Java Interview Questions

PREPARED BY PROF S. DAN , RERF, BARRACKPUR Page 1
Object Oriented Programming WITH JAVA
Object Oriented Programming (OOP)
Java is a computer programming language that is concurrent, class-based and object-oriented. The
advantages of object oriented software development are shown below:
 Modular development of code, which leads to easy maintenance and modification.
 Reusability of code.
 Improved reliability and flexibility of code.
 Increased understanding of code.
Object-oriented programming contains many significant features, such
as encapsulation, inheritance, polymorphismand abstraction. We analyze each feature separately
in the following sections.
Encapsulation
Encapsulation provides objects with the ability to hide their internal characteristics and behavior. Each
object provides a number of methods, which can be accessed by other objects and change its internal
data. In Java, there are three access modifiers: public, private and protected. Each modifier imposes
different access rights to other classes, either in the same or in external packages. Some of the
advantages of using encapsulation are listed below:
 The internal state of every objected is protected by hiding its attributes.
 It increases usability and maintenance of code, because the behavior of an object can be
independently changed or extended.
 It improves modularity by preventing objects to interact with each other, in an undesired way.
You can refer to our tutorial here for more details and examples on encapsulation.
Polymorphism
Polymorphism is the ability of programming languages to present the same interface for differing
underlying data types. A polymorphic type is a type whose operations can also be applied to values of
some other type.
Inheritance

Inheritance provides an object with the ability to acquire the fields and methods of another class, called
base class. Inheritance provides re-usability of code and can be used to add additional features to an
existing class, without modifying it.
Abstraction
Abstraction is the process of separating ideas from specific instances and thus, develop classes in terms
of their own functionality, instead of their implementation details. Java supports the creation and
existence of abstract classes that expose interfaces, without including the actual implementation of all
methods. The abstraction technique aims to separate the implementation details of a class from its
behavior.
Differences between Abstraction and Encapsulation
Abstraction and encapsulation are complementary concepts. On the one hand, abstraction focuses on the
behavior of an object. On the other hand, encapsulation focuses on the implementation of an object’s
behavior. Encapsulation is usually achieved by hiding information about the internal state of an object and
thus, can be seen as a strategy used in order to provide abstraction.
General Questions about Java
1. What is JVM ? Why is Java called the “Platform Independent Programming Language” ? A
Java virtual machine (JVM) is a process virtual machine that can execute Java bytecode. Each Java source
file is compiled into a bytecode file, which is executed by the JVM. Java was designed to allow application
programs to be built that could be run on any platform, without having to be rewritten or recompiled by
the programmer for each separate platform. A Java virtual machine makes this possible, because it is
aware of the specific instruction lengths and other particularities of the underlying hardware platform.
2. What is the Difference between JDK and JRE ? The Java Runtime Environment (JRE) is basically
the Java Virtual Machine (JVM) where your Java programs are being executed. It also includes browser
plugins for applet execution. The Java Development Kit (JDK) is the full featured Software Development
Kit for Java, including the JRE, the compilers and tools (like JavaDoc, and Java Debugger), in order for a
user to develop, compile and execute Java applications.
3. What does the “static” keyword mean ? Can you override private or static method in Java
? The static keyword denotes that a member variable or method can be accessed, without requiring an
instantiation of the class to which it belongs. A user cannot override static methods in Java, because
method overriding is based upon dynamic binding at runtime and static methods are statically binded at
compile time. A static method is not associated with any instance of a class so the concept is not
applicable.
4. Can you access non static variable in static context ? A static variable in Java belongs to its
class and its value remains the same for all its instances. A static variable is initialized when the class is

loaded by the JVM. If your code tries to access a non-static variable, without any instance, the compiler
will complain, because those variables are not created yet and they are not associated with any instance.
5. What are the Data Types supported by Java ? What is Autoboxing and Unboxing ? The eight
primitive data types supported by the Java programming language are:
 byte
 short
 int
 long
 float
 double
 boolean
 char
Autoboxing is the automatic conversion made by the Java compiler between the primitive types and their
corresponding object wrapper classes. For example, the compiler converts an int to an Integer, a double
to a Double, and so on. If the conversion goes the other way, this operation is called unboxing.
6. What is Function Overriding and Overloading in Java ? Method overloading in Java occurs when
two or more methods in the same class have the exact same name, but different parameters. On the
other hand, method overriding is defined as the case when a child class redefines the same method as a
parent class. Overridden methods must have the same name, argument list, and return type. The
overriding method may not limit the access of the method it overrides.
7. What is a Constructor, Constructor Overloading in Java and Copy-Constructor ? A
constructor gets invoked when a new object is created. Every class has a constructor. In case the
programmer does not provide a constructor for a class, the Java compiler (Javac) creates a default
constructor for that class. The constructor overloading is similar to method overloading in Java. Different
constructors can be created for a single class. Each constructor must have its own unique parameter list.
Finally, Java does support copy constructors like C++, but the difference lies in the fact that Java doesn’t
create a default copy constructor if you don’t write your own.
8. Does Java support multiple inheritance ? No, Java does not support multiple inheritance. Each
class is able to extend only on one class, but is able to implement more than one interfaces.
9. What is the difference between an Interface and an Abstract class ? Java provides and
supports the creation both of abstract classes and interfaces. Both implementations share some common
characteristics, but they differ in the following features:
 All methods in an interface are implicitly abstract. On the other hand, an abstract class may contain
both abstract and non-abstract methods.
 A class may implement a number of Interfaces, but can extend only one abstract class.
 In order for a class to implement an interface, it must implement all its declared methods. However, a
class may not implement all declared methods of an abstract class. Though, in this case, the sub-class
must also be declared as abstract.

