JAVA INTERVIEW QUESTIONS (Volume :- 1)

Q:	What is the difference between an Interface and an Abstract class?
A:	An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods. .

Q:	What is the purpose of garbage collection in Java, and when is it used?
A:	The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

Q:	Describe synchronization in respect to multithreading.
A:	With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

Q:	Explain different way of using thread?
A:	The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.

Q:	What are pass by reference and passby value?
A:	Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.

Q:	What is HashMap and Map?
A:	Map is Interface and Hashmap is class that implements that.

Q:	Difference between HashMap and HashTable?
A:	The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

Q:	Difference between Vector and ArrayList?
A:	Vector is synchronized whereas arraylist is not.

Q:	Difference between Swing and Awt?
A:	AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.

Q:	What is the difference between a constructor and a method?
A:	A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

Q:	What is an Iterator?
A:	Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

Q:	State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
A:	public : Public class is visible in other packages, field is visible everywhere (class must be public too) private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.

Q:	What is an abstract class?
A:	Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

Q:	What is static in java?
A:	Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.

Q:

What is final?

A:

A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant). Q: What if the main method is declared as private? A: The program compiles properly but at runtime it will give "Main method not public." message. Q: What if the static modifier is removed from the signature of the main method? A: Program compiles. But at runtime throws an error "NoSuchMethodError". Q: What if I write static public void instead of public static void? A: Program compiles and runs properly. Q: What if I do not provide the String array as the argument to the method? A: Program compiles but throws a runtime error "NoSuchMethodError". Q: What is the first argument of the String array in main method? A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. Q: If I do not provide any arguments on the command line, then the String array of Main method will be empty or null? A: It is empty. But not null. Q: How can one prove that the array is not null but empty using one line of code? A: Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length. Q: What environment variables do I need to set on my machine in order to be able to run Java programs? A:  PATH is the only variable. Q: Can an application have multiple classes having main method? A: Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method. Q: Can I have multiple main methods in the same class? A: No the program fails to compile. The compiler says that the main method is already defined in the class. Q: Do I need to import java.lang package any time? Why ? A: No. It is by default loaded internally by the JVM. Q: Can I import same package/class twice? Will the JVM load the package twice at runtime? A: One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class. Q: What are Checked and UnChecked Exception? A: A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be. Q: What is Overriding? A: When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private. Q: What are different types of inner classes? A: Nested top-level classes, Member classes, Local classes, Anonymous classes Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety. Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class. Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable. Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor. Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile? A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD; Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage. Q: What is the difference between declaring a variable and defining a variable? A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions. Q: What is the default value of an object reference declared as an instance variable? A: null unless we define it explicitly. Q: Can a top level class be private or protected? A: No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected. Q: What type of parameter passing does Java support? A: In Java the arguments are always passed by value . Q: Primitive data types are passed by reference or pass by value? A: Primitive data types are passed by value. Q: Objects are passed by value or by reference? A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object . Q: What is serialization? A: Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream. Q: How do I serialize an object to a file? A: The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file. Q: Which methods of Serializable interface should I implement? A: The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods. Q: How can I customize the seralization process? i.e. how can one have a control over the serialization process? A: Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process. Q: What is the common usage of serialization? A: Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed. Q: What is Externalizable interface? A: Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods. Q: When you serialize an object, what happens to the object references included in the object? A: The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized along with the original object. Q: What one should take care of while serializing the object? A: One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException. Q: What happens to the static fields of a class during serialization? A: There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are 1. Serialization ignores static fields, because they are not part of ay particular state state. 2. Base class fields are only hendled if the base class itself is serializable. 3. Transient fields.

How to Convert a .BAT file or .VBS file into .EXE to Enable Pinning to XP Start Menu

Windows XP will not allow you to pin a shortcut to the start menu that points to a network drive. For this reason it can be necessary to write a batch file to launch a program. The issue is that a shortcut pointed to a .bat file also contains no “Pin to Start Menu” option. While it can be dragged onto the start menu, this leaves no option for scripting these shortcuts. The simple solution is to convert the .bat file to an .exe file and lucky for us Windows XP includes a tool that can do it…

  
Step 1
Navigate to C:\Windows\System32 and locate the file named IEXPRESS.EXE

Step 2
Double Click to launch IEXPRESS.EXE

Step 3
You will be presented with the initial welcome screen and be given two choices. Select “Create new Self Extraction Directive file.” Click Next.

