Activity and It's Life Cycle

Activity : An activity represents visible component of android. It is used by  an application developer to interact with end user.

          Android framework provides a class by the name android.app.Activity this class provides us life cycle method of activity these methods are override by application developer to perform there tasks. An application developer can use this class by defining sub class of activities.

The life cycle of activity can be divided in three state:
1. Active State
2. Visible State
3. Inactive State/Invisible State

Active State : An active state of an activity represents activity is visible as well as it has user focus. The life cycle of this state is managed by onResume() and onPause() life cycle method of activity.

Visible State : Visible sate of activity represents that activity is visible it don't have user focus life cycle of this state is managed by onStart() and onStop() life cycle method of activity.

Inactive State/Invisible State : The invisible state of activity represents that niether activity is visible nor it have user focus. Life cycle of this state is managed by onCreate() and onDestroy() life cycle method of activity.

Activity Life cycle

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Android Framework Modules

We can divide android framework in five basic modules which are as follows:

1. Activity
2. Intent
3. Broadcast Receiver
4. Services
5. Content Provider

Activity : An activity represents visible component of android which is used to interact with end users. Activities are light frames in awt, swing, of forms in case of web applications.

Intent : An intent is used to provide communication among multiple components of android it works as a communicator.

Broadcast Receiver : A broadcast receiver is used to perform some short time consuming on the occurrence of some particular event.

Services : Services is used to perform long time consuming background performs it doesn't require any user interaction.

Content Provider : It provides facility to share a database among multiple application of android system.

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Android Virtual Machine

Virtual Machine : It is software implementation of any machine. It doesn't exist physically. It is used to execute a program or application.

The most important property of virtual machine is that any application or program running inside virtual machine is restricted to its resource and this abstraction is provided by virtual machine.

Virtual Machine is divided in two parts :
1.  System Virtual Machine 
2.  Process Virtual Machine / Application Virtual Machine

System Virtual Machine represents functionality of operating system. It is used to execute the program or application.

Process Virtual Machine is divided in two parts :
1.  Stack Based Virtual Machine  (JVM)
2.  Register Based Virtual Machine  (DVM)                                 

Stack Based Virtual Machine : It is faster but it consumes large amount of memory.

e.g. : JVM

Register Based Virtual Machine : It is slower but it consumes less amount of memory.

eg. : DVM


Note: Register based virtual machine gives 33.3% better performance then stack based virtual machine in case of mobile application.

Delvik Virtual Machine : It is a register based virtual machine developed by "Den Bronstien" it doesn't execute java byte code rather it executes its own byte code format which is know as delvik code.

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Android Definition

Android : Android is an open source platform for mobile application provided
by google which consist of following :

1. Mobile OS
2. Application Devlopment Platform
3. Android Runtime Environment
4. Net of some dummy applications which are being developed in android

Android is a stack of software provided by google which facilitate to develop,
to deploy and to execute the mobile application which is being developed in android.

Native Application

3rd Party Application

(Application Development Framework)
Set of predefined classes and interfaces which are used
by developer to change the productivity of application
development.

Set of native Libraries which are
written in C/C++ such as
Bluetooth lib. , WIFI lib. , Camera
lib. etc. .

Delvik

Virual

Machine

                                            (Linux Kernels)

Devices Drivers

Memory Management

Process Management

Power Management

                                      Stack of Software Warms 

                                          OR 

                          Architecture of Android 

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Smart Phones and Normal Phones

Nokia _________ Symbian(Normal Phone)
|__________ windows Phone(Smart Phone)
|__________ Android(Smart Phone)

Samsung _______ Wada(Normal Phone)
|__________ Android(Smart Phone)

Black Berry ___ BBX(Smart Phone)

Iphone ________ IOS(Smart Phone)


History of Smart Phones

Smart Phones
|___________ Black Berry --> Product of RIM(Research In Motion)
| Launced in 1999
| It is father of Smart Phone
|
|___________ Iphone --> Launched by Apple Inc. in 2007
|
|___________ Android --> First tried to develop by Android Inc.
Founders of Android --> Andy Robin, Rich Minor, Nick Sears and Chris White
but was getting problem in development then Google dealed with them in
2005 in android development (Google + Android Inc.)
But unable to do so, Google contacted to OHA(Open Handset Alliance) in 2007
Then finally launched the android smart phones in 2008.

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What is JDK, JRE and JVM?

JDK (Java Development Kit)

Java Developer Kit contains tools needed to develop the Java programs, and JRE to run the programs. The tools include compiler (javac.exe), Java application launcher (java.exe), Appletviewer, etc.. Compiler converts java code into byte code. Java application launcher opens a JRE, loads the class, and invokes its main method. You need JDK, if at all you want to write your own programs, and to compile them. For running java programs, JRE is sufficient. JRE is targeted for execution of Java files i.e. JRE = JVM + Java Packages Classes(like util, math, lang, awt,swing etc)+runtime libraries. JDK is mainly targeted for java development. I.e. You can create a Java file (with the help of Java packages), compile a Java file and run a java file.

JRE (Java Runtime Environment)

Java Runtime Environment contains JVM, class libraries, and other supporting files. It does not contain any development tools such as compiler, debugger, etc. Actually JVM runs the program, and it uses the class libraries, and other supporting files provided in JRE. If you want to run any java program, you need to have JRE installed in the system. The Java Virtual Machine provides a platform-independent way of executing code; That mean compile once in any machine and run it any where(any machine).

JVM (Java Virtual Machine)

As we all aware when we compile a Java file, output is not an ‘exe’ but it’s a ‘.class’ file. ‘.class’ file consists of Java byte codes which are understandable by JVM. Java Virtual Machine interprets the byte code into the machine code depending upon the underlying operating system and hardware combination. It is responsible for all the things like garbage collection, array bounds checking, etc… JVM is platform dependent.

The JVM is called “virtual” because it provides a machine interface that does not depend on the underlying operating system and machine hardware architecture. This independence from hardware and operating system is a cornerstone of the write-once run-anywhere value of Java programs.

There are different JVM implementations are there. These may differ in things like performance, reliability, speed, etc. These implementations will differ in those areas where Java specification doesn’t mention how to implement the features, like how the garbage collection process works is JVM dependent, Java spec doesn’t define any specific way to do this.

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