Android Programming for beginners guide

Android Programming for beginners: Smartphones and tablets on Android are increasingly found in our bags and pockets, and Android Programming for beginners is also becoming more popular. This is an excellent platform for development – the API is well documented and easy to use, and just interesting to create something that you can run on your smartphone. Initially, you can do without it, by creating and testing the code using an emulator on your Linux PC. The first of two parts of this introduction explains how to create a simple application with a timer, as well as give the initial information about the Android API. The course implies the availability of initial ideas about Java, XML and programming technologies, but nevertheless feel free to try even if you have a very vague idea about it.

Getting started and development environment

A little bit about the versions: the latest version of Android is 4.2 (Jelly Bean), but, as you can see on this diagram, it is still not widely distributed. It’s best to start development under one of the versions: 4.0 (Ice Cream Sandwich) or 2.3 (Gingerbread), especially considering that Android versions support backward compatibility (ie your code for version 2.3 will work on 4.2), and Applications developed for a later version will not always work on the old one. The code shown here should work on versions 4.0 and 2.3.
The easiest way to get the development environment is to install the Android Bundle, which can be downloaded here. You will also need JDK 6 (not only JRE). Do not forget that Android is not compatible with gcj. If you already use Eclipse or another IDE, then you can try to configure it for Android. How to do this is described here. Now create a project called Countdown with Eclipse, or from the command line. I installed in BuildSDK 4.0.3 and minimum SDK 2.2, and (in Eclipse) I used the BlankActivity template.

My first Android project: interface

Our first program for Android will be a timer, showing a countdown from 10 seconds after pressing the button. Before you write code, you need to create an interface – something that the user sees by running our application. You need to open res/layout / activity_countdown.xml and create an XML template – use the Eclipse editor or the text / XML editor to enter the following:

<RelativeLayoutxmlns:android=”http://schemas.android.com/apk/res/android”

xmlns:tools=”http://schemas.android.com/tools”

android:layout_width=”match_parent”

android:layout_height=”match_parent”>

<TextView

android:id=”@+id/time_display_box”

android:layout_width=”wrap_content”

android:layout_height=”wrap_content”

android:layout_alignParentTop=”true”

android:layout_centerHorizontal=”true”

android:layout_marginTop=”60dp”

android:text=”@string/_00_30″

android:textAppearance=”?android:attr/textAppearanceLarge”/>

<Button

android:id=”@+id/startbutton”

android:layout_width=”wrap_content”

android:layout_height=”wrap_content”

android:layout_below=”@+id/time_display_box”

android:layout_centerHorizontal=”true”

android:layout_marginTop=”41dp”

android:text=”@string/start” />

</RelativeLayout>

Notice the string / start and string / __ 00_30. Their values will be located in res / values / strings.xml:

<string name=”start”>Start</string>

<string name=”_00_30″>00:30</string>

This is a common way to access resources in Android: it’s better to use references to string variables than hard-coded strings.

My first Android project: code

Now open the file CountdownActivity.java in the editor – and we are ready to write the code of our application. You should see the automatically generated “stub” of the onCreate () method. It is always called as soon as an Activity object is created, and you can place any functions in it, which should be executed when the application is launched. (Eclipse can also create an empty method onCreateOptionsMenu (), but we will not pay attention to it yet). Enter the following code:

publicclassCountdownActivityextendsActivity {

privatestaticfinalint MILLIS_PER_SECOND = 1000;

privatestaticfinalint SECONDS_TO_COUNTDOWN = 30;

privateTextViewcountdownDisplay;

privateCountDownTimer timer;

@Override

publicvoidonCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_countdown);

countdownDisplay = (TextView) findViewById(R.id.time_display_box);

Button startButton = (Button) findViewById(R.id.startbutton);

startButton.setOnClickListener(newView.OnClickListener() {

publicvoidonClick(View view) {

try {

showTimer(SECONDS_TO_COUNTDOWN * MILLIS_PER_SECOND);

} catch (NumberFormatException e) {

// method ignores invalid (non-integer) input and waits

// for something it can use

}

}

});

}

}

Android Programming for beginners: You see how simple it was to create our first project: the Android API includes CountDownTimer, which we can use. We declared it and the countdown display field as private properties of the Activity class. In the onCreate () method, we used the setContentView method to attach our XML template. This R.foo.bar syntax is the standard way of accessing XML resources in Android, and we will meet with it many more times.
findViewById is another method that you will often use. Here it returns references to the timer output field and the Start button, described in the XML template. For the button, so that we can process its pressing, the “interceptor” OnClickListener, and its onClick () method must be specified. Here it simply calls the showTimer () method with a specified number of milliseconds (now hard-coded in the code).

So, what does showTimer () do:

privatevoidshowTimer(intcountdownMillis) {

if(timer != null) { timer.cancel(); }

timer = newCountDownTimer(countdownMillis, MILLIS_PER_SECOND) {

@Override

publicvoidonTick(longmillisUntilFinished) {

countdownDisplay.setText(“counting down: ” +

millisUntilFinished / MILLIS_PER_SECOND);

}

@Override

publicvoidonFinish() {

countdownDisplay.setText(“KABOOM!”);

}

}.start();

}

The class CountDownTimer does almost all the work for us, which is very nice. In the beginning, we check if the timer already exists, and if it is, then we reset it. Next, we create a new timer with a specified number of milliseconds for the countdown (from the parameter of the showTimer () method) and with a specified number of milliseconds between the sample intervals. After the time elapses between the intervals, the onTick () method is called.

CountDownTimer is an abstract class, and the __onTick () and __onFinish () methods must be implemented in its subclass. We override the onTick () method, which reduces the output number by one at the end of each interval, and the onFinish () method, which displays a message about the end of the countdown. Then start () starts the timer.

Using the “Run” command in Eclipse, you can run the created application, and the Android emulator will automatically start. Look at the Android documentation if you want to learn more about setting up the emulator, or launching applications from the command line .

Congratulations, you just created your first Android application. In the second part of this introduction, we will take a closer look at the structure of the Android application, and make some improvements to our program: entering the countdown time, the Stop button and the menu. We will also run it on a real device, not on the emulator.