Building Your First Android App

This lesson shows you how to create a new Android project with Android Studio and describes some of the files in the project.

Activities

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 Activities for more information.

1. In Android Studio, create a new project:
   • If you don't have a project opened, in the Welcome screen, click New Project.
   • If you have a project opened, from the File menu, select New Project. The Create New Projectscreen appears.
2. Fill out the fields on the screen. For Application Name use "My First App". For Company Domain, use "example.com". For the other fields, use the default values and click Next

   Here's a brief explanation of each field:

   • Application Name is the app name that appears to users.
   • Company domain provides a qualifier that will be appended to the package name; Android Studio will remember this qualifier for each new project you create.
   • Package name 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 Edit this value independently from the application name or the company domain.
   • Project location is the directory on your system that holds the project files.
3. Under Target Android Devices, accept the default values and click Next.

   The Minimum Required SDK is the earliest version of Android that your app supports, indicated using the API level. 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 Supporting Different Platform Versions).

4. Under Add an Activity to Mobile, select Empty Activity and click Next.
5. Under Customize the Activity, accept the default values and click Finish.

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:

app/src/main/java/com.example.myfirstapp/MainActivity.java

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 Activity starts and loads the layout file that says "Hello World!"

app/src/main/res/layout/activity_main.xml

This XML file defines the layout of the activity. It contains a TextView element with the text "Hello world!".

app/src/main/AndroidManifest.xml

The manifest file 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.

app/build.gradle

Android Studio uses Gradle to compile and build your app. There is a build.gradle file for each module of your project, as well as abuild.gradle file for the entire project. Usually, you're only interested in the build.gradle file for the module, in this case the app or application module. This is where your app's build dependencies are set, including the defaultConfig settings:

• compiledSdkVersion 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.
• applicationId is the fully qualified package name for your application that you specified in the New Project wizard.
• minSdkVersion 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.
• targetSdkVersion 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 Supporting Different Platform Versions.

See Building Your Project with Gradle for more information about Gradle.

Note also the /res subdirectories that contain the resources for your application:

drawable-/

Directories for drawable resources, other than launcher icons, designed for various densities.

layout/

Directory for files that define your app's user interface like activity_main.xml, discussed above, which describes a basic layout for theMainActivity class.

menu/

Directory for files that define your app's menu items.

mipmap/

Launcher icons reside in the mipmap/ folder rather than the drawable/ folders. This folder contains the ic_launcher.png image that appears when you run the default app.

values/

Directory for other XML files that contain a collection of resources, such as string and color definitions

Secound Day...

Running Your App

PreviousNext

This lesson teaches you to

1. Run on a Real Device
2. Run on the Emulator

You should also read

• Using Hardware Devices
• Managing AVDs with AVD Manager

In the previous lesson, 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.

Run on a Real Device

Set up your device as follows:

1. Connect your device to your development machine with a USB cable. If you're developing on Windows, you might need to install the appropriate USB driver for your device. For help installing drivers, see the OEM USB Drivers document.
2. Enable USB debugging on your device by going to Settings > Developer options.

   Note: On Android 4.2 and newer, Developer options is hidden by default. To make it available, go to Settings > About phone and tap Build number seven times. Return to the previous screen to find Developer options.

Run the app from Android Studio as follows:

1. In Android Studio, select your project and click Run  from the toolbar.
2. In the Select Deployment Target window, select your device, and click OK.

Android Studio installs the app on your connected device and starts it.

Run on the Emulator

Before you run your app on an emulator, you need to create an Android Virtual Device (AVD) definition. An AVD definition defines the characteristics of an Android phone, tablet, Android Wear, or Android TV device that you want to simulate in the Android Emulator.

Create an AVD Definition as follows:

1. Launch the Android Virtual Device Manager by selecting Tools > Android > AVD Manager, or by clicking the AVD Manager icon  in the toolbar.
2. On the AVD Manager main screen, click Create Virtual Device.
3. In the Select Hardware page, select a phone device, such as Nexus 6, then click Next.
4. In the Select Image page, choose the desired system image for the AVD and click Next.
5. Verify the configuration settings (for your first AVD, leave all the settings as they are), and then click Finish.

For more information about using AVDs, see Create and Manage Virtual Devices.

Run the app from Android Studio as follows:

1. In Android Studio, select your project and click Run  from the toolbar.
2. In the Select Deployment Target window, select your emulator and click OK.

