Core java questions

1. Can there be an abstract class with no abstract methods in it? - Yes
2. Can an Interface be final? - No
3. Can an Interface have an inner class? - Yes.
4. public interface abc
5. {
6. static int i=0; void dd();
7. class a1
8. {
9. a1()
10. {
11. int j;
12.
System.out.println("inside");
13. };
14. public static void
main(String a1[])
15. {
16. System.out.println("in
interfia");
17. }
18. }
19. }
20. Can we define private and protected modifiers for variables in
interfaces? - No

21. What is Externalizable? - Externalizable is an Interface that extends
Serializable Interface. And sends data into Streams in Compressed
Format. It has two methods, writeExternal(ObjectOuput out) and
readExternal(ObjectInput in)
22. What modifiers are allowed for methods in an Interface? - Only public
and abstract modifiers are allowed for methods in interfaces.
23. What is a local, member and a class variable? - Variables declared
within a method are “local” variables. Variables declared within the class
i.e not within any methods are “member” variables (global variables).
Variables declared within the class i.e not within any methods and are
defined as “static” are class variables

24. What are the different identifier states of a Thread? - The different
identifiers of a Thread are: R - Running or runnable thread, S - Suspended
thread, CW - Thread waiting on a condition variable, MW - Thread
waiting on a monitor lock, MS - Thread suspended waiting on a monitor
lock
25. What are some alternatives to inheritance? - Delegation is an
alternative to inheritance. Delegation means that you include an instance
of another class as an instance variable, and forward messages to the
instance. It is often safer than inheritance because it forces you to think
about each message you forward, because the instance is of a known class,
rather than a new class, and because it doesn’t force you to accept all the
methods of the super class: you can provide only the methods that really
make sense. On the other hand, it makes you write more code, and it is
harder to re-use (because it is not a subclass).
26. Why isn’t there operator overloading? - Because C++ has proven by
example that operator overloading makes code almost impossible to
maintain. In fact there very nearly wasn’t even method overloading in
Java, but it was thought that this was too useful for some very basic
methods like print(). Note that some of the classes like DataOutputStream
have unoverloaded methods like writeInt() and writeByte().
27. What does it mean that a method or field is “static”? - Static variables
and methods are instantiated only once per class. In other words they are
class variables, not instance variables. If you change the value of a static
variable in a particular object, the value of that variable changes for all
instances of that class. Static methods can be referenced with the name of
the class rather than the name of a particular object of the class (though
that works too). That’s how library methods like System.out.println()
work. out is a static field in the java.lang.System class.
28. How do I convert a numeric IP address like 192.18.97.39 into a
hostname like java.sun.com?

String
host=InetAddress.getByName("192.18.97.39").getHostN
ame();
29. Difference between JRE/JVM/JDK?

30. Why do threads block on I/O? - Threads block on i/o (that is enters the
waiting state) so that other threads may execute while the I/O operation is
performed.
31. 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.
32. Is null a keyword? - The null value is not a keyword.
33. Which characters may be used as the second character of an
identifier,but not as the first character of an identifier? - The digits 0
through 9 may not be used as the first character of an identifier but they
may be used after the first character of an identifier.
34. What modifiers may be used with an inner class that is a member of
an outer class? - A (non-local) inner class may be declared as public,
protected, private, static, final, or abstract.
35. How many bits are used to represent Unicode, ASCII, UTF-16, and
UTF-8 characters? - Unicode requires 16 bits and ASCII require 7 bits.
Although the ASCII character set uses only 7 bits, it is usually represented
as 8 bits. UTF-8 represents characters using 8, 16, and 18 bit patterns.
UTF-16 uses 16-bit and larger bit patterns.
36. What are wrapped classes? - Wrapped classes are classes that allow
primitive types to be accessed as objects.
37. What restrictions are placed on the location of a package statement
within a source code file? - A package statement must appear as the first
line in a source code file (excluding blank lines and comments).
38. 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.

39. What is a native method? - A native method is a method that is
implemented in a language other than Java.
40. What are order of precedence and associativity, and how are they
used? - Order of precedence determines the order in which operators are
evaluated in expressions. Associatity determines whether an expression is
evaluated left-to-right or right-to-left
41. What is the catch or declare rule for method declarations? - If a
checked exception may be thrown within the body of a method, the
method must either catch the exception or declare it in its throws clause.
42. Can an anonymous class be declared as implementing an interface
and extending a class? - An anonymous class may implement an
interface or extend a superclass, but may not be declared to do both.
43. What is the range of the char type? - The range of the char type is 0 to
2^16 - 1.

