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Another common way to investigate
our app while it's running is to use

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the Android log.

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We can add calls to different
log methods that will write

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data to the Android system log.

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We can then view the log in our IDE using
logcat, while the app is running or

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even at a later time.

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Even if our app crashes,

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the log remains, often holding helpful
information about why the app crashed.

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Let's check it out.

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Let's start by logging some
information from our app.

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Right now, we're showing a toast when
FunFactsActivity is first created.

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But our users don't really
care about that, do they?

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Let's comment out our Toast and
write a message to the log instead.

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We write to the log
using the log Class and

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then one of its static methods,
just like we did with Toast.makeText.

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These methods also don't return anything,
so

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we don't need to store them in a variable.

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Just like toasts,
we can add these log method calls

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all over the place to log information
about how our app is running.

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Let's take a quick look at
the documentation for the log class.

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This is the reference for the log class
from the Android Developers site.

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If we look at the class overview,

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it tells us that we generally
use one of five methods.

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Log.v, d, i, w, or e.

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These letters stand for
different levels of logging.

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E is for errors, w is for
warnings, i is for informational,

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d for debugging, and v is for verbose.

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Which is used when you're
troubleshooting something and

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want to spam as much information
to the log as possible.

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Android does this so we can chose
the appropriate level for our log entries.

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We can filter only the log levels
we care about when viewing a log.

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For example,
we may be troubleshooting an error and

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only want to look at error messages.

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If we log information about
the error with the log.e method,

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then we can filter the log to see just
error messages and ignore everything else.

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If we scroll down, we can see some
of the constants and other methods.

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Here's one called wtf, that stands for
What a Terrible Failure.

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That's only for [LAUGH] extreme errors,
though, and you'll probably never use it.

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All right, let's take a look
at how to use the log methods.

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They're all used the same way.

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So let's take a look at the debug method.

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On a new line, let's type log.d and

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autocomplete shows us that
method requires two parameters.

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The second parameter is the string
data we want to write to the log.

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But the first parameter is what's
known as a tag, also a string.

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The tag is used to identify
the source of a log message.

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It usually identifies the class or
activity where the log call occurs.

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The log contains a lot of information,
which we'll see in a moment.

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Remember, the log contains messages
from all apps and the operating system.

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So, supplying a tag lets us know
where the message came from.

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And it also lets us filter by the tag and
the logcat.

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For the first parameter, let's type
FunFactsActivity inside quotation marks,

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and first let me hit
Alt+Enter to import the log.

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Okay, "FunFactsActivity".

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Then add a comma and
our log message will be,

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"We're logging from
the onCreate() method!");.

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All right,
let's run the app to see it in action.

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Okay, but where did our log message go?

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Let's take a look at the logcat.

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It usually lives here at
the bottom under the code editor.

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But if it's missing,
just go down here and click on Logcat.

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If that's missing too,
click this little box to make it show up.

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This is the Logcat, and it has all
the log data from the connected device,

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which we can see right
here is the emulator.

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And if we look carefully,
we can even see our log message.

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It's easy to find because the log
is currently only showing messages

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from the selected application,
which up here, is set to our FunFacts app.

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But that's no fun.

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Let's change that from Show only
selected application to No Filters

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to see what the log really looks like.

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Now that we've got a ton of log messages,
let's do some filtering.

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Let's select the Log Level dropdown,
and pick Debug.

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This filters the log to show messages that
are of Debug level importance or higher.

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This means that, by selecting Debug,
we'll not only see Debug messages, but

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also Info, Warn, Error,
and Assert messages.

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Lets switch back to Verbose and
look at another filtering method.

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In this text field, we can search the log.

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I'm not sure why it's so small, but
it's here so let's click in it.

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And let's search for
our tag of FunFactsActivity.

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And wow, that is really frustrating
that we can't see it, and

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I can't seem to make it any bigger.

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So I guess we're stuck with it.

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Anywho, it looks like there's a ton of
messages that say FunFactsActivity.

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So instead, let's search for our message.

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I'll hit the x to clear my box.

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So I'll type we're and once we finish
the first word, there's our message, nice.

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Now let's hide this and
go back to our code.

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A common Android practice is to add
a special field to our class called tag.

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Let's scroll up.

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And here, after we declare our class,
let's add a line and

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type public static final String and
then TAG.

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Let's set it = "FunFactsActivity"
just like we typed below.

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Remember that static final means
that this tag is always available,

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static, and
that its value will never change.

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Final, in other words, it's a constant.

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Let's make one more change.

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What would happen if we changed the name
of our FunFactsActivity class to

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some other activity?

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Well, now our tag would be
a little bit misleading.

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Instead of hardcoding the class
name like we've done here,

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we can use a special method to
get the class name automatically.

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Let's delete FunFactsActivity and
the quotes.

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And instead, type FunFactsActivity,
with no quotes,

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and then .class.getSimpleName.

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We pick getSimpleName because getName
includes the whole package name, whereas

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simpleName leaves that out and gives us
just the class name, FunFactsActivity.

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Now you might be thinking,
we just typed FunFactsActivity again.

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But this time,
since it isn't inside a string,

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the refactoring tools will
recognize it and make the change.

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So if I Refractor rename
on FunFactsActivity and

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rename it to some other activity, notice
that it changes here and my tag too.

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I don't actually want to rename this,
though, so let's hit Escape to cancel.

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Now, notice that our log
constant is still gray,

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because we haven't updated it
down here in our debug log call.

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So instead of this hardcoded string,
let's use our tag constant.

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Then let's run the app to
make sure it still works.

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And there we go, a new log message with
the same FunFactsActivity tag as before.

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Logging can be incredibly helpful for
troubleshooting problems with our apps.

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It's also a great way to make sure
everything is running like we

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expect it to.

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Especially when whatever we're
testing isn't visible on the screen.