page.title=Hello, World @jd:body

As a developer, you know that the first impression of a development framework is how easy it is to write "Hello, World." Well, on Android, it's pretty easy. It's particularly easy if you're using Eclipse as your IDE, because we've provided a great plugin that handles your project creation and management to greatly speed up your development cycles.

If you're not using Eclipse, that's okay. Familiarize yourself with Developing in Other IDEs. You can then come back here and ignore anything about Eclipse.

Before you start, you should already have the latest SDK installed, and if you're using Eclipse, you should have installed the ADT plugin as well. See Installing the Android SDK to get these installed.

Note: In some cases, you might want to click the screenshots below to get a bigger view.

Create the Project

  1. Open a new Android Project.

    From Eclipse, select the File > New > Project menu item. If the Android Plugin for Eclipse has been successfully installed, the resulting dialog should have a folder labeled "Android" which should contain a single entry: "Android Project".

    Selected "Android Project" and click Next.

  2. Fill out the project details.

    The next screen allows you to enter the relevant details for your project:

    Click Finish.

    Here's what each field on this screen means:

    Project Name
    This is the name of the directory or folder on your computer that you want to contain the project.
    Package Name
    This is the package namespace (following the same rules as for packages in the Java programming language) that you want all your source code to reside under. This also sets the package name under which the stub Activity will be generated.

    The package name you use in your application must be unique across all packages installed on the system; for this reason, it's very important to use a standard domain-style package for your applications. In the example above, we used the package domain "com.android"; you should use a different one appropriate to your organization.

    Activity Name
    This is the name for the class stub that will be generated by the plugin. This will be a subclass of Android's Activity class. An Activity is simply a class that can run and do work. It can create a UI if it chooses, but it doesn't need to.
    Application Name
    This is the human-readable title for your application.

    The checkbox for toggling "Use default location" allows you to change the location on disk where the project's files will be generated and stored.

  3. View the auto-generated source code.

    After the plugin completes your project creations, you'll have a class named HelloAndroid (found in your project package, HelloAndroid > src > com.android.hello). It should look like this:

    package com.example.hello;

import android.app.Activity;
import android.os.Bundle;

public class HelloAndroid extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

    Now let's modify some code!

Construct the UI

Take a look at this revised code, below, and make the same changes to your HelloAndroid.java file. We'll dissect it line by line:

package com.android.hello;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;

public class HelloAndroid extends Activity {
   /** Called when the activity is first created. */
   @Override
   public void onCreate(Bundle savedInstanceState) {
       super.onCreate(savedInstanceState);
       TextView tv = new TextView(this);
       tv.setText("Hello, Android");
       setContentView(tv);
   }
}

Tip: If you forgot to import the TextView package, try this: press Ctrl-Shift-O (Cmd-Shift-O, on Mac). This is an Eclipse shortcut to organize imports—it identifies missing packages and adds them for you.

Notice that our class is based on the {@link android.app.Activity} class. An Activity is a single application entity that is used to perform actions. An application may have many, but the user interacts with them only one at a time. An Activity is not required to actually have a user interface, but usually will.

An Android user interface is composed of hierarchies of objects called Views. A {@link android.view.View} is simply a drawable object, such as a radio button, an animation, or (in our case) a text label. The specific name for the View subclass that handles text, which we use here, is {@link android.widget.TextView}.

Here's how you construct a TextView:

TextView tv = new TextView(this);

The argument to TextView's constructor is an Android {@link android.content.Context} instance. The Context is simply a handle to the system; it provides services like resolving resources, obtaining access to databases and preferences, and so on. The Activity class inherits from Context. Since our HelloAndroid class is a subclass of Activity, it is also a Context, and so we can pass the this reference to the TextView.

Once we've constructed the TextView, we need to tell it what to display:

tv.setText("Hello, Android");

Nothing too surprising there.

At this point, we've constructed a TextView and told it what text to display. The final step is to connect this TextView with the on-screen display, like so:

setContentView(tv);

The setContentView() method on Activity indicates to the system which View should be associated with the Activity's UI. If an Activity doesn't call this method, no UI is present at all and the system will display a blank screen. For our purposes, all we want is to display some text, so we pass it the TextView we just created.

There it is — "Hello, World" in Android! The next step, of course, is to see it running.