 Abstract classes can implement interfaces without even providing the implementation of interface
methods.
 Variables declared in a Java interface is by default final. An abstract class may contain non-final
variables.
 Members of a Java interface are public by default. A member of an abstract class can either be private,
protected or public.
 An interface is absolutely abstract and cannot be instantiated. An abstract class also cannot be
instantiated, but can be invoked if it contains a main method.
Also check out the Abstract class and Interface differences for JDK 8.
10. What are pass by reference and pass by value ? When an object is passed by value, this means
that a copy of the object is passed. Thus, even if changes are made to that object, it doesn’t affect the
original value. When an object is passed by reference, this means that the actual object is not passed,
rather a reference of the object is passed. Thus, any changes made by the external method, are also
reflected in all places.
Java Threads
11. What is the difference between processes and threads ? A process is an execution of a
program, while aThread is a single execution sequence within a process. A process can contain multiple
threads. A Thread is sometimes called a lightweight process.
12. Explain different ways of creating a thread. Which one would you prefer and why ? There
are three ways that can be used in order for a Thread to be created:
 A class may extend the Thread class.
 A class may implement the Runnable interface.
 An application can use the Executor framework, in order to create a thread pool.
The Runnable interface is preferred, as it does not require an object to inherit the Thread class. In case
your application design requires multiple inheritance, only interfaces can help you. Also, the thread pool is
very efficient and can be implemented and used very easily.
13. Explain the available thread states in a high-level. During its execution, a thread can reside in
one of the following states:
 NEW: The thread becomes ready to run, but does not necessarily start running immediately.
 RUNNABLE: The Java Virtual Machine (JVM) is actively executing the thread’s code.
 BLOCKED: The thread is in a blocked state while waiting for a monitor lock.
 WAITING: The thread waits for another thread to perform a particular action.
 TIMED_WAITING: The thread waits for another thread to perform a particular action up to a specified
waiting time.
 TERMINATED: The thread has finished its execution.
14. What is the difference between a synchronized method and a synchronized block ? In Java
programming, each object has a lock. A thread can acquire the lock for an object by using the

synchronized keyword. The synchronized keyword can be applied in a method level (coarse grained lock)
or block level of code (fine grained lock).
15. How does thread synchronization occurs inside a monitor ? What levels of
synchronization can you apply ? The JVM uses locks in conjunction with monitors. A monitor is
basically a guardian that watches over a sequence of synchronized code and ensuring that only one
thread at a time executes a synchronized piece of code. Each monitor is associated with an object
reference. The thread is not allowed to execute the code until it obtains the lock.
16. What’s a deadlock ? A condition that occurs when two processes are waiting for each other to
complete, before proceeding. The result is that both processes wait endlessly.
17. How do you ensure that N threads can access N resources without deadlock ? A very
simple way to avoid deadlock while using N threads is to impose an ordering on the locks and force each
thread to follow that ordering. Thus, if all threads lock and unlock the mutexes in the same order, no
deadlocks can arise.
Java Collections
18. What are the basic interfaces of Java Collections Framework ? Java Collections
Framework provides a well designed set of interfaces and classes that support operations on a collections of
objects. The most basic interfaces that reside in the Java Collections Framework are:
 Collection, which represents a group of objects known as its elements.
 Set, which is a collection that cannot contain duplicate elements.
 List, which is an ordered collection and can contain duplicate elements.
 Map, which is an object that maps keys to values and cannot contain duplicate keys.
19. Why Collection doesn’t extend Cloneable and Serializable interfaces
? The Collection interface specifies groups of objects known as elements. Each concrete implementation
of a Collection can choose its own way of how to maintain and order its elements. Some collections allow
duplicate keys, while some other collections don’t. The semantics and the implications of either cloning or
serialization come into play when dealing with actual implementations. Thus, the concrete
implementations of collections should decide how they can be cloned or serialized.
20. What is an Iterator ? The Iterator interface provides a number of methods that are able to iterate
over anyCollection. Each Java Collection contains the iterator method that returns an Iterator instance.
Iterators arecapable of removing elements from the underlying collection during the iteration. 21. What
differences exist between Iterator and ListIterator ? The differences of these elements are listed
below:
 An Iterator can be used to traverse the Set and List collections, while the ListIterator can be used to
iterate only over Lists.
 The Iterator can traverse a collection only in forward direction, while the ListIterator can traverse
a List in both directions.
 The ListIterator implements the Iterator interface and contains extra functionality, such as adding an
element, replacing an element, getting the index position for previous and next elements, etc.

