Introduction
This tutorial is a starting point for developing Android apps. It
will explain the very basics of the Android SDK (Software Development
Kit) and how to use it with Eclipse. To understand this tutorial, you
don't need to have any knowledge about programming in Java, but it might
be helpful for further programming to understand the basics of object
orientated programming. This tutorial explains Android beginners how to
create an Android Project in Eclipse, work with resources, and create
some first code.
Setup Eclipse and the Android SDK
If you don't already have a running environment to develop Android apps, follow the instructions at this
link.
Hint: PATH means the Path Environment Variable. In Windows, you will find it under "
Control Panel/System/Advanced System Settings/Environment Variables" in the lower list box. You can check what version of Java is installed, by going in the command line and typing
java -version.
Let's Get Started
Now we want to create our first application, which is (as always) a
Hello World application. First of all, start Eclipse. Then select
"File/New/Project". In the "New Project" dialog, select "Android/Android
Project" and click "Next".
Here you can set up the project. First of all, we need to give the
project a name, so type "Hello World" in the name box. Next you have to
select the Android version you want to use. Here we choose version 2.2.
As we see in the last column, we need the API Version 8 for this Android
version, so we type an "8" in the Min SDK Version box. Also, the
project requires an application name. (Notice that this name is also
used in code, so the name should have no whitespaces). Usually, you use
the project name and delete all whitespaces (e.g., "helloworld" for this
project). Next, you have to define the package of the project. We will
use "com.test.helloworld" here (a package can group multiple classes;
for more information, see
here).
At least, you need a name for the activity (one App might have multiple
Activities; they are like a single part/screen of the app). In this
example, we simply use "HelloWorldApp".
Before we can finally start our first project, we need to create a
configuration. This configuration specifies under which circumstances
the app will be started. E.g., you can control the network speed the
emulator/app can use. Also, you can choose different emulators to test
the app with different versions of Android or in different screen sizes.
To create the configuration, go to "Run/Run Configurations". Now click
the "Android Application" tab at the side and then the New button above
the tabs. Call the new configuration "HelloWorldConfig" and select our
project over the Browse button. Now move on to the target tab. Here you
can select the network speed and which emulator will be used.
Since we haven't created an emulator till now, we need to do that
first. Click the automatic control to enable the buttons at the side and
then click on the manager-button. Here, click the new button to the
right to create a new emulator. In the following screen, you can enter a
name for the emulator (I have used "DefaultAndroidEmulator") and
specify the details (like Android version, SD card size, and much more).
You can control every little detail of the emulator over the hardware
section.
Once you are done with that, click "Create AVD" and close the manager
window. Now we have successfully created the run configurations. Click
"Apply" and close the configurations. At least run your first Android
project.
Notice: It may take the emulator some time to get started, so be
patient! Also, I have cropped the image so that you can't see the
keyboard or the D-pad.
Congratulations! You just created your first App!
Coding is Fun
After we have set up everything, it's (finally) time to actually
getting started with the code, because we all know: Coding is fun!
But before we can actually jump into the Java code, we need to
understand the structure of an Android Application. Go to your Package
Explorer and enlarge the "Hello World" project. You will see five
folders and two files. Let's get started with the one of these two
files, the AndroidManifest file. This file contains all the information
about your project, like the icon, the name of the author. To open it,
make a right click on it and choose "Open With/Android Manifest Editor".
In the upcoming tab, you can specify the package name and the version
of your project. At the bottom, you will find additional tabs. I think
most of the settings you will find are pretty much self-explanatory.
Note the @ in front of some attributes. This shows that the following
string is a reference to a resource. You can find the resources in the "
res" folder of your project. If you enlarge it, you will notice that it has some subfolders. To be specific, the
res folder can have seven types of subfolders:
values,
drawable,
layout,
animations,
xml,
styles, and
raw.
Let's focus on the
values folder first. Here you can store
all kinds of simple resources (like strings, colors, numbers,
dimensions, arrays, etc.). By default, you will find the
strings.xml
file in there. When you open it (with right click, "Open with/Android
Layout Editor"), you will see that it contains two values. The first is
the message you see when you run your project, and the second is the
name of your app. You can add new values if you want to use them later
on in code (or in the Manifest or Layout files). You can also create
specific resources using quantifiers. If you add a - to the folder's
name, you can add a quantifier to the name. E.g., you can rename the
values folder to
values-en
which means that the content of the folder is only used by Android
phones with English language activated. If you do not add a quantifier,
the resources are default. The default resources are used if no specific
resources for the current system are found. If the project is started,
all resources will be compiled as efficiently as possible and added to
the package. Also, a reference will be created (called R) which allows
you to access the resources in code. Since this is only a tutorial, I
will not focus on all the types of resources here. You can find more
information on resources and quantifiers
here.
