Friday, December 16, 2011

Difference Between Interface and Abstract Class


Interface: Java Interfaces are equivalent to protocols. They basically represent an agreed-upon behavior to facilitate interaction between unrelated objects. For example, the buttons on a Remote Controller form the interface for outside world to interact with TV. How this interface is implemented by different vendors is not specified and you’re hardly aware of or bothered about how these buttons have been implemented to work internally. Interface is plainly a contract between the producer and the consumer. How the producer implements the exposed behavior is normally not cared by the consumer.

In Java, an Interface is normally a group of methods with empty bodies. You can have constant declarations in a Java Interface as well. A class that implements the interface agrees to the exposed behavior by implementing all the methods of the interface.

interface TVRemoteController{
void power();
void setChannel(int channelNumber);
void upChannel();
void downChannel();
void upVolume();
void downVolume();
……
}

A sample implementation of this interface by a vendor, say Sony:

public class SonyTVRemoteController implements TVRemoteController{
/*…this class can have other methods, properties as well …*/
……
void power(){
//implementation of power() method of the interface
}
void setChannel(int channelNumber){
//implementation of setChannel(int) method of the interface
}
//similarly, implementation of other methods of the interface
……
}

Implementing an interface means the class will support at least the exposed behavior. It can definitely add any number of extra behaviors/properties for its clients. That’s why few Remote Controllers have hell lot of buttons :-)

Abstract Class: In Java, abstract class is a class which has been declared ‘abstract’. By declaring ‘abstract’ we ensure that the class can’t be instantiated. Why to have such a class then? Because, you would not be having implementation of all the methods in that class and you need to leave it to the subclass to decide how to implement them. In this case, there is no point instantiating an incomplete class.

An abstract method is a method which doesn’t have any implementation. If a class has even a single abstract method, then you got to declare the class ‘abstract’. Though, you don’t need to have at least one abstract method to declare a class abstract. You can declare a complete class as ‘
abstract’ as well. This practice is seldom used. One possible reason may be that you never want your clients to instantiate your class directly even though you’ve already provided default implementation of all the methods. Strange! Yeah… it is. The designer of such a class may like to provide the default implementation of at least one method just to serve as a template (and not the actual implementation) for the client and thus making the class incomplete. So, a client first needs to subclass and implement the method(s) by overriding them. Now the subclass will be a concrete/complete class. Does it make some sense? Okay… Let me try to give another example. Think of a hypothetical situation, where you need to design a class, which will have ‘n’ methods and ‘n’ clients, where every single client wants default implementation of ‘n-1’ methods and it needs to implement only one (unique to every client) of the methods. In such a situation, you may not like to declare any of the methods ‘abstract’ as it’ll be required to be a non-complete method only for one of the clients and a complete implementation for other ‘n-1’ clients. If you declare it ‘abstract’ then every client will need to implement it and you’ll end up getting ‘n-1’ same piece of code. On the other hand, if you don’t declare ‘abstract’ then you simply need to override this method in corresponding sub class. Since, the base class is incomplete in all the ‘n’ cases. Assuming that this class will have only these many forms of usage, you’ll never require having an instance of it. That’s why you would declare it ‘abstract’. Confused? Read this paragraph once more :-)

public abstract class SampleAbstractClass{
//…fields
……
//…non-abstract methods, if any
……
//…abstract method, if any J
abstract void sampleAbstractMethod(); //… ends with ‘;’
}

public class SubClassOfSampleAbstractClass extends SampleAbstractClass{
//… fields, and non-abstract methods (if any)
……
//…implementation of the abstract method
void sampleAbstractMethod(){
……
}
}


Feature
Interface
Abstract class
Multiple inheritance
A class may inherit several interfaces.
A class may inherit only one abstract class.
Default implementation
An interface cannot provide any code, just the signature.
An abstract class can provide complete, default code and/or just the details that have to be overridden.
Access Modfiers
An interface cannot have access modifiers for the subs, functions, properties etc everything is assumed as public
An abstract class can contain access modifiers for the subs, functions, properties
Core VS Peripheral
Interfaces are used to define the peripheral abilities of a class. In other words both Human and Vehicle can inherit from a IMovableinterface.
An abstract class defines the core identity of aclass and there it is used for objects of the same type.
Homogeneity
If various implementations only share method signatures then it is better to use Interfaces.
If various implementations are of the same kind and use common behaviour or status then abstractclass is better to use.
Speed
Requires more time to find the actual method in the corresponding classes.
Fast
Adding functionality (Versioning)
If we add a new method to an Interface then we have to track down all the implementations of theinterface and define implementation for the new method.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly.
Fields and Constants
No fields can be defined ininterfaces
An abstract class can have fields and constrants defined

When to use an Interface: it asks you to start everything from scratch. You need to provide implementation of all the methods. So, you should use it to define the contract, which you’re unsure of how the different vendors/producers will implement. So, you can say that Interfaces can be used to enforce certain standards.