Step 4
Next you will be presented with the Package Purpose screen. For our purposes select “Extract Files and run an installation command” and click the Next button.

Step 5
You will be presented with the Package Title screen, which will give you the opportunity to give your project a name. If you are so inclined give it a meaningful name. If like me you are never going to come back to this, name it whatever you want.

Step 6
You will next be presented with the Confirmation Prompt Screen. We would like the batch file to just be extracted and run so just choose “No Prompt” and click the Next Button.

Step 7
You are presented with the License Agreement window. If you don’t want your users to have to answer a prompt select “Do not display a license.”

Step 8
The Packaged Files window is where you will select your batch file (or .vbs). Click the Add button and browse to your desired file. Then click next.

Step 9
Here you are presented with a window titled Install Program to Launch. Use the drop down control next to “Install Program and choose the only option that will be present, the .bat or .vbs file that you chose in the previous window.

Step 10
The Show Window screen is next. I didn’t want my users to be prompted in any way so I chose Hidden. Click Next.

Step 11
No Finished Message for my users. Select “No message” and choose Next.

Step 12
The Package Name and Options window is where the new .exe specified. Type in a path or browse to the folder you would like your .exe in, type a name in the file name box and click save. Also check the box that says “Hide File Extraction Progress Animation from User.” If you’re worried about long file names go ahead and click the other box as well.

Step 13
Since we really aren’t installing anything we probably want to tell the Configure Restart window to not restart. So choose the option that says “No restart” and hit Next.

Step 14
This window is where you have a chance to save all of the options you have chosen into a project file so that if necessary you may later return and make modifications. I have no need to retain the file, but if you would like to be able to come back to it, by all means choose “Save Self Extraction Directive (SED) file” and tell it where to put it. As always, click Next.

Step 15
Here’s where you’re new .exe is born, on the Create Package screen. Explore to the directory you told it to put the file in, click Next and then watch your little .exe’s first moments as it pops into the big digital world.

Step 16
You’re done! Click finish and go try it out. Pin it directly to the start menu. Point a shortcut to it and pin that to the start menu. Most importantly, script it and see how slick it is. If you’re not familiar with how to script start menu pinning the scripting guys will tell you how. Check it out here!

IP addresses and Commands to get network information

• What is an IP?
  
  An IP is an abbreviation of Internet Protocol IP serves a role of a digital computer identification number that is assigned by a network it's connected to Without an IP address networking and IP tracking would not be possible.
  
  
• Why does a computer's IP address change from time to time?
  
Every time people establish a new connection with their Internet Service Provider account either using a wired home / business connection or a wireless connection, the ISP's network assigns an individual IP number to your computer in order to provide you with the data your computer requests from web servers This is not associated with an account privacy matters but rather a technical aspect of user account digital id distribution .


• How is IPv4 different from IPv6?
  
  IPv6 IP address format is the successor to IPv4 which is the initial IP address format standard The main difference with IPv6 address is its availability It also contains additional features, such as simplified IP address assignment, network renumbering and IP announcements for the router nodes While the 32bit IPv4 address pool is virtually exhausted and allowed somewhere around 43 billion addresses to be assigned, IPv6 is 128bits and can produce 34W1038 unique IP addresses .
  
  Currently, most of the web servers are not configured to handle IPv6 connections, meaning an IPv6 client cannot connect to an IPv4 server Thus, an implementation called IPv4-6 tunneling is created mostly on an ISP level to allow an IPv6 client to connect to an IPv4 host .
  
  
• Who can see and track my IP address?
  
When you send emails, post on web forums, use instant messengers or access any online accounts - your computer's IP is logged by the network receiving communication requests This is an essential step that cannot be avoided and contributes to various technical, security and privacy aspects .


• What can be done to protect my IP's security and privacy?
  
  
It is possible to hide an IP address by using a private Proxy server This helps to conceal your private IP information to a certain extent, however it is not considered a good practice Read below .