1. What is garbage collection? What is the process that is responsible for
doing that in java? - Reclaiming the unused memory by the invalid objects.
Garbage collector is responsible for this process
2. What kind of thread is the Garbage collector thread? - It is a daemon
thread.
3. What is a daemon thread? - These are the threads which can run without
user intervention. The JVM can exit when there are daemon thread by killing
them abruptly.
4. How will you invoke any external process in Java? -
Runtime.getRuntime().exec(….)
5. What is the finalize method do? - Before the invalid objects get garbage
collected, the JVM give the user a chance to clean up some resources before
it got garbage collected.
6. What is mutable object and immutable object? - If a object value is
changeable then we can call it as Mutable object. (Ex., StringBuffer, …) If
you are not allowed to change the value of an object, it is immutable object.
(Ex., String, Integer, Float, …)

7. What is the basic difference between string and stringbuffer object? -
String is an immutable object. StringBuffer is a mutable object.
8. What is the purpose of Void class? –

The Void class is an uninstantiable placeholder class to hold a reference to
the Class object representing the primitive Java type void.

9. What is reflection? - Reflection allows programmatic access to information
about the fields, methods and constructors of loaded classes, and the use
reflected fields, methods, and constructors to operate on their underlying
counterparts on objects, within security restrictions.
10. What is the base class for Error and Exception? - Throwable

11. What is the byte range? -128 to 127
12. What is the implementation of destroy method in java.. is it native or
java code? - This method is not implemented.
13. What is a package? - To group set of classes into a single unit is known as
packaging. Packages provides wide namespace ability.
14. What are the approaches that you will follow for making a program
very efficient? - By avoiding too much of static methods avoiding the
excessive and unnecessary use of synchronized methods Selection of related
classes based on the application (meaning synchronized classes for multiuser
and non-synchronized classes for single user) Usage of appropriate design
patterns Using cache methodologies for remote invocations Avoiding
creation of variables within a loop and lot more.
15. What is a DatabaseMetaData? - Comprehensive information about the
database as a whole.
16. What is Locale? - A Locale object represents a specific geographical,
political, or cultural region
17. How will you load a specific locale? - Using
ResourceBundle.getBundle(…);
18. What is JIT and its use? - Really, just a very fast compiler… In this
incarnation, pretty much a one-pass compiler — no offline computations. So
you can’t look at the whole method, rank the expressions according to which

ones are re-used the most, and then generate code. In theory terms, it’s an
on-line problem.
19. Is JVM a compiler or an interpreter? - Interpreter
20. When you think about optimization, what is the best way to findout the
time/memory consuming process? - Using profiler
21. What is the purpose of assert keyword used in JDK1.4.x? - In order to
validate certain expressions. It effectively replaces the if block and
automatically throws the AssertionError on failure. This keyword should be
used for the critical arguments. Meaning, without that the method does
nothing.
22. How will you get the platform dependent values like line separator, path
separator, etc., ? - Using Sytem.getProperty(…) (line.separator,
path.separator, …)
23. What is skeleton and stub? what is the purpose of those? - Stub is a
client side representation of the server, which takes care of communicating
with the remote server. Skeleton is the server side representation. But that is
no more in use… it is deprecated long before in JDK.
24. What is the final keyword denotes? - final keyword denotes that it is the
final implementation for that method or variable or class. You can’t override
that method/variable/class any more.
25. What is the significance of ListIterator? - You can iterate back and forth.
26. What is the major difference between LinkedList and ArrayList? -
LinkedList are meant for sequential accessing. ArrayList are meant for
random accessing.
27. What is nested class? - If all the methods of a inner class is static then it is a
nested class.
28. What is inner class? - If the methods of the inner class can only be
accessed via the instance of the inner class, then it is called inner class.
29. What is composition? - Holding the reference of the other class within
some other class is known as composition.

30. What is aggregation? - It is a special type of composition. If you expose all
the methods of a composite class and route the method call to the composite
method through its reference, then it is called aggregation.
31. What are the methods in Object? –

clone, equals, wait, finalize, getClass, hashCode, notify, notifyAll, toString

32. Can you instantiate the Math class? - You can’t instantiate the math class.
All the methods in this class are static. And the constructor is not public.
33. What is singleton? - It is one of the design pattern. This falls in the
creational pattern of the design pattern. There will be only one instance for
that entire JVM. You can achieve this by having the private constructor in
the class. For eg., public class Singleton { private static final Singleton s =
new Singleton(); private Singleton() { } public static Singleton getInstance()
{ return s; } // all non static methods … }
34. What is DriverManager? - The basic service to manage set of JDBC
drivers.

