Ques: 1. What do you understand by Private, Protected and Public?
Answer:
Private,
Protected and Public are the accessibility modifiers. Private is the most
restrictive, while public is the least restrictive. There is no real difference
between protected and the default type (also known as package protected) within
the context of the same package, however the protected keyword allows
visibility to a derived class in a different package.
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Ques: 2. What is a java package? How is it used?
Answer:
A Java package
is a naming context for classes and interfaces.
- A package is used to
create a separate name space for groups of classes and interfaces.
- Packages are used to
organize related classes and interfaces into a single API unit and to
control accessibility to these classes and interfaces.
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Ques: 3. What do you understand by EAR, JAR and WAR File?
Answer:
Enterprise Archives (EAR): An EAR file contains all the components
that make up a J2EE application.
Java Archives (JAR): A JAR file encapsulates one or more Java classes, a
manifest, and a descriptor. JAR files are the lowest level of archive. JAR
files are used in J2EE for packaging EJBs and client-side Java Applications.
Web Archives (WAR): WAR files are like JAR files, except that they
are specifically for web applications made from Servlets, JSPs, and supporting
classes.
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Ques: 4. How Java Source Code files are named?
Answer:
A source code
file may contain at most one public class or interface. A Java source code file
takes the name of a public class or interface that is defined within the file.
Source code files use the .java extension.
If a public class or interface is defined within a source code file, then the
source code file must take the name of the public class or interface. If no
public class or interface is defined within a source code file, then the file
must take on a name that is different than its classes and interfaces.
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Ques: 5. What do you understand by Numeric Promotion?
Answer:
In numerical
promotion, byte, char, and short values are converted to int values. It is the
conversion of a smaller numeric type to a larger numeric type, so that integer
and floating-point operations may take place. if required, the int values are
also converted to long values. The long and float values can be converted to
double values.
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Ques: 6. What is the difference between Time Slicing and Preemptive
Scheduling?
Answer:
Under time
slicing in java, a task executes for a predefined slice of time and then
reenters the pool of ready tasks.
But under preemptive scheduling, the highest priority task executes until it
enters the waiting or dead states, or a higher priority task comes into
existence.
The scheduler then determines which task should execute next, based on priority
and other factors.
Ques: 7. What do you mean by a Task's Priority? How is it used in
Scheduling?
Answer:
A task's
priority is an integer value that identifies the relative order in which it
should be executed with respect to other tasks. The scheduler attempts to
schedule higher priority tasks before lower priority tasks.
Ques: 8. What technologies are included in J2EE?
Answer:
The various
technologies in J2EE are:
- Enterprise JavaBeansTM
(EJBsTM).
- JavaServer PagesTM
(JSPsTM).
- Java Servlets.
- The Java Naming and
Directory InterfaceTM (JNDITM).
- The Java Transaction
API (JTA).
- CORBA.
- The JDBCTM data access
API.
Ques: 9. What is the main purpose of the Wait(), Notify() and
Notifyall() methods in java?
Answer:
These methods
are used to provide an efficient way for threads to wait for a shared resource.
When a thread in java executes an object's wait() method, it enters the waiting
state. It only enters the ready state after another thread invokes the object's
notify() or notifyAll() methods.
Ques: 10. What happens when you invoke a Thread's interrupt method while it
is Sleeping or Waiting?
Answer:
When a task's
interrupt() method is executed, the task enters the ready state. The next time
the task enters the running state, an InterruptedException is thrown.
Ques: 11. What are the various restrictions placed on method overriding?
Answer:
- The overriding method
may not limit the access of the method it overrides.
- Overridden methods
must have the same name, argument list, and return type.
- The overriding method
may not throw any exceptions that may not be thrown by the overridden
method.
Ques: 12. What do you understand by Java Swing?
Answer:
Swing is
basically a type of Toolkit which is GUI toolkit for Java. It is one part of
the Java Foundation Classes (JFC). Swing includes graphical user interface
(GUI) widgets such as text boxes, buttons, split-panes, and tables.
Swing supports pluggable look and feel, not by using the native platform's
facilities, but by roughly emulating them. This means you can get any supported
look and feel on any platform.
Swing widgets provide more sophisticated GUI components than the earlier
Abstract Window Toolkit. Since they are written in pure Java, they run the same
on all platforms. The advantage is uniform behavior on all platforms. The
disadvantage of lightweight components is slower execution.
