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Part 6

How do we debug?

In normal Java programs debugging is just looking at the console output. Android apps are very similar. Instead of the "Output" we have the Logcat.

Look at the bottom of Android Studio, you'll see a few panes, one will say Logcat. Logcat

Note: If you don't see the LogCat by default, you can show it by either going to View -> Tool Windows -> Logcat or simply pressing Alt+6.

The LogCat is where the output of your app will go (and all other apps, if unfiltered).

Some quick notes:

If we want to Log to the LogCat for debugging we can use Log.

For example, Log.w("TAG", "MESSAGE") will print out a warning to the output.

You can also use Log.v for Verbose, Log.e for an error, and even [Log.wtf](https://developer.android.com/reference/android/util/Log.html#wtf(java.lang.String, java.lang.String, java.lang.Throwable)) for "What a Terrible Failure: Report an exception that should never happen."

What does a crash look like?

Here I forced a crash with some pretty bad code. This is what it looks like in the LogCat Bad error booo crash

There's really only ever a couple lines pertaining to what we need

    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.tristanwiley.cse410/com.tristanwiley.cse410.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toLowerCase()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2778)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toLowerCase()' on a null object reference
        at com.tristanwiley.cse410.MainActivity.onCreate(MainActivity.java:15)
        at android.app.Activity.performCreate(Activity.java:6999)
        at android.app.Activity.performCreate(Activity.java:6990)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2731)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807) 

The first line tells us that there was a NullPointerException when trying to call toLowerCase() on something.

Note: You'll notice in the screenshot that one link in the LogCat is blue. If you click on that it will usually take you directly to the issue. Which as you can see, is on line 15 of our MainActivity..

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