revisions to the Building Your First app class with feedback from mpawlan (thank you!)

<h2 id="LinearLayout">Create a Linear Layout</h2>

<ol>

  <li>From the <code>res/layout/</code> directory, open the

    <code>activity_main.xml</code> file.

  <li>In Android Studio's <b>Project</b> window, open <b>app > res >

    layout > activity_main.xml</b>.

    <p>This XML file defines the layout of your activity. It contains the

      default "Hello World" text view.</p>

  </li>

  <li>When you open a layout file, you’re first shown the design editor in the

    <a href="/studio/write/layout-editor.html">Layout Editor</a>. For this lesson,

    you work directly with the XML, so click the <b>Text</b> tab to switch to

    the text editor.

    you work directly with the XML, so click the <b>Text</b> tab at the bottom

    of the window to switch to the text editor.

  </li>

  <li>Replace the contents of the file with the following XML:

  <li>Delete everything and insert the following XML:

    <pre>&lt;?xml version="1.0" encoding="utf-8"?&gt;

<LinearLayout

    xmlns:android="http://schemas.android.com/apk/res/android"

</LinearLayout>

</pre>

<p>Don't worry about the error that appears for

<code>&#64;string/edit_message</code>; you'll fix that soon.</p>

<p>Here is a description of the attributes in the

  {@link android.widget.EditText &lt;EditText>} you added:</p>

  <p>A resource object is a unique integer name that's associated with an app resource,

such as a bitmap, layout file, or string.</p>

  <p>Every resource has a

corresponding resource object defined in your project's {@code gen/R.java} file. You can use the

corresponding resource object defined in your project's {@code R.java} file. You can use the

object names in the {@code R} class to refer to your resources, such as when you need to specify a

string value for the <a

href="{@docRoot}reference/android/widget/TextView.html#attr_android:hint">{@code android:hint}</a>

<p>The plus sign (<code>+</code>) before the resource type is needed only when you're defining a

resource ID for the first time. When you compile the app,

the SDK tools use the ID name to create a new resource ID in

your project's {@code gen/R.java} file that refers to the {@link

your project's {@code R.java} file that refers to the {@link

android.widget.EditText} element. With the resource ID declared once this way,

other references to the ID do not

need the plus sign. Using the plus sign is necessary only when specifying a new resource ID and not

<h2 id="Strings">Add String Resources</h2>

<p>By default, your Android project includes a string resource file at

<code>res/values/strings.xml</code>. Here, you'll add two new strings.</p>

<b>res > values > strings.xml</b>. Here, you'll add two new strings.</p>

<ol>

<li>From the <code>res/values/</code> directory, open <code>strings.xml</code>.</li>

<li>From the <b>Project</b> window, open <b>res > values > strings.xml</b>.</li>

<li>Add two strings so that your file looks like this:

<pre>&lt;?xml version="1.0" encoding="utf-8"?>

<resources>

<h2>Run Your App</h2>

<p>This layout is applied by the default {@link android.app.Activity} class

that the SDK tools generated when you created the project.</p>

<p>To run the app and see the results,

  click <strong>Run 'app'</strong>

<p>To see how the app now looks on your device or emulator,

  click <strong>Run</strong>

    <img src="{@docRoot}images/tools/as-run.png"

    style="vertical-align:baseline;margin:0; max-height:1em" /> in the

    toolbar.</p>

<p>Continue to the <a href="starting-activity.html">next

lesson</a> to learn how to respond to button presses, read content

from the text field, start another activity, and more.</p>

 No newline at end of file

<p>To add app behaviors such as responding to a button and starting

another activity, continue to the <a href="starting-activity.html">next

lesson</a>.</p>

 No newline at end of file

<ol>

  <li>In Android Studio, create a new project:

    <ul>

      <li>If you don't have a project opened, in the <strong>Welcome</strong> screen, click <strong>

        New Project</strong>.</li>

      <li>If you have a project opened, from the <strong>File</strong> menu, select <strong>New

        Project</strong>. The <em>Create New Project</em> screen appears.</li>

      <li>If you don't have a project opened, in the <strong>Welcome to Android Studio</strong> window, click <strong>

        Start a new Android Studio project</strong>.</li>

      <li>If you have a project opened, select <strong>File > New Project</strong>.</li>

    </ul>

  </li>

  <li>Fill out the fields on the screen. For <strong>Application Name</strong>

    use "My First App". For <strong>Company Domain</strong>, use "example.com".