What is Class.forName() does and how it is useful? - It loads the class into
the ClassLoader. It returns the Class. Using that you can get the instance
( “class-instance”.newInstance() ).

35. Inq adds a question: Expain the reason for each keyword of public static
void main(String args[])

.
1. Are enterprise beans allowed to use Thread.sleep()? - Enterprise beans
make use of the services provided by the EJB container, such as life-cycle
management. To avoid conflicts with these services, enterprise beans are
restricted from performing certain operations: Managing or synchronizing
threads
2. Is it possible to write two EJB’s that share the same Remote and Home
interfaces, and have different bean classes? if so, what are the
advantages/disadvantages? - It’s certainly possible. In fact, there’s an
example that ships with the Inprise Application Server of an Account
interface with separate implementations for CheckingAccount and
SavingsAccount, one of which was CMP and one of which was BMP.

3. Is it possible to specify multiple JNDI names when deploying an EJB? -
No. To achieve this you have to deploy your EJB multiple times each
specifying a different JNDI name.
4. Is there any way to force an Entity Bean to store itself to the db? I don’t
wanna wait for the container to update the db, I want to do it NOW! Is
it possible? - Specify the transaction attribute of the bean as RequiresNew.
Then as per section 11.6.2.4 of the EJB v 1.1 spec EJB container
automatically starts a new transaction before the method call. The container
also performs the commit protocol before the method result is sent to the
client.
5. I am developing a BMP Entity bean. I have noticed that whenever the
create method is invoked, the ejbLoad() and the ejbStore() methods are
also invoked. I feel that once my database insert is done, having to do a
select and update SQL queries is major overhead. is this behavior
typical of all EJB containers? Is there any way to suppress these
invocations? - This is the default behaviour for EJB. The specification states
that ejbLoad() will be called before every transaction and ejbStore() after
every transaction. Each Vendor has optimizations, which are proprietary for
this scenario.
6. Can an EJB send asynchronous notifications to its clients? -
Asynchronous notification is a known hole in the first versions of the EJB
spec. The recommended solution to this is to use JMS, which is becoming
available in J2EE-compliant servers. The other option, of course, is to use
client-side threads and polling. This is not an ideal solution, but it’s
workable for many scenarios.
7. How can I access EJB from ASP? - You can use the Java 2 Platform,
Enterprise Edition Client Access Services (J2EETM CAS) COM Bridge 1.0,
currently downloadable from Sun
8. Is there a guarantee of uniqueness for entity beans? - There is no such
guarantee. The server (or servers) can instantiate as many instances of the
same underlying Entity Bean (with the same PK) as it wants. However, each
instance is guaranteed to have up-to-date data values, and be transactionally
consistent, so uniqueness is not required. This allows the server to scale the
system to support multiple threads, multiple concurrent requests, and
multiple hosts.

9. How do the six transaction attributes map to isolation levels like “dirty
read”? Will an attribute like “Required” lock out other readers until
I’m finished updating? - The Transaction Attributes in EJB do not map to
the Transaction Isolation levels used in JDBC. This is a common
misconception. Transaction Attributes specify to the container when a
Transaction should be started, suspended(paused) and committed between
method invocations on Enterprise JavaBeans. For more details and a
summary of Transaction Attributes refer to section 11.6 of the EJB 1.1
specification.
10. I have created a remote reference to an EJB in FirstServlet. Can I put
the reference in a servlet session and use that in SecondServlet? - Yes.
The EJB client (in this case your servlet) acquires a remote reference to an
EJB from the Home Interface; that reference is serializable and can be
passed from servlet to servlet. If it is a session bean, then the EJB server will
consider your web client’s servlet session to correspond to a single EJB
session, which is usually (but not always) what you want.
11. Can the primary key in the entity bean be a Java primitive type such as
int? - The primary key can’t be a primitive type–use the primitive wrapper
classes, instead. For example, you can use java.lang.Integer as the primary
key class, but not int (it has to be a class, not a primitive)
12. What’s new in the EJB 2.0 specification? - Following are the main
features supported in EJB 2.0: Integration of EJB with JMS, Message
Driven Beans, Implement additional Business methods in Home interface
which are not specific for bean instance, EJB QL.
13. How many types of protocol implementations does RMI have? - RMI
has at least three protocol implementations: Java Remote Method
Protocol(JRMP), Internet Inter ORB Protocol(IIOP), and Jini Extensible
Remote Invocation(JERI). These are alternatives, not part of the same thing,
All three are indeed layer 6 protocols for those who are still speaking OSI
reference model.
14. What is the need of Remote and Home interfaces. Why can’t there be
one? - In a few words, I would say that the main reason is because there is a
clear division of roles and responsabilities between the two interfaces. The
home interface is your way to communicate with the container, that is who is
responsable of creating, locating even removing one or more beans. The
remote interface is your link to the bean, that will allow you to remotely
access to all its methods and members. As you can see there are two distinct