When to use an Abstract Class: it is used mostly when you’ve partial implementation ready with you, but not the complete. So, you may declare the incomplete methods as ‘abstract’ and leave it to the clients to implement it the way they actually want. Not all the details can be concrete at the base class level or different clients may like to implement the method differently.

When to use both: if you want to implement multiple inheritance where you have the luxury of providing partial implementation as well. You’ll then put all that code in an abstract class (this can be a concrete class as well… but here we assume that the class is also only partially implemented and hence an abstract class), extend that class, and implement as may interfaces as you want.

Thursday, December 15, 2011

DVM vs JVM


1.    Dalvik is the process virtual machine (VM) in Google's Android operating system. It is the software that runs the apps on Android devices.
2.    Dalvik is thus an integral part of Android, which is typically used on mobile devices such as mobile phones and tablet computers. Programs are commonly written in a dialect of Java and compiled to bytecode. Then they are converted from Java Virtual Machine-compatible .class files to Dalvik-compatible .dex (Dalvik Executable) files before installation on a device.
3.    The compact Dalvik Executable format is designed to be suitable for systems that are constrained in terms of memory and processor speed.
4.    Dalvik is open-source software. It was originally written by Dan Bornstein, who named it after the fishing village of Dalvík in EyjafjörðurIceland, where some of his ancestors lived.
5.    A tool called dx is used to convert some (but not all) Java .class files into the .dex format. Multiple classes are included in a single .dex file.
6.      DVM is Register based which is designed to run on low memory, uses its own byte code and runs .Dex file .
7.    From Android 2.2 SDK Dalvik has got a Just in Time compiler
8.    JVM is Stack based which uses java byte code and runs .class file having JIT.
JVM vs DVM

DVM
          JVM
Architecture
Register
Stack
OS Support
Android
Multiple
Re- Tools
few
many
Executables
APK
JAR
Constant-Pool
Per Application
Per class
Memory
Less
More


Dalvik is said to be a clean-room implementation rather than a development on top of a standard Java runtime, which would mean it does not inherit copyright-based license restrictions from either the standard-edition or open-source-edition Java runtimes. Dalvik is published under the Apache 2 license


Android Interview Questions


What is Android?

Android is a stack of software for mobile devices which includes an Operating System, middleware and some key applications. The application executes within its own process and its own instance of Dalvik Virtual Machine. Many Virtual Machines run efficiently by a DVM device. DVM executes Java languages byte code which later transforms into .dex format files.

What is .apk extension?
The extension for an Android package file, which typically contains all of the files related to a single Android application. The file itself is a compressed collection of an AndroidManifest.xml file, application code (.dex files), resource files, and other files. A project is compiled into a single .apk file.

What is .dex extension?
Android programs are compiled into .dex (Dalvik Executable) files, which are in turn zipped into a single .apk file on the device. .dex files can be created by automatically translating compiled applications written in the Java programming language.

What is an activity? 
A. A single screen in an application, with supporting Java code. An activity presents a visual user interface for one focused endeavour the user can undertake. For example, an activity might present a list of menu items users can choose from or it might display photographs along with their captions. 

What is an Action?
A description of something that an Intent sender wants done. An action is a string value assigned to an Intent. Action strings can be defined by Android or by a third-party developer. For example, android.intent.action.VIEW for a Web URL, or com.example.rumbler.SHAKE_PHONE for a custom application to vibrate the phone.

What are the advantages of Android?
1.    The customer will be benefited from wide range of mobile applications to choose, since the monopoly of wireless carriers like AT&T and Orange will be broken by Google Android.
2.    Features like weather details, live RSS feeds, opening screen, icon on the opening screen can be customized
3.     Innovative products like the location-aware services, location of a nearby convenience store etc., are some of the additive facilities in Android.
 
