Ques: 1. What do you understand by Private, Protected and Public?
Ans: 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.
Ques: 2. What is a java package? How is it used?
Ans: 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.
Ques: 3. What do you understand by EAR, JAR and WAR File?
Ans:
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.
Ques: 4. How Java Source Code files are named?
Ans: 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.
Ques: 5. What do you understand by Numeric Promotion?
Ans: 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.
Ques: 6. What is the difference between Time Slicing and
Preemptive Scheduling?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans:
- 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans:
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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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?
Ans: 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()?
Ans: 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 ?
Ans: 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?
Ans: 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?
Ans: 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