elements (the container and the beans) and you need two different interfaces
for accessing to both of them.
15. What is the difference between Java Beans and EJB? - Java Beans are
client-side objects and EJBs are server side object, and they have completely
different development, lifecycle, purpose.
16. Question With regard to Entity Beans, what happens if both my EJB
Server and Database crash, what will happen to unsaved changes? Is
there any transactional log file used? - Actually, if your EJB server
crashes, you will not even be able to make a connection to the server to
perform a bean lookup, as the server will no longer be listening on the port
for incoming JNDI lookup requests. You will lose any data that wasn’t
committed prior to the crash. This is where you should start looking into
clustering your EJB server. Any unsaved and uncommited changes are lost
the moment your EJB Server crashes. If your database also crashes, then all
the saved changes are also lost unless you have some backup or some
recovery mechanism to retrieve the data. So consider database replication
and EJB Clustering for such scenarios, though the occurence of such a thing
is very very rare. Thx, Uma All databse have the concept of log files(for
exampe oracle have redo log files concept). So if data bases crashes then on
starting up they fill look up the log files to perform all pending jobs. But is
EJB crashes, It depend upon the container how frequenlty it passivates or
how frequesntly it refreshes the data with Database.

17. Question Can you control when passivation occurs? - The developer,
according to the specification, cannot directly control when passivation
occurs. Although for Stateful Session Beans, the container cannot passivate
an instance that is inside a transaction. So using transactions can be a a
strategy to control passivation. The ejbPassivate() method is called during
passivation, so the developer has control over what to do during this exercise
and can implement the require optimized logic. Some EJB containers, such
as BEA WebLogic, provide the ability to tune the container to minimize
passivation calls. Taken from the WebLogic 6.0 DTD - “The passivation-
strategy can be either “default” or “transaction”. With the default setting the
container will attempt to keep a working set of beans in the cache. With the
“transaction” setting, the container will passivate the bean after every
transaction (or method call for a non-transactional invocation).”

18. Does RMI-IIOP support dynamic downloading of classes? - No, RMI-
IIOP doesn’t support dynamic downloading of the classes as it is done with
CORBA in DII (Dynamic Interface Invocation).Actually RMI-IIOP
combines the usability of Java Remote Method Invocation (RMI) with the
interoperability of the Internet Inter-ORB Protocol (IIOP).So in order to
attain this interoperability between RMI and CORBA,some of the features
that are supported by RMI but not CORBA and vice versa are eliminated
from the RMI-IIOP specification.
19. Does EJB 1.1 support mandate the support for RMI-IIOP ? What is the
meaning of “the client API must support the Java RMI-IIOP
programming model for portability, but the underlying protocol can be
anything” ? - EJB1.1 does mandate the support of RMI-IIOP. There are 2
types of implementations that an EJB Server might provide: CORBA-based
EJB Servers and Proprietry EJB Servers. Both support the RMI-IIOP API
but how that API is implemented is a different story. (NB: By API we mean
the interface provided to the client by the stub or proxy). A CORBA-based
EJB Server actually implements its EJB Objects as CORBA Objects (it
therefore encorporates an ORB and this means that EJB’s can be contacted
by CORBA clients (as well as RMI-IIOP clients) A proprietry EJB still
implements the RMI-IIOP API (in the client’s stub) but the underlying
protocol can be anything. Therefore your EJB’s CANNOT be contacted by
CORBA clients. The difference is that in both cases, your clients see the
same API (hence, your client portability) BUT how the stubs communicate
with the server is different.
20. The EJB specification says that we cannot use Bean Managed
Transaction in Entity Beans. Why? - The short, practical answer is…
because it makes your entity beans useless as a reusable component. Also,
transaction management is best left to the application server - that’s what
they’re there for. It’s all about atomic operations on your data. If an
operation updates more than one entity then you want the whole thing to
succeed or the whole thing to fail, nothing in between. If you put commits in
the entity beans then it’s very difficult to rollback if an error occurs at some
point late in the operation.
21. Can I invoke Runtime.gc() in an EJB? - You shouldn’t. What will happen
depends on the implementation, but the call will most likely be ignored. You
should leave system level management like garbage collection for the
container to deal with. After all, that’s part of the benefit of using EJBs, you
don’t have to manage resources yourself.