It can take a few minutes for the emulator to start. You may have to unlock the screen. When you do, My First App appears on the emulator screen.

Third Day...

Building a Simple User Interface

PreviousNext

This lesson teaches you to

1. Create a Linear Layout
2. Add a Text Field
3. Add String Resources
4. Add a Button
5. Make the Input Box Fill in the Screen Width

You should also read

• Layouts

In this lesson, you create a layout in XML that includes a text field and a button. In the next lesson, your app responds when the button is pressed by sending the content of the text field to another activity.

The graphical user interface for an Android app is built using a hierarchy of View and ViewGroupobjects. View objects are usually UI widgets such as buttons or text fields. ViewGroup objects are invisible view containers that define how the child views are laid out, such as in a grid or a vertical list.

Android provides an XML vocabulary that corresponds to the subclasses of View and ViewGroup so you can define your UI in XML using a hierarchy of UI elements.

Layouts are subclasses of the ViewGroup. In this exercise, you'll work with a LinearLayout.

Alternative Layouts

Declaring your UI layout in XML rather than runtime code is useful for several reasons, but it's especially important so you can create different layouts for different screen sizes. For example, you can create two versions of a layout and tell the system to use one on "small" screens and the other on "large" screens. For more information, see the class about Supporting Different Devices.

Figure 1. Illustration of how ViewGroup objects form branches in the layout and contain other View objects.

Create a Linear Layout

1. From the res/layout/ directory, open the activity_main.xml file.

   This XML file defines the layout of your activity. It contains the default "Hello World" text view.

2. When you open a layout file, you’re first shown the design editor in the Layout Editor. For this lesson, you work directly with the XML, so click theText tab to switch to the text editor.
3. Replace the contents of the file with the following XML:

   <?xml version="1.0" encoding="utf-8"?>
   
   

LinearLayout is a view group (a subclass of ViewGroup) that lays out child views in either a vertical or horizontal orientation, as specified by theandroid:orientation attribute. Each child of a LinearLayout appears on the screen in the order in which it appears in the XML.

Two other attributes, android:layout_width and android:layout_height, are required for all views in order to specify their size.

Because the LinearLayout is the root view in the layout, it should fill the entire screen area that's available to the app by setting the width and height to "match_parent". This value declares that the view should expand its width or height to match the width or height of the parent view.

For more information about layout properties, see the Layout guide.

Add a Text Field

In the activity_main.xml file, within the  element, add the following  element:

Here is a description of the attributes in the  you added:

android:id

This provides a unique identifier for the view, which you can use to reference the object from your app code, such as to read and manipulate the object (you'll see this in the next lesson).

The at sign (@) is required when you're referring to any resource object from XML. It is followed by the resource type (id in this case), a slash, then the resource name (edit_message).

Resource Objects

A resource object is a unique integer name that's associated with an app resource, such as a bitmap, layout file, or string.

Every resource has a corresponding resource object defined in your project's gen/R.java file. You can use the object names in the R class to refer to your resources, such as when you need to specify a string value for the android:hint attribute. You can also create arbitrary resource IDs that you associate with a view using theandroid:id attribute, which allows you to reference that view from other code.

The SDK tools generate the R.javafile each time you compile your app. You should never modify this file by hand.

For more information, read the guide to Providing Resources.

The plus sign (+) 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 gen/R.java file that refers to the 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 needed for concrete resources such as strings or layouts. See the sidebox for more information about resource objects.

android:layout_width and android:layout_height

Instead of using specific sizes for the width and height, the "wrap_content" value specifies that the view should be only as big as needed to fit the contents of the view. If you were to instead use "match_parent", then the EditText element would fill the screen, because it would match the size of the parent LinearLayout. For more information, see the Layouts guide.

android:hint

This is a default string to display when the text field is empty. Instead of using a hard-coded string as the value, the "@string/edit_message" value refers to a string resource defined in a separate file. Because this refers to a concrete resource (not just an identifier), it does not need the plus sign. However, because you haven't defined the string resource yet, you’ll see a compiler error at first. You'll fix this in the next section by defining the string.

Note: This string resource has the same name as the element ID: edit_message. However, references to resources are always scoped by the resource type (such as id or string), so using the same name does not cause collisions.

Add String Resources

By default, your Android project includes a string resource file at res/values/strings.xml. Here, you'll add two new strings.

