Debugging Android NullPointer Exceptions

A lot of the errors that you will get while writing programs in Java to run on Android will be NullPointer exceptions. They are easy to debug with the tools that Eclipse and ADT give you. If you resume the suspended Hello World Program we left in the last post you will get a NullPointer Exception and be returned to the debug perspective of Eclipse.



In the debug window, you will see the call stack starts with Thread [<3> main] (Suspended (exception RuntimeException)) – giving you a clear indication of the cause of the runtime error. If you click on the top stack frame icon(horizonal blue bars) in the variables window you will see a grey circle labelled e with value Nullpointer Exception – again giving you a clear indication of the error.

The Java source code window is blank, because the tab is showing code relevant to the stack frame which is inside the Dalvik virtual machine and not displayed. If you click on the hello android tab in this window you will see that line 13 is still highlighted as the exception was caused by this line and occurred before the following line could be executed.

This example is rather artificial in that you set a breakpoint on the line causing the run-time exception. In a real world example you are unlikely to have put a breakpoint on the line causing the problem. In this situation LogCat is your most powerful tool. Try running the “hello world” program in run rather than debug mode. You will get the same error message on the screen of the emulator.

Now choose the DDMS perspective in Eclipse and click on the emulator in the top left-hand window. You should get the following screen.


You will see a lot of red lines in LogCat. These are messages that have the severity of error, shown by the uppercase E in column 2. The first two are:

05-24 10:19:41.546: ERROR/AndroidRuntime(206): Uncaught handler: thread main exiting due to uncaught exception
05-24 10:19:41.646: ERROR/AndroidRuntime(206): java.lang.RuntimeException: Unable to start activity ComponentInfo{my.examples.helloandroidV2/my.examples.helloandroidV2.HelloAndroid}: java.lang.NullPointerException
These make it pretty clear that the runtime error is a Nullpointer Exception.

Now look down the error messages and you will see a message:
05-24 10:19:41.646: ERROR/AndroidRuntime(206): Caused by: java.lang.NullPointerException
And the one below is:
05-24 10:19:41.646: ERROR/AndroidRuntime(206): at my.examples.helloandroidV2.HelloAndroid.onCreate(HelloAndroid.java:13)
So whenever there is a Nullpointer exception DDMS and LogCat are going to make it straightforward to determine the relevant line of code. Now all you have to do is determine which object or variable on this line is null. Switch to debug perspective and you will see Eclipse is giving you plenty of clues. The relevant line is highlighted and has a yellow warning triangle on its left. Hovering over this gives the warning:
- Null pointer access: The variable o can only be null at this location
The object o has a yellow wavy line under it, and hovering over it gives the same warning.

Next we will be exploring the Notepad tutirial.

Comments

Post a Comment

Popular posts from this blog

Using Log.v, LogCat and Filters when debugging an Android Application

Image
When debugging an Android Application one of the most useful tools is Log.v. This is a means of sending debug messages to the log file, which can be read in real-time with LogCat. I will illustrate this using a real bug in the Android Notepad tutorial. If you have been following this blog you will now have a version of Notepad (NotepadV4) that makes it easy to add edit and delete Notes. If you run it in the emulator, chose the DDMS perspective in Java and select the emulator then you will see LogCat showing the log messages from Android. Try editing a number of notes within the Notepad application. After editing 4 the following error messages appeared in LogCat. I have maxmised the LogCat Window as the error messages are quite long. The crucial error messages are: 06-07 14:54:31.530: ERROR/Database(221): java.lang.IllegalStateException: /data/data/my.android.demo.notepad4/databases/data SQLiteDatabase created and never closed and 06-07 14:54:31.530: ERROR/Database(221): at my.andr...
Read more

Examining Android database tables using an adb shell and sqlite3

Image
In this post we see how easy it is to use an ADB shell and sqlite3 to examine the contents of a SQLite database on an Android device. I will be using the Android Notepad tutorial as an example. To start build and run the Notepad tutorial using the emulator. I am using the Notepad V4 version I created by upgrading the NotepadV3solution, as this removes a couple of annoying bugs. Instructions on how to build this are given in the previous post. While the application is running on the emulator, create a number of notes with known contents. I created 4 notes with titles of 1, 2, 3 and 4 and bodies of “Body 1”, “Body 2” etc… Now on a windows command line open a shell on the emulator using the ADB command Adb –e shell You will get a # prompt, showing that you are now running a linux shell on the emulator. Android stores SQLite databases in /data/data/{application package name}/databases. So we need to navigate to this directory. I do this in a few steps. (1) cd data/data (2) ls (3) cd my.and...
Read more