22. What is clustering? What are the different algorithms used for
clustering? - Clustering is grouping machines together to transparantly
provide enterprise services.The client does not now the difference between
approaching one server or approaching a cluster of servers.Clusters provide
two benefits: scalability and high availability. Further information can be
found in the JavaWorld article J2EE Clustering.
23. What is the advantage of using Entity bean for database operations,
over directly using JDBC API to do database operations? When would I
use one over the other? - Entity Beans actually represents the data in a
database. It is not that Entity Beans replaces JDBC API. There are two types
of Entity Beans Container Managed and Bean Mananged. In Container
Managed Entity Bean - Whenever the instance of the bean is created the
container automatically retrieves the data from the DB/Persistance storage
and assigns to the object variables in bean for user to manipulate or use
them. For this the developer needs to map the fields in the database to the
variables in deployment descriptor files (which varies for each vendor). In
the Bean Managed Entity Bean - The developer has to specifically make
connection, retrive values, assign them to the objects in the ejbLoad() which
will be called by the container when it instatiates a bean object. Similarly in
the ejbStore() the container saves the object values back the the persistance
storage. ejbLoad and ejbStore are callback methods and can be only invoked
by the container. Apart from this, when you use Entity beans you dont need
to worry about database transaction handling, database connection pooling
etc. which are taken care by the ejb container. But in case of JDBC you have
to explicitly do the above features. what suresh told is exactly perfect.
ofcourse, this comes under the database transations, but i want to add this.
the great thing about the entity beans of container managed, whenever the
connection is failed during the transaction processing, the database
consistancy is mantained automatically. the container writes the data stored
at persistant storage of the entity beans to the database again to provide the
database consistancy. where as in jdbc api, we, developers has to do
manually.
24. What is the role of serialization in EJB? - A big part of EJB is that it is a
framework for underlying RMI: remote method invocation. You’re invoking
methods remotely from JVM space ‘A’ on objects which are in JVM space
‘B’ — possibly running on another machine on the network. To make this
happen, all arguments of each method call must have their current state
plucked out of JVM ‘A’ memory, flattened into a byte stream which can be
sent over a TCP/IP network connection, and then deserialized for

reincarnation on the other end in JVM ‘B’ where the actual method call
takes place. If the method has a return value, it is serialized up for streaming
back to JVM A. Thus the requirement that all EJB methods arguments and
return values must be serializable. The easiest way to do this is to make sure
all your classes implement java.io.Serializable.
25. What is EJB QL? - EJB QL is a Query Language provided for navigation
across a network of enterprise beans and dependent objects defined by
means of container managed persistence. EJB QL is introduced in the EJB
2.0 specification. The EJB QL query language defines finder methods for
entity beans with container managed persistenceand is portable across
containers and persistence managers. EJB QL is used for queries of two
types of finder methods: Finder methods that are defined in the home
interface of an entity bean and which return entity objects. Select methods,
which are not exposed to the client, but which are used by the Bean Provider
to select persistent values that are maintained by the Persistence Manager or
to select entity objects that are related to the entity bean on which the query
is defined.
26. What is the query used to display all tables names in SQL Server
(Query analyzer)?
27. select * from information_schema.tables
28. How many types of JDBC Drivers are present and what are they?-
There are 4 types of JDBC Drivers
1. JDBC-ODBC Bridge Driver
2. Native API Partly Java Driver
3. Network protocol Driver
4. JDBC Net pure Java Driver
29. Can we implement an interface in a JSP?- No
30. What is the difference between ServletContext and PageContext?-
ServletContext: Gives the information about the container. PageContext:
Gives the information about the Request
31. What is the difference in using request.getRequestDispatcher() and
context.getRequestDispatcher()?- request.getRequestDispatcher(path): In
order to create it we need to give the relative path of the resource,