Ques: 13. What are the differences between Swing and AWT?
Answer:
There are many
differences between Swing and AWT:
- AWT is heavy-weight
components, but Swing is light-weight components.
- AWT is OS dependent
because it uses native components, But Swing components are OS
independent.
- Can change the look
and feel in Swing which is not possible in AWT.
- Swing takes less
memory compared to AWT.
- For drawing AWT uses
screen rendering where Swing uses double buffering.
Ques: 14. What are the Types of Scaling?
Answer:
In java, there
are two types of scaling: Horizontal Scaling and Vertical Scaling.
Horizontal Scaling: When Clones of an application server are defined on
multiple physical m/c, it is called Horizontal Scaling. The objective is to use
more than one less powerful m/c more efficiently.
Vertical Scaling: When multiple server clones of an application server are
defined on the same physical m/c, it is called Vertical Scaling. The objective
is to use the processing power of that m/c more efficiently.
Ques: 15. What are the types of Class Loaders in java?
Answer:
In java, there
are three types of class loader as bootstarp class loader, extension class
loader and system class loader.
Bootstrap Class Loader: Bootstrap class loader loads java’s core classes like
java.lang, java.util etc. These are classes that are part of java runtime
environment. Bootstrap class loader is native implementation and so they may
differ across different JVMs.
Extensions Class Loader: JAVA_HOME/jre/lib/ext
contains jar packages that are extensions of standard core java classes.
Extensions class loader loads classes from this ext folder. Using the system
environment property java.ext.dirs you can add ‘ext’ folders and jar files to
be loaded using extensions class loader.
System Class Loader: Java classes that are available in the java classpath are loaded
using System class loader.
Ques: 16. What are differences between the boolean & operator and the
&& operator?
Answer:
If an
expression involving the Boolean & operator is evaluated, both operands are
evaluated. Then the & operator is applied to the operand. When an
expression involving the && operator is evaluated, the first operand is
evaluated.
If the first operand returns a value of true, then the second operand is
evaluated. The && operator is then applied to the first and second
operands. If the first operand evaluates to false, the evaluation of the second
operand is skipped.
Ques: 17. What do you mean by Casting?
Answer:
In java, there
are two types of casting:
1). casting between primitive numeric types: Casting between
numeric types is used to convert larger values, such as double values, to
smaller values, such as byte values.
2). casting between object references: Casting between object
references is used to refer to an object by a compatible class, interface, or
array type reference.
Ques: 18. How does a Try statement determine which Catch clause should be
used to handle an exception?
Answer:
When an
exception is thrown within the body of a try statement, the catch clauses of
the try statement are examined in the order in which they appear. The first
catch clause that is capable of handling the exception is executed. The
remaining catch clauses are ignored.
Ques: 19. What will happens if Remove( ) is never invoked on a session bean?
Answer:
In case of
stateful session bean, the bean may be kept in cache till either the session
times out, in which case the bean is removed or when there is a requirement for
memory in which case the data is cached and the bean is sent to free pool.
In case of a stateless session bean it may not matter if we call or not as in
both cases nothing is done. The number of beans in cache is managed by the
container.
Ques: 20. What Is Java Reflection Api?
Answer:
Reflection is
one of the most powerful api which help to work with classes, methods and
variables dynamically. Basically, it inspects the class attributes at runtime.
Also, we can say it provides a metadata about the class.
Ques: 21. What is the Collection API in java?
Answer:
The Collection
API is a set of classes and interfaces that support operation on collections of
objects. These classes and interfaces are more flexible, more powerful, and
more regular than the vectors, arrays, and hashtables if effectively replaces.
Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and
TreeMap.
Example of interfaces: Collection, Set, List and Map.
Ques: 22. What is the difference between Session and Entity Beans?
Answer:
An entity bean
represents persistent global data from the database; a session bean represents
transient user-specific data that will die when the user disconnects (ends his
session). Generally, the session beans implement business methods that call
entity beans .
Ques: 23. What do you understand by Abstract Schema in java?
Answer:
In java, you
can specify the name of the Abstract schema name in the deployment descriptor.
The queries written in EJB QL for the finder methods references this name. It
is a part of an entity bean’s deployment descriptor which defines the bean’s
persistent fields and their relationship. Abstract schema is specified for
entity beans with container managed persistence. The information provided in
this Abstract Schema is used by the container for persistence management and
relationship management.
