JAVA Interview Questions with Answer
1. What is a virtual function in C++?
Simply put, the virtual keyword enables a function to be ‘virtual’ which
then gives possibility for that function to be overridden (redefined)
in one or more descendant classes. It is a good feature since the
specific function to call is determined at run-time. In other words, a
virtual function allows derived classes to replace the implementation
provided by the base class.
2. What is the difference between private, protected, and public?
These keywords are for allowing privilages to components such as functions and variables.
Public: accessible to all classes
Private: accessible only to the class to which they belong
Protected: accessible to the class to which they belong and any subclasses.
3. What is a cartesian product in PL/SQL?
When a Join condition is not specified by the programmer or is invalid(fails), PL/SQL forms a Cartesian product.
In a Cartesian product, all combinations of rows will be displayed.
For example, All rows in the first table are joined to all rows in the
second table. It joins a bunch of rows and it’s result is rarely useful
unless you have a need to combine all rows from all tables.
4. What is mutual exclusion? How can you take care of mutual exclusion using Java threads?
Mutual exclusion is where no two processes can access critical regions of memory at the same time.
Java provides many utilities to deal with mutual exclusion with the use of threaded programming.
For mutual exclusion, you can simply use the synchronized keyword and
explicitly or implicitly provide an Object, any Object, to synchronize
on.
The runtime system/Java compiler takes care of the gruesome details for
you. The synchronized keyword can be applied to a class, to a method, or
to a block of code. There are several methods in Java used for
communicating mutually exclusive threads such as wait( ), notify( ), or
notifyAll( ). For example, the notifyAll( ) method wakes up all threads
that are in the wait list of an object.
5. What are some advantages and disadvantages of Java Sockets?
Some advantages of Java Sockets:
- Sockets are flexible and sufficient. Efficient socket based programming can be easily implemented for general communications.
- Sockets cause low network traffic. Unlike HTML forms and CGI scripts
that generate and transfer whole web pages for each new request, Java
applets can send only necessary updated information.
Some disadvantages of Java Sockets:
- Security restrictions are sometimes overbearing because a Java
applet running in a Web browser is only able to establish connections to
the machine where it came from, and to nowhere else on the network
- Despite all of the useful and helpful Java features, Socket based
communications allows only to send packets of raw data between
applications. Both the client-side and server-side have to provide
mechanisms to make the data useful in any way.
- Since the data formats and protocols remain application specific, the re-use of socket based implementations is limited.
1. What is the difference between an Interface and an Abstract class?
An Abstract class declares have at least one instance method that is
declared abstract which will be implemented by the subclasses. 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.
2. What is the purpose of garbage collection in Java, and when is it used?
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.
3. Describe synchronization in respect to multithreading.?
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.
4. Explain different way of using thread?
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.
5. What are pass by reference and passby value?
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.
6. What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.
7. Difference between HashMap and HashTable?
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 non
synchronized and Hashtable is synchronized.
8. Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.
9. Difference between Swing and Awt?
AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.
10. What is the difference between a constructor and a method?
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.
11. What is an Iterators?
Some of the collection classes provide traversal of their contents via a
java.util.Iterator interface. This interface allows you to walk 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.
12. 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.?
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.
13. What is an abstract class?
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.
14. What is static in java?
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.
1. What is final?
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).
2. Are the imports checked for validity at compile
time? e.g. will the code containing an import such as java.lang.ABCD
compile?
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;
3. Does importing a package imports the subpackages as well?
e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?
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.
4. What is the difference between declaring a variable and defining a variable?
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.
5. What is the default value of an object reference declared as an instance variable?
null unless we define it explicitly.
6. Can a level class be private or protected?
No. A 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 level class is declared as private the
compiler will complain that the “modifier private is not allowed here”.
This means that a level class can not be private. Same is the case with
protected.
7. What type of parameter passing does Java support?
In Java the arguments are always passed by value .
8. Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.
9. Objects are passed by value or by reference?
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 .
10. What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
11. How do I serialize an object to a file?
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.
12. Which methods of Serializable interface should I implement?
The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.
13. How can I customize the seralization process?
i.e. how can one have a control over the serialization process?
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.
14. What is an abstract class?
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.
1. What is the common usage of serialization?
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.
2. What is Externalizable interface?
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.
3. What happens to the object references included in the object?
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 alongwith the
original obect.
4. What one should take care of while serializing the object?
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.
5. What if the main method is declared as private?
The program compiles properly but at runtime it will give “Main method not public.” message.
6. What if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error “NoSuchMethodError”.
7. What if I write static public void instead of public static void?
Program compiles and runs properly.
8. What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error “NoSuchMethodError”.
9. What is the first argument of the String array in main method?
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.
10. If I do not provide any arguments on the command line, then the String array of Main method will be empty of null?
It is empty. But not null.
11. How can one prove that the array is not null but empty?
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.
12. What environment variables do I need to set on my machine in order to be able to run Java programs?
CLASSPATH and PATH are the two variables.
13. Can an application have multiple classes having main method?
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.
14. Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is already defined in the class.
15. Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
16. Can I import same package/class twice?
Will the JVM load the package twice at runtime?
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.
17. What are Checked and UnChecked Exception?
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
checked 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.
18. What is Overriding?
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.
19. What are different types of inner classes?
They are Nested -level classes, Member classes, Local classes, Anonymous classes
Nested -level classes- If you declare a class within
a class and specify the static modifier, the compiler treats the class
just like any other -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. -level inner classes implicitly have
access only to static variables.There can also be inner interfaces. All
of these are of the nested -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 -level class. The primary
difference between member classes and nested -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.
1. Are the imports checked for validity at compile time? e.g.
will the code containing an import such as java.lang.ABCD compile? 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;
2. Does importing a package imports the subpackages as well?
e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? 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.
3. What is the difference between declaring a variable and defining a variable?
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.
4. What is the default value of an object reference declared as an instance variable? null unless we define it explicitly.
5. Can a level class be private or protected?
No. A 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 level class is declared as private the
compiler will complain that the “modifier private is not allowed here”.
This means that a level class can not be private. Same is the case with
protected.
6. What type of parameter passing does Java support? In Java the arguments are always passed by value .
7. Primitive data types are passed by reference or pass by value? Primitive data types are passed by value.
8. Objects are passed by value or by reference?
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 .
9. What is serialization? Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
10. How do I serialize an object to a file? 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.
11. Which methods of Serializable interface should I implement? The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.
12. How can I customize the seralization process? i.e.
how can one have a control over the serialization process? 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.
13. What is the common usage of serialization?
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.
14. What is Externalizable interface?
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.
15. What happens to the object references included in the object?
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
alongwith the original obect.
16. What one should take care of while serializing the object?
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.
17. What happens to the static fields of a class during serialization?
Are these fields serialized as a part of each serialized object? Yes
the static fields do get serialized. If the static field is an object
then it must have implemented Serializable interface. The static fields
are serialized as a part of every object. But the commonness of the
static fields across all the instances is maintained even after
serialization.
18. How are Observer and Observable used? Objects
that subclass the Observable class maintain a list of observers. When
an Observable object is updated it invokes the update() method of each
of its observers to notify the observers that it has changed state. The
Observer interface is implemented by objects that observe Observable
objects. [Received from Venkateswara Manam]
19. What is synchronization and why is it important?
With respect to multithreading, synchronization is the capability to
control the access of multiple threads to shared resources. Without
synchronization, it is possible for one thread to modify a shared object
while another thread is in the process of using or updating that
object’s value. This often leads to significant errors.
20. How does Java handle integer overflows and underflows? It uses those low order bytes of the result that can fit into the size of the type allowed by the operation.
21. Does garbage collection guarantee that a program will not run out of memory? Garbage
collection does not guarantee that a program will not run out of
memory. It is possible for programs to use up memory resources faster
than they are garbage collected. It is also possible for programs to
create objects that are not subject to garbage collection .
22. What is the difference between preemptive scheduling and time slicing?
Under preemptive scheduling, the highest priority task executes until
it enters the waiting or dead states or a higher priority task comes
into existence. Under time slicing, a task executes for a predefined
slice of time and then reenters the pool of ready tasks. The scheduler
then determines which task should execute next, based on priority and
other factors.
23. When a thread is created and started, what is its initial state? A thread is in the ready state after it has been created and started.
24. What is the purpose of finalization? The
purpose of finalization is to give an unreachable object the opportunity
to perform any cleanup processing before the object is garbage
collected.
25. What is the Locale class? The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
26. What is the difference between a while statement and a do statement?
A while statement checks at the beginning of a loop to see whether the
next loop iteration should occur. A do statement checks at the end of a
loop to see whether the next iteration of a loop should occur. The do
statement will always execute the body of a loop at least once.
27. What is the difference between static and non-static variables?
A static variable is associated with the class as a whole rather than
with specific instances of a class. Non-static variables take on unique
values with each object instance.
28. How are this() and super() used with constructors? this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.
1. Explain the life cycle methods of a Servlet.
The javax.servlet.Servlet interface defines the three methods known as
life-cycle method. public void init(ServletConfig config) throws
ServletException public void service( ServletRequest req,
ServletResponse res) throws ServletException, IOException public void
destroy() First the servlet is constructed, then initialized wih the
init() method. Any request from client are handled initially by the
service() method before delegating to the doXxx() methods in the case of
HttpServlet. The servlet is removed from service, destroyed with the
destroy() methid, then garbaged collected and finalized.
2. What is the difference between the
getRequestDispatcher(String path) method of javax.servlet.ServletRequest
interface and javax.servlet.ServletContext interface?
The getRequestDispatcher(String path) method of
javax.servlet.ServletRequest interface accepts parameter the path to the
resource to be included or forwarded to, which can be relative to the
request of the calling servlet. If the path begins with a “/” it is
interpreted as relative to the current context root.
The getRequestDispatcher(String path) method of
javax.servlet.ServletContext interface cannot accepts relative paths.
All path must sart with a “/” and are interpreted as relative to curent
context root.
