What is Android?
Android is an open source and Linux-based Operating System for mobile devices such as smartphones and tablet computers. Android was developed by the Open Handset Alliance, led by Google, and other companies.
Android offers a unified approach to application development for mobile devices which means developers need only develop for Android, and their applications should be able to run on different devices powered by Android.
The first beta version of the Android Software Development Kit (SDK) was released by Google in 2007 where as the first commercial version, Android 1.0, was released in September 2008.
On June 27, 2012, at the Google I/O conference, Google announced the next Android version, 4.1 Jelly Bean. Jelly Bean is an incremental update, with the primary aim of improving the user interface, both in terms of functionality and performance.
The source code for Android is available under free and open source software licenses. Google publishes most of the code under the Apache License version 2.0 and the rest, Linux kernel changes, under the GNU General Public License version 2.
Features of Android
Android is a powerful operating system competing with Apple 4GS and supports great features. Few of them are listed below:
Feature
|
Description
|
Beautiful UI
|
Android OS basic screen provides a beautiful and intuitive user interface.
|
Connectivity
|
GSM/EDGE, IDEN, CDMA, EV-DO, UMTS, Bluetooth, Wi-Fi, LTE, NFC and WiMAX.
|
Storage
|
SQLite, a lightweight relational database, is used for data storage purposes.
|
Media support
|
H.263, H.264, MPEG-4 SP, AMR, AMR-WB, AAC, HE-AAC, AAC 5.1, MP3, MIDI, Ogg Vorbis, WAV, JPEG, PNG, GIF, and BMP
|
Messaging
|
SMS and MMS
|
Web browser
|
Based on the open-source WebKit layout engine, coupled with Chrome's V8 JavaScript engine supporting HTML5 and CSS3.
|
Multi-touch
|
Android has native support for multi-touch which was initially made available in handsets such as the HTC Hero.
|
Multi-tasking
|
User can jump from one task to another and same time various application can run simultaneously.
|
Resizable widgets
|
Widgets are resizable, so users can expand them to show more content or shrink them to save space
|
Multi-Language
|
Supports single direction and bi-directional text.
|
GCM
|
Google Cloud Messaging (GCM) is a service that lets developers send short message data to their users on Android devices, without needing a proprietary sync solution.
|
Wi-Fi Direct
|
A technology that lets apps discover and pair directly, over a high-bandwidth peer-to-peer connection.
|
Android Beam
|
A popular NFC-based technology that lets users instantly share, just by touching two NFC-enabled phones together.
|
Android Applications
Android applications are usually developed in the Java language using the Android Software Development Kit.
Once developed, Android applications can be packaged easily and sold out either through a store such as Google Play,SlideME,Opera Mobile Store,Mobango,F-droid and the Amazon Appstore.
Android powers hundreds of millions of mobile devices in more than 190 countries around the world. It's the largest installed base of any mobile platform and growing fast. Every day more than 1 million new Android devices are activated worldwide.
This tutorial has been written with an aim to teach you how to develop and package Android application. We will start from environment setup for Android application programming and then drill down to look into various aspects of Android applications.
Categories of Android applications
There are many android applications in the market. The top categories are:
History of Android
The code names of android ranges from A to L currently, such as Aestro, Blender, Cupcake, Donut, Eclair, Froyo, Gingerbread, Honeycomb, Ice Cream Sandwitch, Jelly Bean, KitKat and Lollipop. Let's understand the android history in a sequence.
What is API level?
API Level is an integer value that uniquely identifies the framework API revision offered by a version of the Android platform.
Platform Version
|
API Level
|
VERSION_CODE
| |
Android 5.1
|
22
|
LOLLIPOP_MR1
| |
Android 5.0
|
21
|
LOLLIPOP
| |
Android 4.4W
|
20
|
KITKAT_WATCH
| |
Android 4.4
|
19
|
KITKAT
| |
Android 4.3
|
18
|
JELLY_BEAN_MR2
| |
Android 4.2, 4.2.2
|
17
|
JELLY_BEAN_MR1
| |
Android 4.1, 4.1.1
|
16
|
JELLY_BEAN
| |
Android 4.0.3, 4.0.4
|
15
|
ICE_CREAM_SANDWICH_MR1
| |
Android 4.0, 4.0.1, 4.0.2
|
14
|
ICE_CREAM_SANDWICH
| |
Android 3.2
|
13
|
HONEYCOMB_MR2
| |
Android 3.1.x
|
12
|
HONEYCOMB_MR1
| |
Android 3.0.x
|
11
|
HONEYCOMB
| |
Android 2.3.4
Android 2.3.3
|
10
|
GINGERBREAD_MR1
| |
Android 2.3.2
Android 2.3.1
Android 2.3
|
9
|
GINGERBREAD
| |
Android 2.2.x
|
8
|
FROYO
| |
Android 2.1.x
|
7
|
ECLAIR_MR1
| |
Android 2.0.1
|
6
|
ECLAIR_0_1
| |
Android 2.0
|
5
|
ECLAIR
| |
Android 1.6
|
4
|
DONUT
| |
Android 1.5
|
3
|
CUPCAKE
| |
Android 1.1
|
2
|
BASE_1_1
| |
Android 1.0
|
1
|
BASE
|
Android - User Interface
The basic building block for user interface is a View object which is created from the View class and occupies a rectangular area on the screen and is responsible for drawing and event handling. View is the base class for widgets, which are used to create interactive UI components like buttons, text fields, etc.
The ViewGroup is a subclass of View and provides invisible container that hold other Views or other ViewGroups and define their layout properties.