Ques: 24. What is the use of the Finally block? Is Finally block
in Java guaranteed to be called? When will it is not called?
Answer:
The code in
finally block will execute even if an exception is occurred. Finally, is the
block of code that executes always. Finally block is NOT called in following
conditions:
- If the JVM exits while
the try or catch code is being executed, then the finally block may not
execute. This may happen due to System.exit() call.
- if the thread
executing the try or catch code is interrupted or killed, the finally
block may not execute even though the application as a whole continues.
- If an exception is
thrown in finally block and not handled, then remaining code in finally
block may not be executed.
Ques: 25. What do you mean by Local Interface. How values will be passed to
them?
Answer:
An EJB can use
local client view only if it is really guaranteed that other enterprise beans
or clients will only address the bean within a single JVM. With local client
view, you can do pass-by-reference, which means your bean, as well as the
client, will work directly with one copy of the data. Any changes made by the
bean will be seen by the client and vice versa. Pass-by-reference eliminates
time/system expenses for copying data variables, which provides a performance
advantage.
Ques: 26. What is the main difference between Serializable and
Externalizable Interfaces?
Answer:
Both interfaces
are used for implementing serialization. But, the basic difference is
Serializable interface does not have any method (it’s a marker interface) and
Externalizable interface having 2 methods such as readExternal() and
writeExternal(). Serializable interface is the super interface for
Externalizable interface.
Ques: 27. What is the relation between Local Interfaces and
Container-managed relationships?
Answer:
To be the
target of a container-managed relationship, an entity bean with
container-managed persistence must provide a local interface. Entity beans that
have container-managed relationships with other entity beans, must be accessed
in the same local scope as those related beans, and therefore typically provide
a local client view.
Ques: 28. What are the differences between Creating String as New() and
Literal?
Answer:
When we create
string with new() Operator, it’s created in heap and not added into string pool
while String created using literal are created in String pool itself which
exists in PermGen area of heap.
String s = new String(“Test”);
It does not put the object in String pool, we need to call String.intern()
method which is used to put them into String pool explicitly. its only
when you create String object as String literal e.g. String s = “Test” Java
automatically put that into String pool.
Ques: 29. What is the main difference between Final, Finally And Finalize?
Answer:
Final is used to apply restrictions on class, method and variable.
Final class can’t be inherited, final method can’t be overridden, and final
variable value can’t be changed.
Finally is used to place important code, it will be executed
whether exception is handled or not.
Finalize is used to perform clean up processing just before object is garbage
collected.
Ques: 30. How can an Object's Finalize() Method be invoked while it is reachable?
Answer:
An object's
finalize() method cannot be invoked by the garbage collector while the object
is still reachable. However, an object's finalize() method may be invoked by
other objects.
Ques: 31. What do you mean by an Object’s Lock? Which Object Have
Locks?
Answer:
A thread may
execute a synchronized method of an object only after it has acquired the
object’s lock. An object’s lock is a mechanism that is used by multiple threads
to obtain synchronized access to the object. All objects and classes have
locks. A class’s lock is acquired on the class’s Class object.
Ques: 32. What are the uses of Observer and Observable?
Answer:
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.
Ques: 33. How ConcurrentHashMap Works?
Answer:
Basically,
ConcurrentHashMap locks each of the box (by default 16) which can be locked
independently and thread safe for operation. The basic design of
ConcurrentHashMap is to handling threading. And it does not expose the internal
lock process.
Ques: 34. What are Transient Variables in java?
Answer:
In Java,
Transient variables can’t be serialized. For example, if a variable is declared
as transient in a Serializable class and the class is written to an
ObjectStream, the value of the variable can’t be written to the stream instead
when the class is retrieved from the ObjectStream the value of the variable
becomes null.
Ques: 35. What do you understand by JFC?
Answer:
JFC stands for
Java Foundation Classes. The Java Foundation Classes (JFC) are a set of Java
class libraries provided as part of Java 2 Platform, Standard Edition (J2SE) to
support building graphics user interface (GUI) and graphics functionality for
client applications that will run on popular platforms such as Microsoft
Windows, Linux, and Mac OSX.
Ques: 36. What is the main difference between Find and Select methods In
EJB?