22. What is difference between fail-fast and fail-safe ? The Iterator's fail-safe property works
with the clone of the underlying collection and thus, it is not affected by any modification in the collection.
All the collection classes in java.util package are fail-fast, while the collection classes in
java.util.concurrent are fail-safe. Fail-fast iterators throw aConcurrentModificationException, while fail-safe
iterator never throws such an exception.
23. How HashMap works in Java ? A HashMap in Java stores key-value pairs. The HashMap requires a
hash function and uses hashCode and equals methods, in order to put and retrieve elements to and from
the collection respectively. When the put method is invoked, the HashMap calculates the hash value of the
key and stores the pair in the appropriate index inside the collection. If the key exists, its value is updated
with the new value. Some important characteristics of aHashMap are its capacity, its load factor and the
threshold resizing.
24. What is the importance of hashCode() and equals() methods ? In Java, a HashMap uses
the hashCode andequals methods to determine the index of the key-value pair and to detect duplicates.
More specifically, the hashCodemethod is used in order to determine where the specified key will be stored.
Since different keys may produce the same hash value, the equals method is used, in order to determine
whether the specified key actually exists in the collection or not. Therefore, the implementation of both
methods is crucial to the accuracy and efficiency of the HashMap.
25. What differences exist between HashMap and Hashtable ? Both
the HashMap and Hashtable classes implement the Map interface and thus, have very similar characteristics.
However, they differ in the following features:
 A HashMap allows the existence of null keys and values, while a Hashtable doesn’t allow neither null keys,
nor null values.
 A Hashtable is synchronized, while a HashMap is not. Thus, HashMap is preferred in single-threaded
environments, while a Hashtable is suitable for multi-threaded environments.
 A HashMap provides its set of keys and a Java application can iterate over them. Thus, a HashMap is fail-
fast. On the other hand, a Hashtable provides an Enumeration of its keys.
 The Hashtable class is considered to be a legacy class.
26. What is difference between Array and ArrayList ? When will you use Array over ArrayList
? The Arrayand ArrayList classes differ on the following features:
 Arrays can contain primitive or objects, while an ArrayList can contain only objects.
 Arrays have fixed size, while an ArrayList is dynamic.
 An ArrayListprovides more methods and features, such as addAll, removeAll, iterator, etc.
 For a list of primitive data types, the collections use autoboxing to reduce the coding effort. However,
this approach makes them slower when working on fixed size primitive data types.
27. What is difference between ArrayList and LinkedList ? Both
the ArrayList and LinkedList classes implement the List interface, but they differ on the following
features:
 An ArrayList is an index based data structure backed by an Array. It provides random access to its
elements with a performance equal to O(1). On the other hand, a LinkedList stores its data as list of

elements and every element is linked to its previous and next element. In this case, the search
operation for an element has execution time equal to O(n).
 The Insertion, addition and removal operations of an element are faster in a LinkedList compared to
an ArrayList, because there is no need of resizing an array or updating the index when an element is
added in some arbitrary position inside the collection.
 A LinkedList consumes more memory than an ArrayList, because every node in a LinkedList stores two
references, one for its previous element and one for its next element.
Check also our article ArrayList vs. LinkedList.
28. What is Comparable and Comparator interface ? List their differences. Java provides
the Comparableinterface, which contains only one method, called compareTo. This method compares two
objects, in order to impose an order between them. Specifically, it returns a negative integer, zero, or a
positive integer to indicate that the input object is less than, equal or greater than the existing object.
Java provides the Comparator interface, which contains two methods, called compare and equals. The first
method compares its two input arguments and imposes an order between them. It returns a negative
integer, zero, or a positive integer to indicate that the first argument is less than, equal to, or greater
than the second. The second method requires an object as a parameter and aims to decide whether the
input object is equal to the comparator. The method returns true, only if the specified object is also a
comparator and it imposes the same ordering as the comparator.
29. What is Java Priority Queue ? The PriorityQueue is an unbounded queue, based on a priority heap
and its elements are ordered in their natural order. At the time of its creation, we can provide a
Comparator that is responsible for ordering the elements of the PriorityQueue. A PriorityQueue doesn’t
allow null values, those objects that doesn’t provide natural ordering, or those objects that don’t have any
comparator associated with them. Finally, the Java PriorityQueue is not thread-safe and it requires
O(log(n)) time for its enqueing and dequeing operations.
30. What do you know about the big-O notation and can you give some examples with
respect to different data structures ? The Big-O notation simply describes how well an algorithm
scales or performs in the worst case scenario as the number of elements in a data structure increases.
The Big-O notation can also be used to describe other behavior such as memory consumption. Since the
collection classes are actually data structures, we usually use the Big-O notation to chose the best
implementation to use, based on time, memory and performance. Big-O notation can give a good
indication about performance for large amounts of data.
31. What is the tradeoff between using an unordered array versus an ordered array ? The
major advantage of an ordered array is that the search times have time complexity of O(log n), compared
to that of an unordered array, which is O (n). The disadvantage of an ordered array is that the insertion
operation has a time complexity of O(n), because the elements with higher values must be moved to
make room for the new element. Instead, the insertion operation for an unordered array takes constant
time of O(1).
32. What are some of the best practices relating to the Java Collection framework ?

