DataOutputStream nullpointer exception

Question

I am getting a NullPointerException on a DataOutputStream in Android.

I first wrote some code to write out files in Java using DataOutputStream . Everything worked perfectly. The I imported the code to an Android project and I am getting some weird behavior.

The only code I changed to be android specific is how I get the directory.

So what exactly is happening is: I am creating the DataOutputStream and then I am closing it. That is all for now. I figure it is a good first step to test if stream was opened correctly is to close it.

DataOutputStream is a class instance variable.

When I call close on it in same method it closes fine. When I call close on it from a different method, it gets NullPointerException . But doing these same things in Java did not cause a problem, though they did in Android.

public class WavFileOutStream {

    private DataOutputStream stream;

    public WavFileOutStream(String fileName)
    {

        // Get the directory for the user's public pictures directory. 
        File file = new File(Environment.getExternalStoragePublicDirectory(
            Environment.DIRECTORY_RINGTONES), fileName);
        if (!file.mkdirs()) {
            Log.e("trace", "Directory not created");
        }

        try {
            stream = new DataOutputStream(new FileOutputStream(file));

            if(stream==null)
            {
                Log.i("trace","Null Stream!");
            }

            //<--PROBLEM LINE 1-->
            //If I call close from this line I get NO nullPointer
            //stream.close();


        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } 

        //<--PROBLEM LINE 1-->
        //If I call close from this line I get a nullPointer
        //I comment out other close() first
        close();

    }

    public void close()
    {
        try {
            stream.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Error:

02-24 12:49:39.229: E/AndroidRuntime(11539): FATAL EXCEPTION: main
02-24 12:49:39.229: E/AndroidRuntime(11539): Process: com.example.ringtonegenerator, PID: 11539
02-24 12:49:39.229: E/AndroidRuntime(11539): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.ringtonegenerator/com.example.ringtonegenerator.MainActivity}: java.lang.NullPointerException
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.os.Handler.dispatchMessage(Handler.java:102)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.os.Looper.loop(Looper.java:136)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.app.ActivityThread.main(ActivityThread.java:5017)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at java.lang.reflect.Method.invokeNative(Native Method)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at java.lang.reflect.Method.invoke(Method.java:515)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at dalvik.system.NativeStart.main(Native Method)
02-24 12:49:39.229: E/AndroidRuntime(11539): Caused by: java.lang.NullPointerException
02-24 12:49:39.229: E/AndroidRuntime(11539):    at com.example.ringtonegenerator.WavFileOutStream.close(WavFileOutStream.java:178)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at com.example.ringtonegenerator.WavFileOutStream.<init>(WavFileOutStream.java:43)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at com.example.ringtonegenerator.WavFileWriter.writeWave(WavFileWriter.java:10)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at com.example.ringtonegenerator.MainActivity.onCreate(MainActivity.java:23)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.app.Activity.performCreate(Activity.java:5231)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
02-24 12:49:39.229: E/AndroidRuntime(11539):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
02-24 12:49:39.229: E/AndroidRuntime(11539):    ... 11 more

Answer 1

If you throw an exception when the stream is created, the stream will still be null and you will still call close. There is no check for null there, so it will throw NPE.

public void close()
{
    if(stream != null){
        try {
            stream.close();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

Answer 2

You have code after a catch block that should be inside the try block. That's why you get the NPE.

The underlying problem is that you are calling file.mkdirs() when you should be calling file.getParentFile().mkdirs(), so you are creating the target file as a directory, so you can't create the target file, so you are getting an IOException instead of creating the FileOutputStream, so the DataOutputStream reference is still null, so you get an NPE when the code after the catch block, that should be in the try block, executes.

NB the result of 'new' cannot be null, so testing it straight afterwards is futile.