Run the Code: Hello, Android

The Eclipse plugin makes it very easy to run your applications. Begin by selecting the Run > Open Run Dialog menu entry (in Eclipse 3.4, it's Run > Run Configurations). You should see a dialog like this:

Next, highlight the "Android Application" entry, and then click the icon in the top left corner (the one depicting a sheet of paper with a plus sign in the corner) or simply double-click the "Android Application" entry. You should have a new launcher entry named "New_configuration".

Change the name to something expressive, like "Hello, Android", and then pick your project by clicking the Browse button. (If you have more than one Android project open in Eclipse, be sure to pick the right one.) The plugin will automatically scan your project for Activity subclasses, and add each one it finds to the drop-down list under the "Activity:" label. Since your "Hello, Android" project only has one, it will be the default, and you can simply continue.

Click the "Apply" button. Here's an example:

Now click Run, and the Android Emulator should start and open the application. Once it's booted up your application will appear. (Once booted, you may need to unlock the emulator's phone screen by pressing the device MENU key.) When all is said and done, you should see something like this:

That's "Hello, World" in Android. Pretty straightforward, eh? The next sections of the tutorial offer more detailed information that you may find valuable as you learn more about Android.

Upgrading the UI to an XML Layout

The "Hello, World" example you just completed uses what we call "programmatic" UI layout. This means that you construct and build your application's UI directly in source code. If you've done much UI programming, you're probably familiar with how brittle that approach can sometimes be: small changes in layout can result in big source-code headaches. It's also very easy to forget to properly connect Views together, which can result in errors in your layout and wasted time debugging your code.

That's why Android provides an alternate UI construction model: XML-based layout files. The easiest way to explain this concept is to show an example. Here's an XML layout file that is identical in behavior to the programmatically-constructed example you just completed:

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  android:text="@string/hello"/>

The general structure of an Android XML layout file is simple: it's a tree of XML elements, where each element is the name of a View class (this example, however, is just one element). You can use the name of any class that extends {@link android.view.View} as an element in your XML layouts, including custom View classes you define in your own code. This structure makes it very easy to quickly build up UIs, using a more simple structure and syntax than you would in source code. This model is inspired by the web development model, where you can separate the presentation of your application (its UI) from the application logic used to fetch and fill in data.

In this example, there's just one View element, the TextView, which has four XML attributes. Here's a summary of what they mean:

Attribute Meaning
xmlns:android This is an XML namespace declaration that tells the Android tools that you are going to refer to common attributes defined in the Android namespace. The outermost tag in every Android layout file must have this attribute.
android:layout_width This attribute defines how much of the available width on the screen this View should consume. In this case, it's our only View so we want it to take up the entire screen, which is what a value of "fill_parent" means.
android:layout_height This is just like android:layout_width, except that it refers to available screen height.
android:text This sets the text that the TextView should display. In this example, we use a string resource instead of a hard-coded string value. The hello string is defined in the res/values/strings.xml file. This is the recommended practice for inserting strings to your application, because it makes the localization of your application to other languages graceful, without need to hard-code changes to the layout file. For more information, see Resources and Internationalization.

These layout files belong in the res/layout/ directory in your project. The "res" is short for "resources" and that directory contains all the non-code assets that your application requires. Resources also include things like images, localized strings, and XML layout files.

Landscape layout

When you want a different design for landscape, put your layout XML file in res/layout-land/. Android will automatically look here when the layout changes. Without it the layout will just be stretched.

The Eclipse plugin automatically creates one of these layout files for you (main.xml). In our example above, we just ignored it and created our layout programmatically. We did so just to teach you a little more about the framework, but you should almost always define your layout in an XML file instead of in your code.

So, let's put it to use and change the "Hello, Android" sample to use the XML layout.

  1. In the Eclipse Package Explorer, expand the folder res/layout/, and open the file main.xml (once opened, you might need to click the "main.xml" tab at the bottom to see the XML source). Replace its contents with the following XML: 
    <?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"
  android:text="@string/hello"/>

    Save the file.

  2. Inside the project folder res/values/, open the file strings.xml. This is where you should save all default text strings for your user interface. If you're using Eclipse, then ADT will have started you with two strings, hello and app_name. Revise hello to something else. Perhaps "Hello, Android! I am a string resource!" The entire file should now look like this: 
    <?xml version="1.0" encoding="utf-8"?>
<resources>
    <string name="hello">Hello, Android! I am a string resource!</string>
    <string name="app_name">Hello, Android</string>
</resources>
  3. Now open and modify your HelloAndroid class source code to read the XML layout, instead of the hard-coded version. Edit the file to look like this: 
    package com.example.hello;

import android.app.Activity;
import android.os.Bundle;

public class HelloAndroid extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
    }
}

    When you make this change, type it by hand to try the code-completion feature on that R class. You'll probably find that it helps a lot.

