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[MUSIC]

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(University of Maryland)

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(Programming Mobile Applications for[br]Android Handheld Systems)

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One of the defining characteristics of [br]modern hand held systems is that they can

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keep us connected and networked without [br]tethering us to a single location.

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In this lesson, we'll explore [br]the software and

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programming practices that you'll need to [br]connect your applications to the network.

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

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That discussion will focus on connecting [br]your applications to the internet using

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the Hypertext Transfer Protocol or HTTP [br]specifically by using HTTP GET requests.

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After that, I'll present [br]several classes that

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Android provides to support [br]this kind of networking.

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And lastly, I'll discuss how your [br]applications can process the

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data they receive in response [br]to these HTTP GET requests.

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In particular, I'll talk about two [br]popular data formatting languages.

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One, the Javascript [br]Object Notation Language, or JSON.

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And two, the Extensible Markup Language, [br]or XML.

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And I'll talk about how you parse, [br]or make sense, of

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these HTTP responses when they're [br]formatted in one of these languages.

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So early handheld devices [br]gave us mobility.

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You could move from one place to another, [br]and still perform useful computation.

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However,

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their networking capabilities [br]were primitive by today's standards.

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Now moving forward, today's devices [br]combine powerful processors

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with fast network connections [br]over WiFi and cellular networks.

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Handheld applications will therefore [br]often want to make use

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of these networking capabilities to [br]access and provide data, and services. [br]

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Now, to help you do this, Android includes

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a variety of networking support classes, [br]including the Socket and URL classes,

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in the Java.net packages.

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The HttpRequest and HttpResponse classes, [br]in the org.apache packages.

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And the URI, AndroidHttpClient,[br]and AudioStream classes,

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in the android.net packages.

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In this lesson we're going to look at [br]several of these classes,

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using each of them to implement [br]the same example application.

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And this application interacts [br]with an internet service

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to get information about [br]earthquakes that have occurred

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in a particular geographic region.

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

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Now initially we'll just display the [br]downloaded text as is.

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Later on in the lesson, I'll show you [br]how to process that data

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to extract [br]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,

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because this data includes [br]geographic information,

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

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Now, we won't do that in this lesson, [br]but keep this in mind,

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

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So in order to make this application [br]work, the code needs to create

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an HTTP request, send it to a [br]server computer, retrieve the results,

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and then display those results. [br]Android provides several classes for

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helping with this. [br]Three we'll talk about now are

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the Socket class, the HttpUrlConnection [br]class

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and the AndroidHttpClient.

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I'll launch the networking [br]socket application.

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As you can see,

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this application initially displays [br]a single button labeled LoadData.

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When I press that button, [br]the application will issue

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an HTTP GET request to an [br]external server.

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And that server will respond [br]with some complex text

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containing the requested [br]earthquake data.

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Okay.

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So now I'll press the Load Data button [br]and there you can see the requested data.

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

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Now here I've opened the application [br]in the IDE.

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Now I'll open the main activity [br]for this application.

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And here I'm showing the listener [br]for the Load Data button.

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When this button is pressed, the [br]application will create, and then execute,

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an AsyncTask called HttpGetTask.[br]Let's look at that class.

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The HttpGetTask class first declares [br]some variables,

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they're used in creating [br]an HTTP GET Request.

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When the execute method is called, on[br]the HttpGetTask,

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the doInBackground method is called.

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And that method begins [br]by creating a new socket

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that will be connected to [br]the host computer, api.geonames.org,

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on the standard http port, port 80.[br]Next, the code gets

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the socket's output stream, [br]and then writes the HTTP<u>GET</u>COMMAND.

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And this string will be sent to [br]the host computer, which interprets it

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as an HTTPGetRequest, and then responds

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by sending back [br]the appropriate response data.

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And then this code continues [br]by getting the socket's input stream

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And by passing it to a method [br]called readStream.

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The readStream method ultimately [br]reads the response data

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from the socket's InputStream and then [br]returns the response as a single string.

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And this string is passed to the

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onPostExecute method which executes [br]on the main thread

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and which displays the response [br]in the text view.