Answer:
A select method
can return a persistent field (or a collection thereof) of a related entity
bean. A finder method can return only a local or remote interface (or a
collection of interfaces).
A select method is defined in the entity bean class. For bean-managed
persistence, a finder method is defined in the entity bean class, but for
container-managed persistence it is not.
Because it is not exposed in any of the local or remote interfaces, a select
method cannot be invoked by a client. It can be invoked only by the methods
implemented within the entity bean class. A select method is usually invoked by
either a business or a home method.
Ques: 37. If some new data has entered in the database, how can a servlet
refresh automatically?
Answer:
It depends on
the scenario, how you can handle them. You need to handle this in dao
layer, when doing insert operation, you can call a utility method which will
load the context ServletContextListener. Because, servlets are basically used
for handling request and give the response.
Ques: 38. What do you mean by In-memory replication?
Answer:
The process by
which the contents in the memory of one physical m/c are replicated in all the
m/c in the cluster is called in-memory replication.
Ques: 39. What do you understand by synchronization? Why is it so Important?
Answer:
In respect of
multithreading, synchronization is the capability to control the access of
multiple threads to shared resources. In the absence of 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.
Ques: 40. How many Bits are used to represent Unicode, ASCII,
UTF-16 and UTF-8 characters?
Answer:
Although the
ASCII character set uses only 7 bits, it is usually represented as 8 bits.
Unicode requires 16 bits and ASCII require 7 bits. UTF-8 represents characters
using 8, 16, and 18 bits patterns. UTF-16 uses 16-bit and larger bit patterns.
Ques: 41. What is a Clone in java?
Answer:
In java, the
copies of a server group are called Clones. But unlike a Server Group Clones
are associated with a node and are real server process running in that node.
Ques: 42. How does the Garbage Collection guarantee that a program will not
run out of memory?
Answer:
In java,
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.
Ques: 43. What do you mean by AWT?
Answer:
AWT is stands
for Abstract Window Toolkit. AWT enables programmers to develop Java
applications with GUI components, such as windows, and buttons. The Java
Virtual Machine (JVM) is responsible for translating the AWT calls into the
appropriate calls to the host operating system.
Ques: 44. What Advantage do Java's Layout Managers provide over traditional
Windowing Systems?
Answer:
Java uses
layout managers to lay out components in a consistent manner across all
windowing platforms. Since Java's layout managers aren't tied to absolute
sizing and positioning, they are able to accommodate platform-specific
differences among windowing systems.
Ques: 45. Is Decorator an EJB design pattern?
Answer:
Decorator
design pattern is not an EJB design pattern. It is the one which exhibits very
low-level runtime polymorphism, for the specific and single object (Instance of
the class). But not for at least for a class. It is the stuff to add specific
functionality to a single & pointed object and leaves others like it
unmodified. It is having close similarities like AspectJ stuff, but not with
EJB stuff.
Ques: 46. What is the main purpose of the Enableevents() method?
Answer:
The
enableEvents() method is used to enable an event for a particular object.
Normally, an event is enabled when a listener is added to an object for a
particular event.
The enableEvents() method is used by objects that handle events by overriding
their event-dispatch methods.
Ques: 47. What is the main difference between Paint() and PaintComponent()?
Answer:
The main point
is that the paint() method invokes three methods in the following order:
- PaintComponent()
- PaintBorder()
- paintChildren()
As a general
rule, in Swing, we should be overriding the paintComponent method unless we
know what we are doing paintComponent() paints only component (panel) but
paint() paints component and all its children.
Ques: 48. What do you understand by double buffering ?
Answer:
Double
buffering is the process of using two buffers rather than one to temporarily
hold data being moved to and from an I/O device. Double buffering increases
data transfer speed because one buffer can be filled while the other is being
emptied.
Ques: 49. What is the main difference between the File and RandomAccessFile
Classes?
Answer:
The File class
encapsulates the files and directories of the local file system.
The RandomAccessFile class provides the methods needed to directly access data
contained in any part of a file.
Ques: 50. Can you Name some of the Layoutmanagers In Java?
Answer:
Some of the
LayoutManagers in java are:
- Flow Layout Manager
- Grid Layout Manager
- Box Layout Manager
- Border Layout Manager
- Card Layout Manager
- GridBag Layout
Manager
