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[MUSIC]

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One of the defining characteristics,
of modern handheld systems, is that they

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can keep us connected and networked,
without tethering us to a single location.

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In this lesson we'll
explore the software and

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programing practices that we'll need to
connect your applications to the network.

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So I'll start this lesson by
discussing networking in general.

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That discussion will focus on connecting
your applications to the internet

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using the hypertext transfer protocol or

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HTTP, Specifically by
using HTTP get requests.

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After that I'll present several classes
that Android provides to support this

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kind of networking, And lastly I'll
discuss how your applications can

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process the data they receive in
response to these HTTP get request.

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In particular I'll talk about two
popular data formatting languages.

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One, the JavaScript Object
Notation Langauge or

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JSON, And two,
the extensible markup language or XML.

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And I'll talk about how you parse or
make sense of

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these HTTP responses when they're
formated in one of these languages.

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So early hand-held devices,
gave us mobility.

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You could move from one place to another
and still perform useful computation.

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However, their networking capabilities
were primitive by todays standards.

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Now moving forward todays devices
combine power processors with

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fast network connections over WiFi and
cellular networks.

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Handheld applications will
therefore often want to make use,

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are these networking capabilities to
access and provide data and services.

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Now to help you do this, Android includes
a variety of networking support classes

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including the socket and
URL classes in the Java.net packages.

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The HttpRequest and HttpResponse classes
in the org.appache packages, and

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the URI AndroidHttpClient and AudioStream
classes in the android.net packages.

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In this lesson we're going to
look at several of these classes.

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Using each of them to implement
the same example application.

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This application interacts with an
internet service to get information about

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earthquakes that have occurred in
a particular geographic Region.

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And as you'll see,
that data is returned in various formats.

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Now, initially we'll just display
the downloaded text as is.

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Later on in the lesson,
I'll show you how to process that

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data to extract just
the information that you want.

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Oh, and and one other thing.

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As you'll see in a second, because this
data includes geographic information,

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it's really begging to be displayed
on a map, rather than as text.

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Now, we won't do that in this lesson,
but keep this in

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mind because we'll come back to this when
we get to the lesson on maps and location.

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So in order to make this application work,
the code needs to create an http request,

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send it to a server computer, retrieve the
results, and then display those results.

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Android provides several classes for
helping with this.

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Three we'll talk about
now are the socket class.

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The HTTPURLConnection class and
the AndroidHTTPClient.

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I'll launch the networking
socket application.

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As you can see,

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this application initially displays
a single button labeled Load Data.

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When I press that button, the application
will issue an HTTP GET request

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to an external server, and that server
will respond with some complex text,

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containing the requested earth quake data.

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Okay, so
now I'll press the Load Data button,

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and there you can see the requested data.

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Let's look at the source code to
see what it took to get that data.

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Now, here I've opened
the application in the IDE.

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Now, I'll open the main activity for

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this application, and here I'm showing
the listener for the load data button.

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When this button is pressed,
the application will create and

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then execute an async task
called HTTP get task.

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Let's look at that class.

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The HTTP get task class first
declares some variables that

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are used in creating an HTTP get request.

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When the execute method is
called on the HTTP get task.

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The do in background method is called.

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That method begins by creating a new
socket that will be connected to the host

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computer API.geoname.org on
the standard http port, port 80.

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Next, the code gets
the sockets output stream, and

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then writes the http get command, and this
string will be sent to the host computer.

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Which interprets it as a HTTP GET request,
and

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then responds by sending back
the appropriate, response data, and

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then this code continues by getting
the socket's input stream and

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by passing it to a method
called read stream.

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The read stream method ultimately reads
the response data from the socket's input

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stream, and then returns
the response as a single string.

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And this string is then passed to
the on post execute method which

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executes on the main thread, and which
displays the response in the text view.

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We turn back to the application.

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You'll notice that the response text
includes not only the earthquake data but

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also the HTTP response headers.

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Now normally I wouldn't want
to display that text here.

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I'd really just want to show
you the earthquake data.

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So in this case I should have
parsed the response, and

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pulled out just the data that I wanted.

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In addition, you might have
noticed that I didn't write any of

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the error handling code that you'd really
need to make this application robust.

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And these points capture pretty well
the tradeoffs of using sockets.

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They're very low level.

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You can write whatever you
want on the socket, but

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in return, you have to handle all the many
details of making the http requests,

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all the error handling, and
all the processing of the http responses.

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The next implementation we'll look at
uses the http URL connection class.

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This class provides a higher level
interface that handles more of

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the networking details than the socket
class does, but, as we'll see in a moment,

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it also has a less flexible API
than our last option, the H,

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the Android HTTP client class.

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Now having said that, I'll also point
out that the Android team is not

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actively working on
the Android HTTP Client anymore, and

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it's putting its efforts into
improving this class going forward.

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So, let's look at the example
application implemented this time

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with the HTTP URL connection class.

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Now I'll launch the networking
URL application.

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As before, this application
initially displays a single button

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labeled load data, and as before,
when I press on that button,

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the application will issue
an HTTP get request.

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To an external server.

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And that server will respond
with some complex text

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containing the requested earthquake data.

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Okay.

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So now I'll press the Low Data button.

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There you can see the requested
data appearing in the text view.

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Notice however that this time the HTTP
response headers have been stripped out.

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Let's look at the source code and
see how this works.

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Now here I've got the application
opened in the IDE.

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Now I'll open the main activity for

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this application, and here I'm showing
the listener for the load data button.

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As before, when this button is pressed
the application will create and

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then execute an asynch
task called httpGetTask.

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Let's look at that class.

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When the execute method
is called on HttpGetTask,

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the doInBackground method is invoked.

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That method begins by
creating a new URL object and

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passing a URL string for
the desired service as a parameter.

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The code then calls the open
connection method on the url object.

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Which returns an http url connection.

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This object is then stored in
a variable called http url connection.

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The code continues by getting
the http url connection's

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input stream, and
bypassing it to the read stream method.

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And as before, the read stream
method reads the response data from

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the socket's input stream, and then
returns the response as a single string.

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This time, however, the http URL
connection strips off the http

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response headers, and
handles the error checking for you.

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Now this string is then passed
to the onPost execute method.

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Which displays the response
in the text view.

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The third class is AndroidHTTPClient.

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This class is an implementation of
the Apache Project's DefaultHttpClient.

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And it allows a great
deal of customization.

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In particular,
the class breaks an HTTP transaction into

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a request object, and
into a response object.

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So you can create subclasses that
customize the handling of requests and

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their responses.

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Now by this point you know what
the application looks like so

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let's jump straight into the code and
look at the implementation.

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Now here I've got the
NetworkingAndroidHttpClient application.

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Open in the IDE.

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Now, I'll open the main activity for
this application, and let's go right to

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the http get task class, that class
begins by creating a new Android

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http client object, by calling
the classes new instance method.

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Now, when the doInBackground method is
called, the code creates an HttpGet

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object, passing in the URL string for
that request.

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Next, it creates a ResponseHandler object.

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This object is responsible for
handling the response.

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To the http get request, in this case
the response handler is of type,

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Basic Response Handler,
which will return the responses body.

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Now we'll see a more complex response
handler later in this lesson.

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And finally the request and the response
handler are passed into the execute method

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which sends the request, gets the
response, passing it through the response

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handler, and the result of all this
is then passed on to on post execute.

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Which displays the response
in a text view.