    For the other fields, use the default values and click <strong>Next</strong>

    <p>Here's a brief explanation of each field:</p>

  <li>In the <b>New Project</b> screen, enter the following values:</p>

    <ul>

      <li><strong>Application Name</strong> is the app name that appears to users.</li>

      <li><strong>Company domain</strong> provides a qualifier that will be appended to the package

        name; Android Studio will remember this qualifier for each new project you create.</li>

      <li><strong>Package name</strong> is the fully qualified name for the project (following the

        same rules as those for naming packages in the Java programming language). Your package name

        must be unique across all packages installed on the Android system. You can <strong>

        Edit</strong> this value independently from the application name or the company

        domain.</li>

      <li><strong>Project location</strong> is the directory on your system that holds the project

        files.</li>

      <li><strong>Application Name</strong>: "My First App" </li>

      <li><strong>Company Domain</strong>: "example.com"</li>

    </ul>

    <p>Android Studio fills in the package name and project location for you,

    but you can edit these if you'd like.

  </li>

  <li>Under <strong>Target Android Devices</strong>, accept the default values

    and click <strong>Next</strong>.

    <p>The Minimum Required SDK is the earliest version of Android that your app supports,

      indicated using the <a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#ApiLevels">

  <li>Click <b>Next</b>.</li>

  <li>In the <b>Target Android Devices</b> screen, keep the default values and

    click <b>Next</b>.

    <p>The <b>Minimum Required SDK</b> is the earliest version of Android that your app supports,

      which is indicated by the <a href="{@docRoot}guide/topics/manifest/uses-sdk-element.html#ApiLevels">

      API level</a>. To support as many devices as possible, you should set this to the lowest

      version available that allows your app to provide its core feature set. If any feature of your

      app is possible only on newer versions of Android and it's not critical to the app's core

      feature set, you can enable the feature only when running on the versions that support it (as

      discussed in <a href="{@docRoot}training/basics/supporting-devices/platforms.html">

      app is possible only on newer versions of Android and it's not critical to the core

      feature set, enable that feature only when running on the versions that support it (see

      <a href="{@docRoot}training/basics/supporting-devices/platforms.html">

      Supporting Different Platform Versions</a>).</p>

    </li>

  <li>Under <strong>Add an Activity to Mobile</strong>, select <strong>Empty

  <li>In the <strong>Add an Activity to Mobile</strong> screen, select <strong>Empty

    Activity</strong> and click <strong>Next</strong>.

  </li>

  <div class="sidebox-wrapper">

    <div class="sidebox">

      <h3>Activities</h3>

      <p>An activity is one of the distinguishing features of the Android framework. Activities

        provide the user with access to your app, and there may be many activities. An application

        will usually have a main activity for when the user launches the application, another

        activity for when she selects some content to view, for example, and other activities for

        when she performs other tasks within the app. See <a href="{@docRoot}guide/components/activities.html">

        Activities</a> for more information.</p>

    </div>

  </div>

  <li>Under <strong>Customize the Activity</strong>, accept the default values

  <li>In the <strong>Customize the Activity</strong> screen, keep the default values

    and click <strong>Finish</strong>.

</ol>

<p>Your Android project is now a basic "Hello World" app that contains some default files. Take a

moment to review the most important of these:</p>

<p>After some processing, Android Studio opens and displays a "Hello World" app

with default files. You will add functionality to some of

these files in the following lessons.</p>

<p>Now take a moment to review the most important files. First, be sure that

the <b>Project</b> window is open (select <b>View > Tool Windows > Project</b>)

and the <b>Android</b> view is selected from the drop-down list at the top.

You can then see the following files:</p>

<dl>

  <dt><code>app/src/main/java/com.example.myfirstapp/MainActivity.java</code></dt>

  <dt><b>app > java > com.example.myfirstapp > MainActivity.java</b></dt>

  <dd>This file appears in Android Studio after the New Project wizard finishes.