1. From the res/values/ directory, open strings.xml.
2. Add two strings so that your file looks like this:

   <?xml version="1.0" encoding="utf-8"?>
   
       My First App
       Enter a message
       Send
   

For text in the user interface, always specify each string as a resource. String resources allow you to manage all UI text in a single location, which makes the text easier to find and update. Externalizing the strings also allows you to localize your app to different languages by providing alternative definitions for each string resource.

For more information about using string resources to localize your app for other languages, see the Supporting Different Devices class.

Add a Button

Go back to the activity_main.xml file and add a button after the . Your file should look like this:

        
        <Button
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="@string/button_send" />

Note: This button doesn't need the android:id attribute, because it won't be referenced from the activity code.

The layout is currently designed so that both the EditText and Button widgets are only as big as necessary to fit their content, as figure 2 shows.

Figure 2. The EditText and Button widgets have their widths set to "wrap_content".

This works fine for the button, but not as well for the text field, because the user might type something longer. It would be nice to fill the unused screen width with the text field. You can do this inside a LinearLayout with the weight property, which you can specify using theandroid:layout_weight attribute.

The weight value is a number that specifies the amount of remaining space each view should consume, relative to the amount consumed by sibling views. This works kind of like the amount of ingredients in a drink recipe: "2 parts soda, 1 part syrup" means two-thirds of the drink is soda. For example, if you give one view a weight of 2 and another one a weight of 1, the sum is 3, so the first view fills 2/3 of the remaining space and the second view fills the rest. If you add a third view and give it a weight of 1, then the first view (with weight of 2) now gets 1/2 the remaining space, while the remaining two each get 1/4.

The default weight for all views is 0, so if you specify any weight value greater than 0 to only one view, then that view fills whatever space remains after all views are given the space they require.

Make the Input Box Fill in the Screen Width

In activity_main.xml, modify the  so that the attributes look like this:

android:layout_weight="1"
    android:layout_width="0dp"
    android:layout_height="wrap_content"
    android:hint="@string/edit_message" />

Setting the width to zero (0dp) improves layout performance because using "wrap_content" as the width requires the system to calculate a width that is ultimately irrelevant because the weight value requires another width calculation to fill the remaining space.

Figure 3. The EditText widget is given all the layout weight, so it fills the remaining space in the LinearLayout.

Here’s how your complete activity_main.xmllayout file should now look:

<?xml version="1.0" encoding="utf-8"?>

    
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/button_send" />

Run Your App

This layout is applied by the default Activity class that the SDK tools generated when you created the project.

To run the app and see the results, click Run 'app'  in the toolbar.

Fourth Day...

Starting Another Activity

This lesson teaches you to

1. Respond to the Send Button
2. Build an Intent
3. Create the Second Activity
4. Display the Message

After completing the previous lesson, you have an app that shows an activity (a single screen) with a text field and a button. In this lesson, you’ll add some code to MainActivitythat starts a new activity when the user clicks the Send button.

Respond to the Send Button

1. In the file res/layout/activity_main.xml, add the android:onClick attribute to the <Button>element as shown below:

   <Button
         android:layout_width="wrap_content"
         android:layout_height="wrap_content"
         android:text="@string/button_send"
          />
       

   This attribute tells the system to call the sendMessage() method in your activity whenever a user clicks on the button.

2. In the file java/com.example.myfirstapp/MainActivity.java, add the sendMessage() method stub as shown below:

   public class MainActivity extends AppCompatActivity {
       @Override
       protected void onCreate(Bundle savedInstanceState) {
           super.onCreate(savedInstanceState);
           setContentView(R.layout.activity_main);
       }
   
       /** Called when the user clicks the Send button */
       public void sendMessage(View view) {
           // Do something in response to button
       }
   }

   In order for the system to match this method to the method name given to android:onClick, the signature must be exactly as shown. Specifically, the method must:

   • Be public
   • Have a void return value
   • Have a View as the only parameter (this will be the View that was clicked)

public class MainActivity extends AppCompatActivity {
    public final static String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    /** Called when the user clicks the Send button */
    public void sendMessage(View view) {
        Intent intent = new Intent(this, DisplayMessageActivity.class);
        EditText editText = (EditText) findViewById(R.id.edit_message);
        String message = editText.getText().toString();
        intent.putExtra(EXTRA_MESSAGE, message);
        startActivity(intent);
    }
}