context.getRequestDispatcher(path): In order to create it we need to give the
absolute path of the resource.
32. How to pass information from JSP to included JSP?- Using <
%jsp:param> tag.
33. What is the difference between directive include and jsp include?- <%@
include>: Used to include static resources during translation time. JSP
include: Used to include dynamic content or static content during runtime.
34. What is the difference between RequestDispatcher and sendRedirect?-
RequestDispatcher: server-side redirect with request and response objects.
sendRedirect : Client-side redirect with new request and response objects.
35. How does JSP handle runtime exceptions?- Using errorPage attribute of
page directive and also we need to specify isErrorPage=true if the current
page is intended to URL redirecting of a JSP.
36. How do you delete a Cookie within a JSP?
37. Cookie mycook = new Cookie("name","value");
38. response.addCookie(mycook);
39. Cookie killmycook = new
Cookie("mycook","value");
40. killmycook.setMaxAge(0);
41. killmycook.setPath("/");
42. killmycook.addCookie(killmycook);
43. How do I mix JSP and SSI #include?- If you’re just including raw HTML,
use the #include directive as usual inside your .jsp file.
44. 

But it’s a little trickier if you want the server to evaluate any JSP code that’s
inside the included file. If your data.inc file contains jsp code you will have
to use

<%@ vinclude="data.inc" %>

The <!–#include file="data.inc"–> is used for including non-JSP files.

45. I made my class Cloneable but I still get Can’t access protected method
clone. Why?- Some of the Java books imply that all you have to do in order
to have your class support clone() is implement the Cloneable interface. Not

so. Perhaps that was the intent at some point, but that’s not the way it works
currently. As it stands, you have to implement your own public clone()
method, even if it doesn’t do anything special and just calls super.clone().
46. Why is XML such an important development?- It removes two
constraints which were holding back Web developments: dependence on a
single, inflexible document type (HTML) which was being much abused for
tasks it was never designed for; the complexity of full SGML, whose syntax
allows many powerful but hard-to-program options. XML allows the
flexible development of user-defined document types. It provides a robust,
non-proprietary, persistent, and verifiable file format for the storage and
transmission of text and data both on and off the Web; and it removes the
more complex options of SGML, making it easier to program for.
47. What is the fastest type of JDBC driver?- JDBC driver performance will
depend on a number of issues:
1. the quality of the driver code,
2. the size of the driver code,
3. the database server and its load,
4. network topology,
5. the number of times your request is translated to a different API.

In general, all things being equal, you can assume that the more your request
and response change hands, the slower it will be. This means that Type 1
and Type 3 drivers will be slower than Type 2 drivers (the database calls are
make at least three translations versus two), and Type 4 drivers are the
fastest (only one translation).

How do I find whether a parameter exists in the request object?
48.
49. boolean hasFoo = !(request.getParameter("foo") ==
null
50. || request.getParameter("foo").equals(""));

or

boolean hasParameter =
request.getParameterMap().contains(theParameter);
//(which works in Servlet 2.3+)

51. How can I send user authentication information while
makingURLConnection?- You’ll want to use
HttpURLConnection.setRequestProperty and set all the appropriate headers
to HTTP authorization.
52. Is “abc” a primitive value? - The String literal “abc” is not a primitive
value. It is a String object.
53. What restrictions are placed on the values of each case of a switch
statement? - During compilation, the values of each case of a switch
statement must evaluate to a value that can be promoted to an int value.
54. What modifiers may be used with an interface declaration? - An
interface may be declared as public or abstract.
55. Is a class a subclass of itself? - A class is a subclass of itself.
56. 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.
57. What modifiers can be used with a local inner class? - A local inner class
may be final or abstract.
58. What is the purpose of the File class? - The File class is used to create
objects that provide access to the files and directories of a local file system.
59. Can an exception be rethrown? - Yes, an exception can be rethrown.
60. When does the compiler supply a default constructor for a class? - The
compiler supplies a default constructor for a class if no other constructors
are provided.
61. If a method is declared as protected, where may the method be
accessed? - A protected method may only be accessed by classes or
interfaces of the same package or by subclasses of the class in which it is
declared.
62. Which non-Unicode letter characters may be used as the first character
of an identifier? - The non-Unicode letter characters $ and _ may appear as
the first character of an identifier