    It contains the class definition for the activity you created earlier. When you build

    and run the app, the {@link android.app.Activity} starts and loads the

    layout file that says "Hello World!"</dd>

  <dt><code>app/src/main/res/layout/activity_main.xml</code></dt>

  <dt><b>app > res > layout > activity_main.xml</b></dt>

  <dd>This XML file defines the layout of the activity. It contains a {@code TextView}

    element with the text "Hello world!".</dd>

  <dt><code>app/src/main/AndroidManifest.xml</code></dt>

  <dt><b>app > manifests > AndroidManifest.xml</b></dt>

  <dd>The <a href="{@docRoot}guide/topics/manifest/manifest-intro.html">manifest file</a> describes

    the fundamental characteristics of the app and defines each of its components. You'll revisit

    this file as you follow these lessons and add more components to your app.</dd>

  <dt><code>app/build.gradle</code></dt>

  <dt><b>Gradle Scripts > build.gradle</b></dt>

  <dd>Android Studio uses Gradle to compile and build your app. There is a <code>build.gradle</code>

    file for each module of your project, as well as a <code>build.gradle</code> file for the entire

    project. Usually, you're only interested in the <code>build.gradle</code> file for the module,

    in this case the <code>app</code> or application module. This is where your app's build dependencies

    are set, including the <code>defaultConfig</code> settings:

    <ul>

      <li><code>compiledSdkVersion</code> is the platform version against which you will compile

        your app. By default, this is set to the latest version of Android available in your SDK.

        By default, this is set to the latest version of Android SDK installed on your

        development machine.

        You can still build your app to support older versions, but setting this to the latest

        version allows you to enable new features and optimize your app for a great user experience

        on the latest devices.</li>

      <li><code>applicationId</code> is the fully qualified package name for your application that

        you specified in the New Project wizard.</li>

      <li><code>minSdkVersion</code> is the Minimum SDK version you specified during the New Project

        wizard. This is the earliest version of the Android SDK that your app supports.</li>

      <li><code>targetSdkVersion</code> indicates the highest version of Android with which you have

        tested your application. As new versions of Android become available, you should

        test your app on the new version and update this value to match the latest API level and

        thereby take advantage of new platform features. For more information, read

        <a href="{@docRoot}training/basics/supporting-devices/platforms.html">Supporting Different

          Platform Versions</a>.</li>

    </ul>

    <p>See <a href="{@docRoot}studio/build/index.html">Building Your Project with Gradle</a>

    for more information about Gradle.</p></dd>

</dl>

<p>Note also the <code>/res</code> subdirectories that contain the

<a href="{@docRoot}guide/topics/resources/overview.html">resources</a> for your application:</p>

<dl>

  <dt><code>drawable<em>-&lt;density&gt;</em>/</code></dt>

    <dd>Directories for <a href="{@docRoot}guide/topics/resources/drawable-resource.html">

    drawable resources</a>, other than launcher icons, designed

    for various <a href="{@docRoot}training/multiscreen/screendensities.html">densities</a>.

</dd>

  <dt><code>layout/</code></dt>

    <dd>Directory for files that define your app's user interface like {@code activity_main.xml},

      discussed above, which describes a basic layout for the {@code MainActivity}

      class.</dd>

  <dt><code>menu/</code></dt>

    <dd>Directory for files that define your app's menu items.</dd>

  <dt><code>mipmap/</code></dt>

    <dd>Launcher icons reside in the {@code mipmap/} folder rather than the

    {@code drawable/} folders. This folder contains the {@code ic_launcher.png} image

    that appears when you run the default app.</dd>

  <dt><code>values/</code></dt>

    <dd>Directory for other XML files that contain a collection of resources, such as

      string and color definitions.</dd>

    project. Usually, you're only interested in the <code>build.gradle</code> file for the module.

    in this case the <code>app</code> or application module. For more information about this file,

    see <a href="{@docRoot}studio/build/index.html">Building Your Project with Gradle</a>.</dd>

</dl>

<p>

parent.link=index.html

trainingnavtop=true

page.tags=emulator

helpoutsWidget=true

@jd:body

<ol>

  <li><a href="#RealDevice">Run on a Real Device</a></li>

  <li><a href="#Emulator">Run on the Emulator</a></li>

  <li><a href="#Emulator">Run on an Emulator</a></li>

</ol>

<h2>You should also read</h2>

<p>In the <a href="creating-project.html">previous lesson</a>, you created an

  Android project. The project contains a default app that displays

  "Hello World". In this lesson, you will run the app on a device or emulator.</p>

Android project that displays "Hello World." You can now run the app on a real

device or on an emulator. If you don't have a real device available, skip to

<a href="#Emulator">Run on an Emulator</a>.</p>

<h2 id="RealDevice">Run on a Real Device</h2>

<p>Android Studio installs the app on your connected device and starts it.</p>

<h2 id="Emulator">Run on the Emulator</h2>

<h2 id="Emulator">Run on an Emulator</h2>

<p>Before you run your app on an emulator, you need to create an

  <a href="{@docRoot}tools/devices/index.html">Android Virtual Device</a> (AVD)

    <strong>Tools &gt; Android &gt; AVD Manager</strong>, or by clicking

    the AVD Manager icon <img src="{@docRoot}images/tools/avd-manager-studio.png"

    style="vertical-align:bottom;margin:0;height:19px"> in the toolbar.</li>

  <li>On the AVD Manager main screen, click <strong>Create Virtual Device</strong>.</li>

  <li>In the Select Hardware page, select a phone device, such as Nexus 6,

    then click <strong>Next</strong>.