3. Explain the directory structure of a web application.
The directory structure of a web application consists of two parts. A
private directory called WEB-INF A public resource directory which
contains public resource folder. WEB-INF folder consists of 1. web.xml
2. classes directory
3. lib directory
4. What are the common mechanisms used for session tracking?
Cookies SSL sessions URL- rewriting
5. Explain ServletContext.
ServletContext interface is a window for a servlet to view it’s
environment. A servlet can use this interface to get information such as
initialization parameters for the web applicationor servlet container’s
version. Every web application has one and only one ServletContext and
is accessible to all active resource of that application.
6. What is preinitialization of a servlet?
A container doesnot initialize the servlets ass soon as it starts up,
it initializes a servlet when it receives a request for that servlet
first time. This is called lazy loading. The servlet specification
defines the element, which can be specified in the deployment descriptor
to make the servlet container load and initialize the servlet as soon
as it starts up. The process of loading a servlet before any request
comes in is called preloading or preinitializing a servlet.
7. What is the difference between Difference between doGet() and doPost()?
doPost() method call doesn’t need a long text tail after a servlet
name in a request. All parameters are stored in a request itself, not in
a request string, and it’s impossible to guess the data transmitted to a
servlet only looking at a request string.
8. What is the difference between HttpServlet and GenericServlet?
A GenericServlet has a service() method aimed to handle requests.
HttpServlet extends GenericServlet and adds support for doGet(),
doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(),
doDelete(), doTrace() methods (HTTP 1.1). Both these classes are
abstract.
1. What is a output comment?
A comment that is sent to the client in the viewable page source.The JSP
engine handles an output comment as uninterpreted HTML text, returning
the comment in the HTML output sent to the client. You can see the
comment by viewing the page source from your Web browser. JSP Syntax
Example 1 Displays in the page source:
2. What is a Hidden Comment?
A comments that documents the JSP page but is not sent to the client.
The JSP engine ignores a hidden comment, and does not process any code
within hidden comment tags. A hidden comment is not sent to the client,
either in the displayed JSP page or the HTML page source. The hidden
comment is useful when you want to hide or “comment out” part of your
JSP page. You can use any characters in the body of the comment except
the closing –%> combination. If you need to use –%> in your
comment, you can escape it by typing –%\>. JSP Syntax Examples
3. What is a _Expression?
An _expression tag contains a scripting language _expression that is
evaluated, converted to a String, and inserted where the _expression
appears in the JSP file. Because the value of an _expression is
converted to a String, you can use an _expression within text in a JSP
file. Like You cannot use a semicolon to end an _expression
4. What is a Declaration?
A declaration declares one or more variables or methods for use later in
the JSP source file. A declaration must contain at least one complete
declarative statement. You can declare any number of variables or
methods within one declaration tag, as long as they are separated by
semicolons. The declaration must be valid in the scripting language used
in the JSP file.
5. What is a Scriptlet?
A scriptlet can contain any number of language statements, variable or
method declarations, or expressions that are valid in the page scripting
language.Within scriptlet tags, you can
1.Declare variables or methods to use later in the file (see also Declaration).
2.Write expressions valid in the page scripting language (see also _Expression).
3.Use any of the JSP implicit objects or any object declared with a tag.
You must write plain text, HTML-encoded text, or other JSP tags
outside the scriptlet. Scriptlets are executed at request time, when the
JSP engine processes the client request. If the scriptlet produces
output, the output is stored in the out object, from which you can
display it.
6. What are implicit objects? List them?
Certain objects that are available for the use in JSP documents without
being declared first. These objects are parsed by the JSP engine and
inserted into the generated servlet. The implicit objects re listed
below request response pageContext session application out config page
exception
7. Difference between forward and sendRedirect?
When you invoke a forward request, the request is sent to another
resource on the server, without the client being informed that a
different resource is going to process the request. This process occurs
completly with in the web container. When a sendRedirtect method is
invoked, it causes the web container to return to the browser indicating
that a new URL should be requested. Because the browser issues a
completly new request any object that are stored as request attributes
before the redirect occurs will be lost. This extra round trip a
redirect is slower than forward.
8. What are the different scope valiues for the ?
The different scope values for are
1. page
2. request
3.session
4.application
9. Explain the life-cycle mehtods in JSP?
THe generated servlet class for a JSP page implements the HttpJspPage
interface of the javax.servlet.jsp package. Hte HttpJspPage interface
extends the JspPage interface which inturn extends the Servlet interface
of the javax.servlet package. the generated servlet class thus
implements all the methods of the these three interfaces.
The JspPage interface declares only two mehtods – jspInit() and
jspDestroy() that must be implemented by all JSP pages regardless of the
client-server protocol.
However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests.
This interface declares one method _jspService(). The jspInit()- The
container calls the jspInit() to initialize te servlet instance.It is
called before any other method, and is called only once for a servlet
instance.
The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects.
The jspDestroy()- The container calls this when it decides take the
instance out of service. It is the last method called n the servlet
instance.
JAVA Interview Questions with Answer