63. What restrictions are placed on method overloading? - Two methods
may not have the same name and argument list but different return types.
64. What is casting? - There are two types of casting, casting between primitive
numeric types and casting between object references. Casting between
numeric types is used to convert larger values, such as double values, to
smaller values, such as byte values. Casting between object references is
used to refer to an object by a compatible class, interface, or array type
reference.
65. What is the return type of a program’s main() method? - A program’s
main() method has a void return type.
66. What class of exceptions are generated by the Java run-time system? -
The Java runtime system generates RuntimeException and Error exceptions.
67. What class allows you to read objects directly from a stream? - The
ObjectInputStream class supports the reading of objects from input streams.
68. What is the difference between a field variable and a local variable? - A
field variable is a variable that is declared as a member of a class. A local
variable is a variable that is declared local to a method.
69. 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.
70. What is the relationship between a method’s throws clause and the
exceptions that can be thrown during the method’s execution? - A
method’s throws clause must declare any checked exceptions that are not
caught within the body of the method.
71. Why are the methods of the Math class static? - So they can be invoked
as if they are a mathematical code library.
72. What are the legal operands of the instanceof operator? - The left
operand is an object reference or null value and the right operand is a class,
interface, or array type.
73. What an I/O filter? - An I/O filter is an object that reads from one stream
and writes to another, usually altering the data in some way as it is passed
from one stream to another.

74. If an object is garbage collected, can it become reachable again? - Once
an object is garbage collected, it ceases to exist. It can no longer become
reachable again.
75. What are E and PI? - E is the base of the natural logarithm and PI is
mathematical value pi.
76. Are true and false keywords? - The values true and false are not keywords.
77. What is the difference between the File and RandomAccessFile classes?
- 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.
78. What happens when you add a double value to a String? - The result is a
String object.
79. What is your platform’s default character encoding? - If you are running
Java on English Windows platforms, it is probably Cp1252. If you are
running Java on English Solaris platforms, it is most likely 8859_1.
80. Which package is always imported by default? - The java.lang package is
always imported by default.
81. What interface must an object implement before it can be written to a
stream as an object? - An object must implement the Serializable or
Externalizable interface before it can be written to a stream as an object.
82. How can my application get to know when a HttpSession is removed? -
Define a Class HttpSessionNotifier which implements
HttpSessionBindingListener and implement the functionality what you need
in valueUnbound() method. Create an instance of that class and put that
instance in HttpSession.
83. Whats the difference between notify() and notifyAll()? - notify() is used
to unblock one waiting thread; notifyAll() is used to unblock all of them.
Using notify() is preferable (for efficiency) when only one blocked thread
can benefit from the change (for example, when freeing a buffer back into a
pool). notifyAll() is necessary (for correctness) if multiple threads should
resume (for example, when releasing a “writer” lock on a file might permit
all “readers” to resume).
84. Why can’t I say just abs() or sin() instead of Math.abs() and Math.sin()?
- The import statement does not bring methods into your local name space. It

lets you abbreviate class names, but not get rid of them altogether. That’s
just the way it works, you’ll get used to it. It’s really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets
you what you want. If your top-level class doesn’t need to inherit from
anything else, make it inherit from java.lang.Math. That *does* bring all the
methods into your local name space. But you can’t use this trick in an applet,
because you have to inherit from java.awt.Applet. And actually, you can’t
use it on java.lang.Math at all, because Math is a “final” class which means
it can’t be extended.
85. Why are there no global variables in Java? - Global variables are
considered bad form for a variety of reasons: Adding state variables breaks
referential transparency (you no longer can understand a statement or
expression on its own: you need to understand it in the context of the
settings of the global variables), State variables lessen the cohesion of a
program: you need to know more to understand how something works. A
major point of Object-Oriented programming is to break up global state into
more easily understood collections of local state, When you add one
variable, you limit the use of your program to one instance. What you
thought was global, someone else might think of as local: they may want to
run two copies of your program at once. For these reasons, Java decided to
ban global variables.
86. What does it mean that a class or member is final? - A final class can no
longer be subclassed. Mostly this is done for security reasons with basic
classes like String and Integer. It also allows the compiler to make some
optimizations, and makes thread safety a little easier to achieve. Methods
may be declared final as well. This means they may not be overridden in a
subclass. Fields can be declared final, too. However, this has a completely
different meaning. A final field cannot be changed after it’s initialized, and it
must include an initializer statement where it’s declared. For example,
public final double c = 2.998; It’s also possible to make a static field final to
get the effect of C++’s const statement or some uses of C’s #define, e.g.
public static final double c = 2.998;
87. What does it mean that a method or class is abstract? - An abstract class
cannot be instantiated. Only its subclasses can be instantiated. You indicate
that a class is abstract with the abstract keyword like this:
88. public abstract class Container extends
Component {