    Now, instead of passing setContentView() a View object, we give it a reference to our layout resource. The resource is identified as R.layout.main, which is actually a compiled object representation of the layout defined in layout/main.xml. The Eclipse plugin automatically creates this reference for us inside the project's R.java class. If you're not using Eclipse, then the R.java class will be generated for you when you run Ant to build the application. (More about the R class in a moment.)

Now that you've made this change, go ahead and re-run your application — all you need to do is click the green Run arrow icon, or select Run > Run History > Hello, Android from the menu. You should see pretty much the same thing you saw before! After all, the point was to show that the two different layout approaches produce identical results.

Tip: Use the shortcut Ctrl-Shift-F11 (Cmd-Shift-F11, on Mac) to run your currently visible application.

You've just completed your first Android application! Continue reading for an introduction to debugging and a little more information on using other IDEs. Once you're ready to move on, please begin by reading Application Fundamentals. Also refer to the Developer's Guide introduction page for an overview of the Dev Guide documentation.

R class

In Eclipse, open the file named R.java in your source code folder in the Package Explorer. It should look something like this:

public final class R {
    public static final class attr {
    }
    public static final class drawable {
        public static final int icon=0x7f020000;
    }
    public static final class layout {
        public static final int main=0x7f030000;
    }
    public static final class string {
        public static final int app_name=0x7f040001;
        public static final int hello=0x7f040000;
    }
}

A project's R.java file is an index into all the resources defined in the file. You use this class in your source code as a sort of short-hand way to refer to resources you've included in your project. This is particularly powerful with the code-completion features of IDEs like Eclipse because it lets you quickly and interactively locate the specific reference you're looking for.

It's possible yours looks slighly different than this (perhaps the hexadecimal values are different). For now, notice the inner class named "layout", and its member field "main". The Eclipse plugin noticed the XML layout file named main.xml and generated a class for it here. As you add other resources to your project (such as strings in the res/values/string.xml file or drawables inside the res/drawable/ direcory) you'll see R.java change to keep up.

When not using Eclipse, this class file will be generated for you at build time (with the Ant tool).

You should never edit this file by hand.

Debugging Your Project

The Android Plugin for Eclipse also has excellent integration with the Eclipse debugger. To demonstrate this, let's introduce a bug into our code. Change your HelloAndroid source code to look like this:

package com.android.hello;

import android.app.Activity;
import android.os.Bundle;

public class HelloAndroid extends Activity {
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Object o = null;
        o.toString();
        setContentView(R.layout.main);
    }
}

This change simply introduces a NullPointerException into your code. If you run your application again, you'll eventually see this:

Press "Force Quit" to terminate the application and close the emulator window.

To find out more about the error, set a breakpoint in your source code on the line Object o = null; (double-click on the marker bar next to the source code line). Then select Run > Debug History > Hello, Android from the menu to enter debug mode. Your app will restart in the emulator, but this time it will suspend when it reaches the breakpoint you set. You can then step through the code in Eclipse's Debug Perspective, just as you would for any other application.

Creating the Project without Eclipse

If you don't use Eclipse (such as if you prefer another IDE, or simply use text editors and command line tools) then the Eclipse plugin can't help you. Don't worry though — you don't lose any functionality just because you don't use Eclipse.

The Android Plugin for Eclipse is really just a wrapper around a set of tools included with the Android SDK. (These tools, like the emulator, aapt, adb, ddms, and others are documented elsewhere.) Thus, it's possible to wrap those tools with another tool, such as an 'ant' build file.

The Android SDK includes a Python script named "activitycreator.py" that can be used to create all the source code and directory stubs for your project, as well as an ant-compatible build.xml file. This allows you to build your project from the command line, or integrate it with the IDE of your choice.

For example, to create a HelloAndroid project similar to the one we just created via Eclipse, you'd use this command:

activitycreator.py --out HelloAndroid com.android.hello.HelloAndroid

To build the project, you'd then run the command 'ant'. When that command successfully completes, you'll be left with a file named HelloAndroid.apk under the 'bin' directory. That .apk file is an Android Package, and can be installed and run in your emulator using the 'adb' tool.

For more information on how to use these tools, please read the documentation cited above.