 Choosing the right type of the collection to use, based on the application’s needs, is very crucial for its
performance. For example if the size of the elements is fixed and know a priori, we shall use an Array,
instead of an ArrayList.
 Some collection classes allow us to specify their initial capacity. Thus, if we have an estimation on the
number of elements that will be stored, we can use it to avoid rehashing or resizing.
 Always use Generics for type-safety, readability, and robustness. Also, by using Generics you avoid
theClassCastException during runtime.
 Use immutable classes provided by the Java Development Kit (JDK) as a key in a Map, in order to
avoid the implementation of the hashCode and equals methods for our custom class.
 Program in terms of interface not implementation.
 Return zero-length collections or arrays as opposed to returning a null in case the underlying collection
is actually empty.
33. What’s the difference between Enumeration and Iterator interfaces ? Enumeration is twice as
fast as compared to an Iterator and uses very less memory. However, the Iterator is much safer
compared to Enumeration, because other threads are not able to modify the collection object that is
currently traversed by the iterator. Also,Iteratorsallow the caller to remove elements from the underlying
collection, something which is not possible withEnumerations.
34. What is the difference between HashSet and TreeSet ? The HashSet is Implemented using a
hash table and thus, its elements are not ordered. The add, remove, and contains methods of
a HashSet have constant time complexity O(1). On the other hand, a TreeSet is implemented using a tree
structure. The elements in a TreeSet are sorted, and thus, the add, remove, and contains methods have
time complexity of O(logn).
Garbage Collectors
35. What is the purpose of garbage collection in Java, and when is it used ? The purpose of
garbage collection is to identify and discard those objects that are no longer needed by the application, in
order for the resources to be reclaimed and reused.
36. What does System.gc() and Runtime.gc() methods do ? These methods can be used as a hint
to the JVM, in order to start a garbage collection. However, this it is up to the Java Virtual Machine (JVM)
to start the garbage collection immediately or later in time.
37. When is the finalize() called ? What is the purpose of finalization ? The finalize method is
called by the garbage collector, just before releasing the object’s memory. It is normally advised to
release resources held by the object inside the finalize method.
38. If an object reference is set to null, will the Garbage Collector immediately free the
memory held by that object ? No, the object will be available for garbage collection in the next cycle
of the garbage collector.
39. What is structure of Java Heap ? What is Perm Gen space in Heap ? The JVM has a heap that
is the runtime data area from which memory for all class instances and arrays is allocated. It is created at

the JVM start-up. Heap memory for objects is reclaimed by an automatic memory management system
which is known as a garbage collector. Heap memory consists of live and dead objects. Live objects are
accessible by the application and will not be a subject of garbage collection. Dead objects are those which
will never be accessible by the application, but have not been collected by the garbage collector yet. Such
objects occupy the heap memory space until they are eventually collected by the garbage collector.
40. What is the difference between Serial and Throughput Garbage collector ? The throughput
garbage collector uses a parallel version of the young generation collector and is meant to be used with
applications that have medium to large data sets. On the other hand, the serial collector is usually
adequate for most small applications (those requiring heaps of up to approximately 100MB on modern
processors).
41. When does an Object becomes eligible for Garbage collection in Java ? A Java object is
subject to garbage collection when it becomes unreachable to the program in which it is currently used.
42. Does Garbage collection occur in permanent generation space in JVM ? Garbage Collection
does occur in PermGen space and if PermGen space is full or cross a threshold, it can trigger a full
garbage collection. If you look carefully at the output of the garbage collector, you will find that PermGen
space is also garbage collected. This is the reason why correct sizing of PermGen space is important to
avoid frequent full garbage collections. Also check our article Java 8: PermGen to Metaspace.
Exception Handling
43. What are the two types of Exceptions in Java ? Which are the differences between them
? Java has two types of exceptions: checked exceptions and unchecked exceptions. Unchecked
exceptions do not need to be declared in a method or a constructor’s throws clause, if they can be thrown
by the execution of the method or the constructor, and propagate outside the method or constructor
boundary. On the other hand, checked exceptions must be declared in a method or a constructor’s throws
clause. See here for tips on Java exception handling.
44. What is the difference between Exception and Error in java ? Exception and Error classes are
both subclasses of the Throwable class. The Exception class is used for exceptional conditions that a user’s
program should catch. The Error class defines exceptions that are not excepted to be caught by the user
program.
45. What is the difference between throw and throws ? The throw keyword is used to explicitly
raise a exception within the program. On the contrary, the throws clause is used to indicate those
exceptions that are not handled by a method. Each method must explicitly specify which exceptions does
not handle, so the callers of that method can guard against possible exceptions. Finally, multiple
exceptions are separated by a comma.
45. What is the importance of finally block in exception handling ? A finally block will always be
executed, whether or not an exception is actually thrown. Even in the case where the catch statement is
missing and an exception is thrown, the finally block will still be executed. Last thing to mention is that
the finally block is used to release resources like I/O buffers, database connections, etc.