• What is a Proxy Server?

A Proxy server is a network service which utilizes a dedicated computer on a remote network allowing internet users such as yourself to make indirect connections to other computers / websites using its own network connection When using a proxy server, the recipient of your communications will see an IP address of the proxy server and not yours There are plenty of proxy servers establishments on the world wide web that offer free online user accounts to hide your online identity This, however, is not a bullet-proof method .


• Is there a risk using a Proxy Server?
  
Definitely ,Using an anonymous proxy server computer to hide your IP actually presents a more serious threat that contributes to the digital information leak Since the digital data has to pass through a third party proxy server, any information you transmit through that server can be intercepted and searched, including messsages you sent, your account login and password information that you may utilize for any online services In attempt to search a better privacy on Internet, people search and find the tools like the proxy servers, not realizing that the danger is actually greater using the proxy servers Furthermore, in order to trace and check on people's illegal activity, some governments setup free proxy servers to monitor activity of those people who are most likely to have something to hide . Never access any of your digitally stored financial account information, including online credit card banking systems, initiate transfer of funds, pay your bills or mortgages online or register any online banking accounts that contain secure information while using a free proxy server As a matter of fact, most of the electronic identity theft cases targeted at financial sectors and being investigated worldwide are the direct result of digital information theft via utilization of so called free proxy servers that offer free online user accounts.


• If someone possesses my private IP information, what type of security risk does it present?
First of all, every single Internet user has an IP number, therefore a chance of you becoming a victim of a malicious attack is minimized by the overwhelming pool of IP addresses available to those who exercise this type of activity Second of all, to protect your privacy, the Internet Service Providers do not release any digitally stored legal or financial bank information to anyone but legal authorities Therefore your private information connected to your ISP provider generally remains very secure.
There is still a chance though that you may become a target of an internet hacker attack The IP address attack spoofs your local Internet connection by trying to forge your computer's IP via software vulnerabilities If have your computer connected directly to a DSL or Cable Modem without a router or a firewall, a hacker may be able to access the private data on your computer, especially if you use file sharing In this situation, it would not be a bad idea to get a good Firewall software along with an Antivirus software commonly used by people such as AVG or QuickHeal!, or a hardware router with a built in digital firewall There is a definite advantage of having a hardware firewall built-in into a router While a software firewall is good and gives you full control, it may be susceptible to viruses, mis-configurations and crashes, while a hardware firewall always runs in the background, allowing only outgoing traffic, giving you somewhat a limited control of your inbound traffic while providing you with the best possible protection from a dangerous inbound traffic.

Below, you'll find a list of the most common IP commands for Windows and DOS You can trace many information about your IP and network through these commands:-

Display Connection Configuration: ipconfig /all

Display DNS Cache Info: ipconfig /displaydns

Clear DNS Cache: ipconfig /flushdns

Release All IP Address Connections: ipconfig /release

Renew All IP Address Connections: ipconfig /renew

Re-Register the DNS connections: ipconfig /registerdns

Change/Modify DHCP Class ID: ipconfig /setclassid

Network Connections: control netconnections

Network Setup Wizard: netsetupcpl

Test Connectivity: ping googlecom

Trace Route: tracert

Displays the TCP/IP protocol sessions: netstat

Display Local Route: route

Display Resolved MAC Addresses: arp

Display Name of Computer Currently on: hostname

Display DHCP Class Information: ipconfig /showclassid

NameServer Lookup: nslookup googlecom

I want to be a Java developer, What skills do I need ???

"I want to be a java developer, What are the all skills that I need to have. I have learned the core java in college but what now?"

Does this sound a familiar question! You too may have a similar question in your mind. Don't worry, This article explains what are all the skills that you will need to be a Java web developer, and what are the skills that you are expected to have when you appear for an interview for a Java/J2EE web developer.

 

Java developer ! But what kind of ? 

Let me clarify, this article is about the Java web development. As most of the time, when beginner say, 'I want to be a java developer', he actually means he wants to be a java web developer and wants to get involved in developing web applications. Otherwise, Java development in general is a vast field, today Java is used in many types of platforms, that includes Personal computers, Main frames, Hand held portable devices, Mobiles, Smart cards, Games and many more thing. But the biggest stream of java development that creates more jobs than any other stream is the JavaEE platform. The JavaEE platform is used to create the enterprise applications (It includes web applications as well).

