Null Pointer exception within a try catch block

Question

Getting the following runtime error, causing my application to crash on launch

E FATAL EXCEPTION: MonitoringThread 13533 AndroidRuntime E Process: foo.com, PID: 13533 13533 AndroidRuntime E java.lang.NullPointerException 13533 AndroidRuntime E at foo.com$MonitoringThread.run(foo.java:125) 13533
AndroidRuntime E at java.lang.Thread.run(Thread.java:841)

The offending line is

ret = mConnection.getInputStream().read(buffer);

in the following code snippet

try {
    ret = mConnection.getInputStream().read(buffer);
    } catch (IOException e) {
    break;
    }

Can anyone suggest next steps in trying to debug? I thought that use of a try catch block would eliminate any null pointer errors.

Answer 1

You should not use try / catch blocks to eliminate null pointer exceptions. Null pointer exceptions should be passed down, to let programmer know that problem arises and where.

In your case, you are catching IOException, so its not NullPointerException.

Also check what is null that is causing this exception, maybe its mConnection ? or getInputStream() returns null.

From this example, you can also see that its best to not execute lots of methods in one line:

ret = mConnection.getInputStream().read(buffer);

its better to write:

InputStream is = mConnection.getInputStream();
ret = is.read(buffer);

this way you will know from callstack where NPE originated,

if your code is unsafe, like you know you can get nullpointer from some method, then simply check it:

InputStream is=null;
if ( mConnection != null ) {
   is = mConnection.getInputStream();
   if ( is != null ) {
     ret = is.read(buffer);
   }
   else {
      // log error?
   }
} 
else {
   // log error?
}