Abstract classes may contain abstract methods. A method declared abstract
is not actually implemented in the current class. It exists only to be
overridden in subclasses. It has no body. For example,

public abstract float price();

Abstract methods may only be included in abstract classes. However, an
abstract class is not required to have any abstract methods, though most of
them do. Each subclass of an abstract class must override the abstract
methods of its superclasses or itself be declared abstract.

89. What is a transient variable? - transient variable is a variable that may not
be serialized.
90. 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.
91. Can a lock be acquired on a class? - Yes, a lock can be acquired on a
class. This lock is acquired on the class’s Class object.
92. What state does a thread enter when it terminates its processing? -
When a thread terminates its processing, it enters the dead state.
93. 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.
94. What is the difference between the >> and >>> operators? - The >>
operator carries the sign bit when shifting right. The >>> zero-fills bits that
have been shifted out.
95. Is sizeof a keyword? - The sizeof operator is not a keyword.
96. 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
97. Can an object’s finalize() method be invoked while it is reachable? - 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.
98. What value does readLine() return when it has reached the end of a
file? - The readLine() method returns null when it has reached the end of a
file.
99. Can a for statement loop indefinitely? - Yes, a for statement can loop
indefinitely. For example, consider the following: for(;;) ;
100. To what value is a variable of the String type automatically
initialized? - The default value of an String type is null.
101. What is a task’s priority and how is it used in scheduling? - 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.
102. What is the range of the short type? - The range of the short type is
-(2^15) to 2^15 - 1.
103. What is the purpose of garbage collection? - The purpose of
garbage collection is to identify and discard objects that are no longer
needed by a program so that their resources may be reclaimed and reused.
104. What do you understand by private, protected and public? -
These are 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.
105. What is Downcasting ? - Downcasting is the casting from a general
to a more specific type, i.e. casting down the hierarchy
106. Can a method be overloaded based on different return type but
same argument type ? - No, because the methods can be called without
using their return type in which case there is ambiquity for the compiler
107. What happens to a static var that is defined within a method of a
class ? - Can’t do it. You’ll get a compilation error
108. How many static init can you have ? - As many as you want, but the
static initializers and class variable initializers are executed in textual order

and may not refer to class variables declared in the class whose declarations
appear textually after the use, even though these class variables are in scope.
109. What is the difference amongst JVM Spec, JVM Implementation,
JVM Runtime ? - The JVM spec is the blueprint for the JVM generated and
owned by Sun. The JVM implementation is the actual implementation of the
spec by a vendor and the JVM runtime is the actual running instance of a
JVM implementation
110. Describe what happens when an object is created in Java? -
Several things happen in a particular order to ensure the object is constructed
properly: Memory is allocated from heap to hold all instance variables and
implementation-specific data of the object and its superclasses.
Implemenation-specific data includes pointers to class and method data. The
instance variables of the objects are initialized to their default values. The
constructor for the most derived class is invoked. The first thing a
constructor does is call the consctructor for its superclasses. This process
continues until the constrcutor for java.lang.Object is called, as
java.lang.Object is the base class for all objects in java. Before the body of
the constructor is executed, all instance variable initializers and initialization
blocks are executed. Then the body of the constructor is executed. Thus, the
constructor for the base class completes first and constructor for the most
derived class completes last.
111. What does the “final” keyword mean in front of a variable? A
method? A class? - FINAL for a variable: value is constant. FINAL for a
method: cannot be overridden. FINAL for a class: cannot be derived
112. What is the difference between instanceof and isInstance? -
instanceof is used to check to see if an object can be cast into a specified
type without throwing a cast class exception. isInstance() Determines if the
specified Object is assignment-compatible with the object represented by
this Class. This method is the dynamic equivalent of the Java language
instanceof operator. The method returns true if the specified Object
argument is non-null and can be cast to the reference type represented by
this Class object without raising a ClassCastException. It returns false
otherwise.
113. Why does it take so much time to access an Applet having Swing
Components the first time? - Because behind every swing component are
many Java objects and resources. This takes time to create them in memory.

JDK 1.3 from Sun has some improvements which may lead to faster
execution of Swing applications.

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