or null.