- Getting Text from an EditText 8:21
- Getting Text from an EditText 1 objective
- Adding a Second Activity 1:57
- Introducing Intents 2:23
- Starting a New Activity 2 objectives
- Sending Data to a New Activity 2:53
- Putting Data in an Intent 1 objective
- Getting Data from an Intent 3:06
- Getting Data from an Intent 2 objectives
- Investigating a NullPointerException 3:21
- Introducing String Resources 2:41
- Getting Resources from the Context 4:20
- Using String Resources 3 objectives
Preview
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Investigating a NullPointerException
3:21We have an error! It's a common one: we have encountered an uncaught NullPointerException. Let's see how to investigate and fix it.
So I just made an intentional but
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very common mistake where I mistyped the
name that we used in the other activity.
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So if we run the app now it starts up and
everything seems fine.
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But if we type our name.
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And start our adventure, we get an error.
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Let's investigate and see what happened.
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Here I want to bring up the Android
monitor which again has our log cat view.
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This is a lot of information and
it's sometimes hard to see.
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There's a button here that can
change at the line breaks.
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But I like to have it here and
scroll over.
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By the way you can see the error,
it's nice and obvious.
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It's highlighted in red whereas the rest
of the stuff was black and blue.
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I'm going to scroll over a little bit and
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take a better look at
the exception itself.
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So let's unpack this a little bit.
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We have a fatal exception
in our main thread.
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It's in our app interactive story and
it is a java.lang.runtime exception.
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Says it was unable to start the activity
and it gives some details about it.
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And this is called a stack
trace because it traces through
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the stack of method calls where the error
handled and then was bubbled up.
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So if we scroll down a little bit we can
see that it was originally caused by
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a NullPointerException where it
says print line needs a message.
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And if we dig back a little bit further,
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we can see that it's
from our log statement.
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And sure enough we get to
a line in StoryActivity
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inside of our onCreate method,
and it's clickable.
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If we click on it,
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we can see the offending line that
threw the initial NullPointerException.
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Our app crashed because we
didn't handle that error.
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The first thing we can do is avoid
a hard crash by adding a null check
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around this code.
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This is good practice whenever we're
getting data from an intent or
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anywhere really.
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We know in this simple app that we
will always start StoryActivity from
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MainActivity.
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But in a more complex app we might be
starting activities from multiple places.
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And if we're not careful, we might not
have the same exact intents from each one.
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So let's reduce this Android monitor.
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And we can still set
the name variable here.
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But after the fact,
we want to add a new if statement.
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if name = null then we should give
the name some kind of default value.
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Let's set name = Friend instead.
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So now, no matter what,
our name variable will have a value.
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Well, that's not totally true, what
happens if the user doesn't type anything?
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In that case,
it would have a blank string.
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It would just be blank.
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That would affect how our story will look,
so
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we should also supply a default
value if there is a blank string.
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We can do that with another check here.
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So if name is null or if name is empty,
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name.isEmpty, we'll check
if it's an empty string.
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So first let's run this and see if it
catches the exception for the wrong key.
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And we can type the name, start the
adventure and it didn't crash this time.
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And if we go to log we should be able to
find yep, there it is, Friend was used.
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Now let's fix the typo with the key,
change this to a lowercase n, and
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run it again.
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This time I'm not going to type anything,
just click on the button.
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And if we go back to the log,
same thing, there we go.
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A new string with the word Friend in it.
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In the next video, we will address the key
name by setting it in one master location,
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the string resource file.
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