46. What will happen to the Exception object after exception handling ? The Exception object
will be garbage collected in the next garbage collection.
47. How does finally block differ from finalize() method ? A finally block will be executed whether
or not an exception is thrown and is used to release those resources held by the application. Finalize is a
protected method of the Object class, which is called by the Java Virtual Machine (JVM) just before an
object is garbage collected.
Java Applets
48. What is an Applet ? A java applet is program that can be included in a HTML page and be executed
in a java enabled client browser. Applets are used for creating dynamic and interactive web applications.
49. Explain the life cycle of an Applet. An applet may undergo the following states:
 Init: An applet is initialized each time is loaded.
 Start: Begin the execution of an applet.
 Stop: Stop the execution of an applet.
 Destroy: Perform a final cleanup, before unloading the applet.
50. What happens when an applet is loaded ? First of all, an instance of the applet’s controlling
class is created. Then, the applet initializes itself and finally, it starts running.
51. What is the difference between an Applet and a Java Application ? Applets are executed
within a java enabled browser, but a Java application is a standalone Java program that can be executed
outside of a browser. However, they both require the existence of a Java Virtual Machine (JVM).
Furthermore, a Java application requires a main method with a specific signature, in order to start its
execution. Java applets don’t need such a method to start their execution. Finally, Java applets typically
use a restrictive security policy, while Java applications usually use more relaxed security policies.
52. What are the restrictions imposed on Java applets ? Mostly due to security reasons, the
following restrictions are imposed on Java applets:
 An applet cannot load libraries or define native methods.
 An applet cannot ordinarily read or write files on the execution host.
 An applet cannot read certain system properties.
 An applet cannot make network connections except to the host that it came from.
 An applet cannot start any program on the host that’s executing it.
53. What are untrusted applets ? Untrusted applets are those Java applets that cannot access or
execute local system files. By default, all downloaded applets are considered as untrusted.
54. What is the difference between applets loaded over the internet and applets loaded via
the file system ?Regarding the case where an applet is loaded over the internet, the applet is loaded by
the applet classloader and is subject to the restrictions enforced by the applet security manager.
Regarding the case where an applet is loaded from the client’s local disk, the applet is loaded by the file
system loader. Applets loaded via the file system are allowed to read files, write files and to load libraries

on the client. Also, applets loaded via the file system are allowed to execute processes and finally, applets
loaded via the file system are not passed through the byte code verifier.
55. What is the applet class loader, and what does it provide ? When an applet is loaded over the
internet, the applet is loaded by the applet classloader. The class loader enforces the Java name space
hierarchy. Also, the class loader guarantees that a unique namespace exists for classes that come from
the local file system, and that a unique namespace exists for each network source. When a browser loads
an applet over the net, that applet’s classes are placed in a private namespace associated with the
applet’s origin. Then, those classes loaded by the class loader are passed through the verifier.The verifier
checks that the class file conforms to the Java language specification . Among other things, the verifier
ensures that there are no stack overflows or underflows and that the parameters to all bytecode
instructions are correct.
56. What is the applet security manager, and what does it provide ? The applet security manager
is a mechanism to impose restrictions on Java applets. A browser may only have one security manager.
The security manager is established at startup, and it cannot thereafter be replaced, overloaded,
overridden, or extended.
Swing
57. What is the difference between a Choice and a List ? A Choice is displayed in a compact form
that must be pulled down, in order for a user to be able to see the list of all available choices. Only one
item may be selected from a Choice. A List may be displayed in such a way that several List items are
visible. A List supports the selection of one or more List items.
58. What is a layout manager ? A layout manager is the used to organize the components in a
container.
59. What is the difference between a Scrollbar and a JScrollPane ? A Scrollbar is a Component, but
not aContainer. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own
scrolling.
60. Which Swing methods are thread-safe ? There are only three thread-safe methods: repaint,
revalidate, and invalidate.
61. Name three Component subclasses that support painting. The Canvas, Frame, Panel, and Applet
classes support painting.
62. What is clipping ? Clipping is defined as the process of confining paint operations to a limited area
or shape.
63. What is the difference between a MenuItem and a CheckboxMenuItem
? The CheckboxMenuItem class extends the MenuItem class and supports a menu item that may be either
checked or unchecked.
64. How are the elements of a BorderLayout organized ? The elements of a BorderLayout are
organized at the borders (North, South, East, and West) and the center of a container.
65. How are the elements of a GridBagLayout organized ? The elements of a GridBagLayout are
organized according to a grid. The elements are of different sizes and may occupy more than one row or
column of the grid. Thus, the rows and columns may have different sizes.

