How to debug the Android mobile application with LogCat

In computer science and especially in programming, things can go wrong easily. Even simple things, that you have done it before, can generate runtime exceptions that crash the application. Most of the time, the reason is the inability to think to all aspects every time you do something. And because this will not happen, the programmer best friend is the debugger.

In this article we will see how to debug the Android mobile application using the Android LogCat. Despite this Android SDK tool, the application can be debugged in Eclipse like any other Java application (i.e using breakpoints).

Other topics that are part of this Android tutorial are accessible through Android Tutorial – Overview and contents.

When developing and testing an Android mobile application you can get a message like this:

Android Application Runtime Error Message

The dialog window announces that the application has crashed, mainly because a runtime exceptions or error. Reasons for this situations are multiple and depends entirely on what you have done in the Android project. So, we now that something is wrong, but we have a a question:

How to find out what is the problem that forced the Android application to stop unexpectedly

Java programmers, that use Eclipse, know that if the application generates a runtime exception, they will get the exception details in the console window.

When developing Android applications in Eclipse the runtime exceptions messages are NOT displayed in the console window. Instead, the Android SDK comes with a tool, called LogCat, that logs all the platform messages.

How to open LogCat window

In Eclipse you can view the LogCat window by selecting Window -> Show View -> Other… and choosing LogCat from the Android category:

Open the Android LogCat View in Eclipse

The LogCat view displays logs generated by the Android emulator and by the mobile application:

LogCat View window in Eclipse

Also, using the LogCat top toolbar you can filter logs based on their Tag, pid or Log Level (Error, Warning, Info, Debug, Verbose) or you can define your own custom filter.

LogCat Custom Filter Editor

How to display messages in the LogCat window from code

A simple debugging technique is to display your own messages in the log window. For Java applications, this is done by printing messages in the console window usingSystem.out.println(String message) method.

For Android projects, there is a better solution. The Log class has some static methods used to print messages in the system log which is displayed by LogCat view. Each method prints a message with a predefined log level:

        Log.e("MyTag", "Error message with my own tag");
        Log.w("dalvikvm", "VM Warning message");
        Log.d("MyTag", "Debug message");
        Log.i("MainActivity","Information message");
        Log.v("MyTag", "Verbose message");
        Log.wtf("WTF", "What a Terrible Failure");

The methods have multiple signatures. The previous example shows only the (String Tag, String Message) one. For tags is recommended to use your own tags or the class name. This will allow you to filter them easily.