What is ADB?
Android Debug Bridge, a command-line debugging application shipped with the SDK. It provides tools to browse the device, copy tools on the device, and forward ports for debugging.

What is Dalvik?
The name of Android’s virtual machine. The Dalvik VM is an interpreter-only virtual machine that executes files in the Dalvik Executable (.dex) format, a format that is optimized for efficient storage and memory-mappable execution. The virtual machine is register-based, and it can run classes compiled by a Java language compiler that have been transformed into its native format using the included “dx” tool. The VM runs on top of Posix-compliant operating systems, which it relies on for underlying functionality (such as threading and low level memory management). The Dalvik core class library is intended to provide a familiar development base for those used to programming with Java Standard Edition, but it is geared specifically to the needs of a small mobile device.

Explain about DDMS?
Dalvik Debug Monitor Service, a GUI debugging application shipped with the SDK. It provides screen capture, log dump, and process examination capabilities. See Using the Dalvik Debug Monitor Server to learn more about this program.

What is Intents?
A class (Intent) that contains several fields describing what a caller would like to do. The caller sends this intent to Android’s intent resolver, which looks through the intent filters of all applications to find the activity most suited to handle this intent. Intent fields include the desired action, a category, a data string, the MIME type of the data, a handling class, and other restrictions.

What is role of Intent Filter?
Activities and intent receivers include one or more filters in their manifest to describe what kinds of intents or messages they can handle or want to receive. An intent filter lists a set of requirements, such as data type, action requested, and URI format, that the Intent or message must fulfill. For Activities, Android searches for the Activity with the most closely matching valid match between the Intent and the activity filter. For messages, Android will forward a message to all receivers with matching intent filters.

How is nine-patch image different from a regular bitmap?
It is a resizable bitmap resource that can be used for backgrounds or other images on the device. The NinePatch class permits drawing a bitmap in nine sections. The four corners are unscaled; the four edges are scaled in one axis, and the middle is scaled in both axes.

What languages does Android support for application development?
Android applications are written using the Java programming language.

 Describe Briefly the Android Application Architecture
Android Application Architecture has the following components:
Services like Network Operation
Intent
 - To perform inter-communication between activities or services
Resource Externalization - such as strings and graphics
Notification signaling users
 - light, sound, icon, notification, dialog etc.
Content Providers - They share data between applications

What dialog boxes are supported in android?
Android supports 4 dialog boxes:

AlertDialog :  An alert dialog box supports 0 to 3 buttons and a list of selectable elements, including check boxes and radio buttons. Among the other dialog boxes, the most suggested dialog box is the alert dialog box.
ProgressDialog :  This dialog box displays a progress wheel or a progress bar. It is an extension of AlertDialog and supports adding buttons.
DatePickerDialog :  This dialog box is used for selecting a date by the user.
TimePickerDialog :  This dialog box is used for selecting time by the user.

How to Translate in Android?
The Google translator translates the data of one language into another language by using XMPP to transmit data. You can type the message in English and select the language which is understood by the citizens of the country in order to reach the message to the citizens.

Thursday, December 1, 2011

Difference between Android Service,Thread,IntentService and AsyncTask



Service
Thread
IntentService
AsyncTask
When to use ?
Task with no UI, but shouldn't be too long. Use threads within service for long tasks.
- Long task in general.

- For tasks in parallel use Multiple threads (traditional mechanisms)
- Long task usually with no communication to main thread.
(Update)- If communication is required, can use main thread handler or broadcast intents

- When callbacks are needed (Intent triggered tasks). 
- Small task having to communicate with main thread.

- For tasks in parallel use multiple instances OR Executor
 
Trigger
Call to method
onStartService()
Thread start() method
Intent
Call to method execute()
Triggered From (thread)
Any thread
Any Thread
Main Thread (Intent is received on main thread and then worker thread is spawed)
Main Thread
Runs On (thread)
Main Thread
Its own thread
Separate worker thread
Worker thread. However, Main thread methods may be invoked in between to publish progress.
Limitations /
Drawbacks
May block main thread
- Manual thread management

- Code may become difficult to read
- Cannot run tasks in parallel.

- Multiple intents are queued on the same worker thread.
- one instance can only be executed once (hence cannot run in a loop) 

- Must be created and executed from the Main thread