So be clear in your mind, This article is about java web development or enterprise application development. Most of the time when you read a job classified for skills, “Spring, Spring MVC, Struts, Hibernate, EJB” they are for java web development.

So, now it’s clear that you want to be a Java web developer and looking for a jump start. So here are the skills that will get you going.

Step 1 : First thing first, Know the basics, Learn core Java

The first thing you must learn is Core Java. Having a solid foundation in core java will help you when you jump into the advanced java. No matter what Java technologies you plan to work on, you will need to use the core java every day. So it is absolutely must to have a good understanding of the core concepts. Learn the basic programming concepts like variables, branching and looping, logical conditions, data types etc.

Good understanding of core java library is a must

Learn core java library, especially collection framework, Threads, java.lang, java.io packages. If you are targeting the web development, you may not want to waste time on the swing.
Get yourself a good core java book that covers the latest java edition JDK 1.6.

Step 2 : Learn to use an IDE (integrated development environment)

While working as a professional, you won’t use the plain text editors to write the code, you will have to know how to use IDE. IDE A.K.A code editor will greatly improve your productivity, and make your life easier. Code editors are used to write, compile, run and debug code. They provide features like syntax highlighting, auto completion, can identify the problems in code even before compiling. These are some of the most common features, but there are many more than just these. I will recommend you to learn eclipse which is one of the most popular IDE amongst java developer, and its free.

Step 3 : your first step toward web development, HTML, CSS and JavaScript

I will not explain what HTML or CSS is, there are plenty of resources available on the internet. But understand, this is the most important parts of any web application. Second most important thing to learn is JavaScript, let me clarify, the JavaScript is not java, the similarity in name is for historical reasons. Get yourself into the world of web applications with this three skills, learn some tutorials on the internet or get some good book.

Step 4 : The java web components, Servlet API and JSP

Almost every web application running on internet or intranet which is developed in Java has their roots in Servlet API. So next thing to learn is servlets. Servlet API is the only standard API for java web application development. JSP is also based on the servlet API. Almost all the web frameworks, like Spring MVC, Struts, Wicket uses the servlet API under the hood. Get yourself a good book like “head first servlet”. Having the solid understanding of the Servlet API is quite important. Understand the core concepts like, life cycle, request, response, filters, session, config, context etc.

Step 5 : Learn a web container

Web container is a run time environment or server that is used to deploy and run the web applications or more specifically Web components (Eg servlets or JSP)
Web containers does not know how to run struts or Spring MVC, they just understand the servlet API, but still you can run them on a web container, because ultimately they are based on the Servlet API. You can learn the tomcat, that would be sufficient initially.

Step 6 : Learn JDBC

Almost every web application will need to connect to a database for storing and retrieving persistent data. JDBC is the standard API for database connectivity. So it is quite important that you understand the JDBC well.

Step 7 : Learn a web application framework and an ORM framework

A web application framework is actually a set of libraries that makes developing web applications easier. Struts and Spring MVC are good choices. If you are just starting, I would recommend going with Spring MVC.

ORM stands for object to relational mapping, they are based on JDBC and used for storing and querying the database. But unlike JDBC, ORM frameworks operates on objects and provides ways for storing and retrieving objects. (But be clear, they are ultimately using JDBC under the hood). ORM frameworks reduces lots of boilerplate code, improves productivity and makes developers life easier. Hibernate is the best choice. If you are just starting, read and understand JPA (Java persistence API) which is the standard java ORM API. You can use hibernate for running your JPA code.

Step 8 : What next!

Once you are familiar with the things mentioned above, The next step would be to learn the JavaEE (Java enterprise edition). Learn EJB (Enterprise java beans), JSF (Java server faces) and other APIs that are part of the JavaEE. Get a good book that covers the latest JavaEE edition 1.6

Resources

• Official JDK download
• Installing JDK
• Servlet 3.0 Specification
• Apache Tomcat official site
• Eclipse IDE official site
• Spring framework documentation
• JavaRanch forums