66. What is the difference between a Window and a Frame ? The Frame class extends
the Window class and defines a main application window that can have a menu bar.
67. What is the relationship between clipping and repainting ? When a window is repainted by
the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.
68. What is the relationship between an event-listener interface and an event-adapter class
? An event-listener interface defines the methods that must be implemented by an event handler for a
particular event. An event adapter provides a default implementation of an event-listener interface.
69. How can a GUI component handle its own events ? A GUI component can handle its own
events, by implementing the corresponding event-listener interface and adding itself as its own event
listener.
70. What advantage do Java’s layout managers provide over traditional windowing systems
? Java uses layout managers to lay out components in a consistent manner, across all windowing
platforms. Since layout managers aren’t tied to absolute sizing and positioning, they are able to
accomodate platform-specific differences among windowing systems.
71. What is the design pattern that Java uses for all Swing components ? The design pattern
used by Java for all Swing components is the Model View Controller (MVC) pattern.
JDBC
72. What is JDBC ? JDBC is an abstraction layer that allows users to choose between databases. JDBC
enables developers to write database applications in Java, without having to concern themselves with the
underlying details of a particular database.
73. Explain the role of Driver in JDBC. The JDBC Driver provides vendor-specific implementations of
the abstract classes provided by the JDBC API. Each driver must provide implementations for the
following classes of the java.sql
package:Connection, Statement, PreparedStatement, CallableStatement, ResultSet and Driver.
74. What is the purpose Class.forName method ? This method is used to method is used to load
the driver that will establish a connection to the database.
75. What is the advantage of PreparedStatement over Statement ? PreparedStatements are
precompiled and thus,their performance is much better. Also, PreparedStatement objects can be reused
with different input values to their queries.
76. What is the use of CallableStatement ? Name the method, which is used to prepare a
CallableStatement.A CallableStatement is used to execute stored procedures. Stored procedures are
stored and offered by a database. Stored procedures may take input values from the user and may return
a result. The usage of stored procedures is highly encouraged, because it offers security and
modularity.The method that prepares a CallableStatement is the following:
1 CallableStament.prepareCall();
77. What does Connection pooling mean ? The interaction with a database can be costly, regarding
the opening and closing of database connections. Especially, when the number of database clients
increases, this cost is very high and a large number of resources is consumed.A pool of database

connections is obtained at start up by the application server and is maintained in a pool. A request for a
connection is served by a connection residing in the pool. In the end of the connection, the request is
returned to the pool and can be used to satisfy future requests.
Remote Method Invocation (RMI)
78. What is RMI ? The Java Remote Method Invocation (Java RMI) is a Java API that performs the
object-oriented equivalent of remote procedure calls (RPC), with support for direct transfer of serialized
Java classes and distributed garbage collection. Remote Method Invocation (RMI) can also be seen as the
process of activating a method on a remotely running object. RMI offers location transparency because a
user feels that a method is executed on a locally running object. Check some RMI Tips here.
79. What is the basic principle of RMI architecture ? The RMI architecture is based on a very
important principle which states that the definition of the behavior and the implementation of that
behavior, are separate concepts. RMI allows the code that defines the behavior and the code that
implements the behavior to remain separate and to run on separate JVMs.
80. What are the layers of RMI Architecture ? The RMI architecture consists of the following layers:
 Stub and Skeleton layer: This layer lies just beneath the view of the developer. This layer is responsible
for intercepting method calls made by the client to the interface and redirect these calls to a remote
RMI Service.
 Remote Reference Layer: The second layer of the RMI architecture deals with the interpretation of
references made from the client to the server’s remote objects. This layer interprets and manages
references made from clients to the remote service objects. The connection is a one-to-one (unicast)
link.
 Transport layer: This layer is responsible for connecting the two JVM participating in the service. This
layer is based on TCP/IP connections between machines in a network. It provides basic connectivity,
as well as some firewall penetration strategies.
81. What is the role of Remote Interface in RMI ? The Remote interface serves to identify
interfaces whose methods may be invoked from a non-local virtual machine. Any object that is a remote
object must directly or indirectly implement this interface. A class that implements a remote interface
should declare the remote interfaces being implemented, define the constructor for each remote object
and provide an implementation for each remote method in all remote interfaces.
82. What is the role of the java.rmi.Naming Class ? The java.rmi.Naming class provides methods
for storing and obtaining references to remote objects in the remote object registry. Each method of the
Naming class takes as one of its arguments a name that is a String in URL format.
83. What is meant by binding in RMI ? Binding is the process of associating or registering a name for
a remote object, which can be used at a later time, in order to look up that remote object. A remote
object can be associated with a name using the bind or rebind methods of the Naming class.
84. What is the difference between using bind() and rebind() methods of Naming Class ? The
bind method bind is responsible for binding the specified name to a remote object, while the rebind