At third level we have different layouts which are subclasses of ViewGroup class and a typical layout defines the visual structure for an Android user interface and can be created either at run time using View/ViewGroup objects or you can declare your layout using simple XML file main_layout.xml which is located in the res/layout folder of your project.
LAYOUT PARAMS
This tutorial is more about creating your GUI based on layouts defined in XML file. A layout may contain any type of widgets such as buttons, labels, textboxes, and so on. Following is a simple example of XML file having LinearLayout:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<TextView android:id="@+id/text"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="This is a TextView" />
<Button android:id="@+id/button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="This is a Button" />
<!-- More GUI components go here -->
</LinearLayout>
Once your layout has created, you can load the layout resource from your application code, in your Activity.onCreate() callback implementation as shown below −
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
Android Layout Types
There are number of Layouts provided by Android which you will use in almost all the Android applications to provide different view, look and feel.
Sr.No
|
Layout & Description
|
1
|
LinearLayout is a view group that aligns all children in a single direction, vertically or horizontally.
|
2
|
RelativeLayout is a view group that displays child views in relative positions.
|
3
|
TableLayout is a view that groups views into rows and columns.
|
4
|
AbsoluteLayout enables you to specify the exact location of its children.
|
5
|
The FrameLayout is a placeholder on screen that you can use to display a single view.
|
6
|
ListView is a view group that displays a list of scrollable items.
|
7
|
GridView is a ViewGroup that displays items in a two-dimensional, scrollable grid.
|
Layout Attributes
Each layout has a set of attributes which define the visual properties of that layout. There are few common attributes among all the layouts and their are other attributes which are specific to that layout. Following are common attributes and will be applied to all the layouts:
Attribute
|
Description
|
android:id
|
This is the ID which uniquely identifies the view.
|
android:layout_width
|
This is the width of the layout.
|
android:layout_height
|
This is the height of the layout
|
android:layout_marginTop
|
This is the extra space on the top side of the layout.
|
android:layout_marginBottom
|
This is the extra space on the bottom side of the layout.
|
android:layout_marginLeft
|
This is the extra space on the left side of the layout.
|
android:layout_marginRight
|
This is the extra space on the right side of the layout.
|
android:layout_gravity
|
This specifies how child Views are positioned.
|
android:layout_weight
|
This specifies how much of the extra space in the layout should be allocated to the View.
|
android:layout_x
|
This specifies the x-coordinate of the layout.
|
android:layout_y
|
This specifies the y-coordinate of the layout.
|
android:layout_width
|
This is the width of the layout.
|
android:layout_width
|
This is the width of the layout.
|
android:paddingLeft
|
This is the left padding filled for the layout.
|
android:paddingRight
|
This is the right padding filled for the layout.
|
android:paddingTop
|
This is the top padding filled for the layout.
|
android:paddingBottom
|
This is the bottom padding filled for the layout.
|
Here width and height are the dimension of the layout/view which can be specified in terms of dp (Density-independent Pixels), sp ( Scale-independent Pixels), pt ( Points which is 1/72 of an inch), px( Pixels), mm ( Millimeters) and finally in (inches).
You can specify width and height with exact measurements but more often, you will use one of these constants to set the width or height −
· android:layout_width=wrap_content tells your view to size itself to the dimensions required by its content.
· android:layout_width=fill_parent tells your view to become as big as its parent view.
Gravity attribute plays important role in positioning the view object and it can take one or more (separated by '|') of the following constant values.
Constant
|
Value
|
Description
|
top
|
0x30
|
Push object to the top of its container, not changing its size.
|
bottom
|
0x50
|
Push object to the bottom of its container, not changing its size.
|
left
|
0x03
|
Push object to the left of its container, not changing its size.
|
right
|
0x05
|
Push object to the right of its container, not changing its size.
|
center_vertical
|
0x10
|
Place object in the vertical center of its container, not changing its size.
|
fill_vertical
|
0x70
|
Grow the vertical size of the object if needed so it completely fills its container.
|
center_horizontal
|
0x01
|
Place object in the horizontal center of its container, not changing its size.
|
fill_horizontal
|
0x07
|
Grow the horizontal size of the object if needed so it completely fills its container.
|
center
|
0x11
|
Place the object in the center of its container in both the vertical and horizontal axis, not changing its size.
|
fill
|
0x77
|
Grow the horizontal and vertical size of the object if needed so it completely fills its container.
|
clip_vertical
|
0x80
|
Additional option that can be set to have the top and/or bottom edges of the child clipped to its container's bounds. The clip will be based on the vertical gravity: a top gravity will clip the bottom edge, a bottom gravity will clip the top edge, and neither will clip both edges.
|
clip_horizontal
|
0x08
|
Additional option that can be set to have the left and/or right edges of the child clipped to its container's bounds. The clip will be based on the horizontal gravity: a left gravity will clip the right edge, a right gravity will clip the left edge, and neither will clip both edges.
|
start
|
0x00800003
|
Push object to the beginning of its container, not changing its size.
|
end
|
0x00800005
|
Push object to the end of its container, not changing its size.
|
View Identification
A view object may have a unique ID assigned to it which will identify the View uniquely within the tree. The syntax for an ID, inside an XML tag is −
android:id="@+id/my_button"
Following is a brief description of @ and + signs −
· The at-symbol (@) at the beginning of the string indicates that the XML parser should parse and expand the rest of the ID string and identify it as an ID resource.
· The plus-symbol (+) means that this is a new resource name that must be created and added to our resources. To create an instance of the view object and capture it from the layout, use the following −
Button myButton = (Button) findViewById(R.id.my_button);
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