Can an assignment statement cause a null-pointer exception?

This code snippet is reading camera picture Exif metadata using ExifInterface: Apparently, one particular picture has no or invalid datetime and .getDateTime() is returning null. In the code I assign it to a long, dt, and that results in the exception that is shown below. Of course, if I un-comment the null-check just prior to the assignment, all is well.

So, I have 1 question and 1 lesson:

I’m assuming getDateTime() is really the culprit. Can an assignment cause such a exception?

As you see, the offending line is within try/catch but it wasn’t catching it because I was catching only IOException. When it was changed to Exception, it caught.

         String latlong = "";
         long dt = 0;
         ExifInterface exifInterface;
         try {
             exifInterface = new ExifInterface(pf.getAbsolutePath());
             if (exifInterface != null)
             {
                 float[] latLng = new float[2];
                 if (exifInterface.getLatLong(latLng)) { //file has exif latlong info
                     //etc, latLng[0] is your latitute value and latLng[1] your longitude value
                     latlong = latLng[0] + "," + latLng[1];
                 }
                 //if (exifInterface.getDateTime() != null)
                   dt = exifInterface.getDateTime();
                 picInfo.comments = exifInterface.getAttribute(ExifInterface.TAG_USER_COMMENT);
             }
             else
             {
                 System.out.println(">>>>>>>>>>>>>ERROR: cameraPicTask.doInBackground");
                 System.out.println(">>>>>>>>>>>>>-----: null exifInterface for: " + pf.getAbsolutePath());
             }
         } catch (IOException e) {
             e.printStackTrace();
         }

    E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #2
    Process: com.example.mypics, PID: 7912
    java.lang.RuntimeException: An error occurred while executing doInBackground()
        at android.os.AsyncTask$4.done(AsyncTask.java:415)
        at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:383)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:252)
        at java.util.concurrent.FutureTask.run(FutureTask.java:271)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:305)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:920)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'long java.lang.Long.longValue()' on a null object reference
        at com.example.mypics.PicsActivity$cameraPicTask.doInBackground(PicsActivity.java:211)
        at com.example.mypics.PicsActivity$cameraPicTask.doInBackground(PicsActivity.java:185)
        at android.os.AsyncTask$3.call(AsyncTask.java:394)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:305) 
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) 
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 
        at java.lang.Thread.run(Thread.java:920)

Answer

It seems exifInterface.getDateTime() returns a Long, and in this case, it returns null. You assign it to long dt which involves an unboxing operation. The compiler emits code to convert the Long to a long by calling longValue() on it, which throws the NPE. You can see it in your stack trace:

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method ‘long java.lang.Long.longValue()’ on a null object reference

Even if you were to assign it to a Long first:

Long dateTime = exifInterface.getDateTime();
long dt = dateTime;

it would still have to unbox it, and thus, yes, an assignment can throw a NullPointerException.

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Answer