method is responsible for rebinding the specified name to a new remote object. In case a binding exists
for that name, the binding is replaced.
85. What are the steps involved to make work a RMI program ? The following steps must be
involved in order for a RMI program to work properly:
 Compilation of all source files.
 Generatation of the stubs using rmic.
 Start the rmiregistry.
 Start the RMIServer.
 Run the client program.
86. What is the role of stub in RMI ? A stub for a remote object acts as a client’s local representative
or proxy for the remote object. The caller invokes a method on the local stub, which is responsible for
executing the method on the remote object. When a stub’s method is invoked, it undergoes the following
steps:
 It initiates a connection to the remote JVM containing the remote object.
 It marshals the parameters to the remote JVM.
 It waits for the result of the method invocation and execution.
 It unmarshals the return value or an exception if the method has not been successfully executed.
 It returns the value to the caller.
87. What is DGC ? And how does it work ? DGC stands for Distributed Garbage Collection. Remote
Method Invocation (RMI) uses DGC for automatic garbage collection. Since RMI involves remote object
references across JVM’s, garbage collection can be quite difficult. DGC uses a reference counting
algorithm to provide automatic memory management for remote objects.
88. What is the purpose of using RMISecurityManager in RMI ? RMISecurityManager provides a
security manager that can be used by RMI applications, which use downloaded code. The class loader of
RMI will not download any classes from remote locations, if the security manager has not been set.
89. Explain Marshalling and demarshalling. When an application wants to pass its memory objects
across a network to another host or persist it to storage, the in-memory representation must be
converted to a suitable format. This process is called marshalling and the revert operation is called
demarshalling.
90. Explain Serialization and Deserialization. Java provides a mechanism, called object serialization
where an object can be represented as a sequence of bytes and includes the object’s data, as well as
information about the object’s type, and the types of data stored in the object. Thus, serialization can be
seen as a way of flattening objects, in order to be stored on disk, and later, read back and reconstituted.
Deserialisation is the reverse process of converting an object from its flattened state to a live object.
Servlets

91. What is a Servlet ? The servlet is a Java programming language class used to process client
requests and generate dynamic web content. Servlets are mostly used to process or store data submitted
by an HTML form, provide dynamic content and manage state information that does not exist in the
stateless HTTP protocol.
92. Explain the architechure of a Servlet. The core abstraction that must be implemented by all
servlets is the javax.servlet.Servlet interface. Each servlet must implement it either directly or indirectly,
either by extending javax.servlet.GenericServlet or javax.servlet.http.HTTPServlet. Finally, each servlet is
able to serve multiple requests in parallel using multithreading.
93. What is the difference between an Applet and a Servlet ? An Applet is a client side java
program that runs within a Web browser on the client machine. On the other hand, a servlet is a server
side component that runs on the web server.An applet can use the user interface classes, while a servlet
does not have a user interface. Instead, a servlet waits for client’s HTTP requests and generates a
response in every request.
94. What is the difference between GenericServlet and HttpServlet ? GenericServlet is a
generalized and protocol-independent servlet that implements the Servlet and ServletConfig interfaces.
Those servlets extending the GenericServlet class shall override the service method. Finally, in order to
develop an HTTP servlet for use on the Web that serves requests using the HTTP protocol, your servlet
must extend the HttpServlet instead. Check Servlet examples here.
95. Explain the life cycle of a Servlet. On every client’s request, the Servlet Engine loads the servlets
and invokes its init methods, in order for the servlet to be initialized. Then, the Servlet object handles all
subsequent requests coming from that client, by invoking the service method for each request separately.
Finally, the servlet is removed by calling the server’s destroy method.
96. What is the difference between doGet() and doPost() ? doGET: The GET method appends the
name-value pairs on the request’s URL. Thus, there is a limit on the number of characters and
subsequently on the number of values that can be used in a client’s request. Furthermore, the values of
the request are made visible and thus, sensitive information must not be passed in that way. doPOST: The
POST method overcomes the limit imposed by the GET request, by sending the values of the request
inside its body. Also, there is no limitations on the number of values to be sent across. Finally, the
sensitive information passed through a POST request is not visible to an external client.
97. What is meant by a Web Application ? A Web application is a dynamic extension of a Web or
application server. There are two types of web applications: presentation-oriented and service-oriented. A
presentation-oriented Web application generates interactive web pages, which contain various types of
markup language and dynamic content in response to requests. On the other hand, a service-oriented
web application implements the endpoint of a web service. In general, a Web application can be seen as
a collection of servlets installed under a specific subset of the server’s URL namespace.
98. What is a Server Side Include (SSI) ? Server Side Includes (SSI) is a simple interpreted server-
side scripting language, used almost exclusively for the Web, and is embedded with a servlet tag. The
most frequent use of SSI is to include the contents of one or more files into a Web page on a Web server.
When a Web page is accessed by a browser, the Web server replaces the servlet tag in that Web page
with the hyper text generated by the corresponding servlet.