At last, it is time to start coding! Go to the "
src" folder. In the folder, you will find the package folder, open the
HelloWorld.java file. You will see the default code for an Android Activity:
Collapse | Copy Code
package com.test.helloworld;
import android.app.Activity;
import android.os.Bundle;
public class HelloWorldApp extends Activity {
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
}
As you can see, we create a new activity by extending the default Android activity class. Then we override the default
onCreate function, which is called when the project is created. In there, we load our own layout from the resources and also call the
onCreate
function of the base class. Now let's take a closer look at the layout
file. You find it in the layout folder under resources. When you open
it, it should look like this:
Collapse | Copy Code
<linearlayout android:layout_height="fill_parent"
android:layout_width="fill_parent" android:orientation="vertical"
xmlns:android="http://schemas.android.com/apk/res/android" />
<textview android:layout_height="wrap_content"
android:layout_width="fill_parent" android:text="@string/hello" />
</linearlayout />
You see the root node is called
LinearLayout. As you you might already have figured out, there are different types of layouts:
FrameLayout: All controls (children) are placed in the upper left corner.LinearLayout: All children are positioned in vertical/horizontal order.RelativeLayout: The positions of the children are specified in relation to the other children.TableLayout: The child elements are placed with a grid.AbsoluteLayout: The child elements are positioned based on absolute coordinates (in pixel).
Once you have chosen a layout type, you can add child elements. In the code given, there is already a
TextView, which is used to display text on the screen. The current content is a reference to a resource defined in the
values.xml
file. As you will see, it uses the whole width of the screen, but is
only as long as it needs to, to display the content. We might start with
some small changes. Let's change the text color of the
TextView to green:
Collapse | Copy Code
<textview android:layout_height="wrap_content"
android:layout_width="fill_parent" android:text="@string/hello"
android:textcolor="#FF00FF00" />
Now, launch the project and see the changes. Next, let's add a new control called
EditText:
Collapse | Copy Code
<linearlayout android:layout_height="fill_parent"
android:layout_width="fill_parent" android:orientation="vertical"
xmlns:android="http://schemas.android.com/apk/res/android" />
<edittext android:layout_height="wrap_content"
android:layout_width="fill_parent" android:textcolor="#FF0000FF"
android:id="@+id/et_Text" />
<textview android:layout_height="wrap_content" android:layout_width="fill_parent"
android:textcolor="#FF00FF00" android:id="@+id/lv_View" />
</linearlayout />
When we want to access the controls in code, they need to have an ID. Next we create some code for the controls. Go to the
helloworld.java file.
Collapse | Copy Code
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
final TextView tv_View = (TextView)findViewById(R.id.tv_View);
final EditText et_Text = (EditText)findViewById(R.id.et_Text);
et_Text.setOnKeyListener(new OnKeyListener()
{
public boolean onKey(View v, int keyCode, KeyEvent event)
{
if (event.getAction() == KeyEvent.ACTION_DOWN)
{
if (keyCode == KeyEvent.KEYCODE_DPAD_CENTER)
{
tv_View.setText(tv_View.getText() + ", " +
et_Text.getText());
et_Text.setText("");
return true;
}
}
return false;
}
});
}
We will analyze the code line by line. First of all, as before, we load the layout. Then we create a
TextView and a
EditText variable and load our interface objects in them (that's what we need the ID for). Finally, we add a new
OnKeyListener to the
EditText control. In this
OnKeyListener, we create the method
onKey,
which is called when a key is pressed, when the control is active. In
the method, we perform two checks: the first to be sure that a key is
pressed down (and not released), and the second to specify the key (in
this case, the center key of the D-pad). If both checks are passed, we
add the text of the
EditText control to the
TextView, and finally the text of the
EditText control is deleted. Run and test the application. Great, you created your first real Android app.
Android App Design
As with every platform, Android has its own design challenges. Always
keep in mind that you are developing for a mobile platform with limited
memory, disk space, and processing power. Therefore, Android
automatically kills processes (each app runs in its own process) to keep
the system responsive. Processes are sorted after importance. The most
important is the currently active process, followed by visible and
stated service processes. The bottommost types of processes in hierarchy
are background and empty processes. Keep that in mind when you design
your application, because you don't want its process to be killed in the
middle of something. Also, you can use whatever hardware is built into
the Android phone. But notice that not all phones might have this
hardware, and so not all might be able to run your app.
That's it for this tutorial. Hopefully, you understood the basics of
Android development. Since this is my first article on CodeProject, I
would really appreciate feedback!