  <li>In the <b>Your Virtual Devices</b> screen, click <strong>Create Virtual Device</strong>.</li>

  <li>In the <b>Select Hardware</b> screen, select a phone device, such as Nexus 6,

    and then click <strong>Next</strong>.

  </li>

  <li>In the Select Image page, choose the desired system image for the AVD and

  <li>In the <b>System Image</b> screen, choose the desired system image for the AVD and

    click <strong>Next</strong>.

    <p>If you don't have a particular system image installed,

    you can get it by clicking the <b>download</b> link.</p>

  </li>

  <li>Verify the configuration settings (for your first AVD, leave all the

    settings as they are), and then click <strong>Finish</strong>.

<h2 id="RespondToButton">Respond to the Send Button</h2>

<ol>

  <li>In the file <code>res/layout/activity_main.xml</code>, add the

  <li>In the file <b>res > layout > activity_main.xml</b>, add the

    <a href="{@docRoot}reference/android/view/View.html#attr_android:onClick">{@code android:onClick}</a>

    attribute to the {@link android.widget.Button <Button>} element as

    shown below:

      method in your activity whenever a user clicks on the button.</p>

  </li>

  <li>In the file <code>java/com.example.myfirstapp/MainActivity.java</code>,

  <li>In the file <b>java > com.example.myfirstapp > MainActivity.java</b>,

    add the <code>sendMessage()</code> method stub as shown below:

    <pre>public class MainActivity extends AppCompatActivity {

<p>Next, you’ll fill in this method to read the contents of the text field and deliver that text to

another activity.</p>

<h2 id="BuildIntent">Build an Intent</h2>

<p>An {@link android.content.Intent} is an object that provides runtime binding

  between separate components (such as two activities). The

  {@link android.content.Intent} represents an app’s "intent to do something."

    }

}</pre>

<p class="note"><strong>Note: </strong>Android Studio will display

  <code>Cannot resolve symbol</code> errors because the code references classes

  like {@link android.content.Intent} and {@link android.widget.EditText}

  that have not been imported. To import these classes, you can either 1)

  use Android Studio's "import class" functionality by pressing Alt + Enter

  (Option + Return on Mac) or 2) manually add import statements at the top of

  the file.</p>

<p>Android Studio will display <b>Cannot

resolve symbol</b> errors because this code references classes that are not

imported. You can solve some of these with Android Studio's "import class"

functionality by pressing Alt + Enter (or Option + Return on Mac).

Your imports should end up as the following:</p>

<pre>

import android.content.Intent;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.view.View;

import android.widget.EditText;

</pre>

<p>An error remains for <code>DisplayMessageActivity</code>, but that's okay;

you'll fix that in the next section.

<p>There’s a lot going on in <code>sendMessage()</code>, so let’s explain

  what's going on.</p>

  method starts an instance of the <code>DisplayMessageActivity</code> specified

  by the {@link android.content.Intent}. Now you need to create the class.</p>

<h2 id="CreateActivity">Create the Second Activity</h2>

<ol>

  <li>Creates the corresponding layout file <code>activity_display_message.xml</code>

    </li>

  <li>Adds the required

    <a href="{@docRoot}guide/topics/manifest/activity-element.html">&lt;activity&gt;</a>

    <a href="{@docRoot}guide/topics/manifest/activity-element.html"

    ><code>&lt;activity&gt;</code></a>

    element in <code>AndroidManifest.xml</code>.

</ul>

   layout.addView(textView);

}</pre>

  </li>

  <li>Press Alt + Enter (option + return on Mac) to import missing classes.</li>

  <li>Press Alt + Enter (or Option + Return on Mac) to import missing classes.

  Your imports should end up as the following:

<pre>

import android.content.Intent;

import android.support.v7.app.AppCompatActivity;

import android.os.Bundle;

import android.view.ViewGroup;

import android.widget.TextView;

</pre>

</li>

</ol>

<p>There’s a lot going on here, so let’s explain:</p>