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If we return back to the application, [br]you'll notice that the response text

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includes not only the earthquake data, [br]but also the HTTP response headers.

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Now normally, I wouldn't want [br]to display that text here.

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I really just want to show you [br]the earthquake data.

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So in the case, I should have [br]parsed the response

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and pulled out [br]just the data that I wanted.

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In addition, you might have noticed [br]I didn't write any of the

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error handling code that you'd really [br]need to make this application robust.

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And these points capture pretty well [br]the trade-offs of using sockets.

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They're very low level: you can write [br]whatever you want on the socket,

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but in return, you have to handle [br]all the many details

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of making the HTTP requests, [br]all the error handling,

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and all the processing [br]of the HTTP responses.

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The next implementation we'll look at [br]uses the HttpUrlConnection class.

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This class provides a higher-level [br]interface that handles more

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of the networking details [br]than the socket class does.

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But as we'll see in a moment, [br]it also has a less flexible API

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than our last option, [br]the Android HTTP client class.

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Now, having said that, I'll also [br]point out that the Android team

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is not actively working on the [br]Android HTTP client anymore.

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And is putting its efforts into [br]improving this class going forward.

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So let's look at the example application

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implemented this time [br]with the HttpURLConnection class.

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...

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...

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Now I'll launch [br]the NetworkingURL application.

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As before, this application [br]initially displays

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a single button labeled Load Data.

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And as before, when I press on [br]that button the application will issue

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an HTTP GET request to an external server, [br]and that server will respond

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with some complex text, containing [br]the requested earthquake data.

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Okay, so now I'll press the

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Load Data button.

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There you can see the requested data, [br]appearing in a text view.

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Notice, however, that this time, the HTTP [br]response headers have been stripped out.

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

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Now, here I've got the application [br]opened in the IDE.

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Now, I'll open the main activity [br]for this application.

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And here, I'm showing the listener [br]for the load data button.

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As before, when this button is pressed, [br]the application will create and then

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execute an AsyncTask called, HttpGetTask. [br]Let's look at that class.

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When the execute method is called on [br]HTTPGetTask, the doInBackground method

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is invoked. That method begins [br]by creating a new URL object,

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and passing a URL string [br]for the desired service as a parameter.

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The code then calls the [br]open connection method

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on the URL object, which returns [br]an httpUrlConnection.

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This object is then stored in a variable [br]called HttpURLConnection.

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The code continues by getting the [br]HttpURLConnection's input stream,

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and by passing it through [br]the readStream method.

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And as before, the readStream method [br]reads the response data

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from the socket's input stream, [br]and then returns the response,

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as a single string. [br]This time however, the HTTP URL connection

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strips off the HTTP response headers [br]and handles the error checking for you.

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Now this string is then passed [br]to the onPostExecute method

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which displays the response [br]in a text view.

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The third class is Android HTTP client.[br]This class is an implementation of the

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Apache project's DefaultHttpClient and [br]it allows a great deal of customization.

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In particular, the class breaks [br]an HTTP transaction into

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a request object and into a response object. [br]So you can create subclasses

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that customize the handling of requests [br]and their responses.

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Now, by this point, you know [br]what the application looks like,

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so let's jump straight into the code [br]and look at the implementation.

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Now, here I've got the networking Android [br]HTTP client application opened in the IDE.

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Now, I'll open the main activity [br]for this application.

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And let's go right to the HTTP get task class.

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That class begins by creating a new

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AndroidHttpClient object by calling [br]the classes newInstance method.

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Now, when the doInBackground method [br]is called, the code creates

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an HttpGet object, passing in [br]the URL string for that request.

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Next, it creates a [br]ResponseHandler object.

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This object is responsible for handling [br]the response to the HttpGet request.

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In this case, the ResponseHandler [br]is of type

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basicResponseHandler, [br]which will return the response's body.

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Now we'll see a more complex [br]ResponseHandler later in this lesson.

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And finally, the request and the ResponseHandler

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are passed into the execute method,

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which sends the request, [br]gets the response,

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passing it through the ResponseHandler.

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And the result of all this

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is then passed on to onPostExecute,

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which displays the response [br]in a text field.

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[BLANK_AUDIO]