99. What is Servlet Chaining ? Servlet Chaining is the method where the output of one servlet is sent
to a second servlet. The output of the second servlet can be sent to a third servlet, and so on. The last
servlet in the chain is responsible for sending the response to the client.
100. How do you find out what client machine is making a request to your servlet ? The
ServletRequest class has functions for finding out the IP address or host name of the client machine.
getRemoteAddr() gets the IP address of the client machine and getRemoteHost() gets the host name of
the client machine. See example here.
101. What is the structure of the HTTP response ? The HTTP response consists of three parts:
 Status Code: describes the status of the response. It can be used to check if the request has been
successfully completed. In case the request failed, the status code can be used to find out the reason
behind the failure. If your servlet does not return a status code, the success status code,
HttpServletResponse.SC_OK, is returned by default.
 HTTP Headers: they contain more information about the response. For example, the headers may
specify the date/time after which the response is considered stale, or the form of encoding used to
safely transfer the entity to the user. See how to retrieve headers in Servlet here.
 Body: it contains the content of the response. The body may contain HTML code, an image, etc. The
body consists of the data bytes transmitted in an HTTP transaction message immediately following the
headers.
102. What is a cookie ? What is the difference between session and cookie ? A cookie is a bit of
information that the Web server sends to the browser. The browser stores the cookies for each Web
server in a local file. In a future request, the browser, along with the request, sends all stored cookies for
that specific Web server.The differences between session and a cookie are the following:
 The session should work, regardless of the settings on the client browser. The client may have chosen
to disable cookies. However, the sessions still work, as the client has no ability to disable them in the
server side.
 The session and cookies also differ in the amount of information the can store. The HTTP session is
capable of storing any Java object, while a cookie can only store String objects.
103. Which protocol will be used by browser and servlet to communicate ? The browser
communicates with a servlet by using the HTTP protocol.
104. What is HTTP Tunneling ? HTTP Tunneling is a technique by which, communications performed
using various network protocols are encapsulated using the HTTP or HTTPS protocols. The HTTP protocol
therefore acts as a wrapper for a channel that the network protocol being tunneled uses to communicate.
The masking of other protocol requests as HTTP requests is HTTP Tunneling.
105. What’s the difference between sendRedirect and forward methods ? The sendRedirect
method creates a new request, while the forward method just forwards a request to a new target. The
previous request scope objects are not available after a redirect, because it results in a new request. On
the other hand, the previous request scope objects are available after forwarding. FInally, in general, the
sendRedirect method is considered to be slower compare to the forward method.

106. What is URL Encoding and URL Decoding ? The URL encoding procedure is responsible for
replacing all the spaces and every other extra special character of a URL, into their corresponding Hex
representation. In correspondence, URL decoding is the exact opposite procedure.
JSP
107. What is a JSP Page ? A Java Server Page (JSP) is a text document that contains two types of
text: static data and JSP elements. Static data can be expressed in any text-based format, such as HTML
or XML. JSP is a technology that mixes static content with dynamically-generated content. See JSP
example here.
108. How are the JSP requests handled ? On the arrival of a JSP request, the browser first requests
a page with a .jsp extension. Then, the Web server reads the request and using the JSP compiler, the
Web server converts the JSP page into a servlet class. Notice that the JSP file is compiled only on the first
request of the page, or if the JSP file has changed.The generated servlet class is invoked, in order to
handle the browser’s request. Once the execution of the request is over, the servlet sends a response
back to the client. See how to get Request parameters in a JSP.
109. What are the advantages of JSP ? The advantages of using the JSP technology are shown
below:
 JSP pages are dynamically compiled into servlets and thus, the developers can easily make updates to
presentation code.
 JSP pages can be pre-compiled.
 JSP pages can be easily combined to static templates, including HTML or XML fragments, with code
that generates dynamic content.
 Developers can offer customized JSP tag libraries that page authors access using an XML-like syntax.
 Developers can make logic changes at the component level, without editing the individual pages that
use the application’s logic.
110. What are Directives ? What are the different types of Directives available in JSP
? Directives are instructions that are processed by the JSP engine, when the page is compiled to a
servlet. Directives are used to set page-level instructions, insert data from external files, and specify
custom tag libraries. Directives are defined between < %@ and % >.The different types of directives are
shown below:
 Include directive: it is used to include a file and merges the content of the file with the current page.
 Page directive: it is used to define specific attributes in the JSP page, like error page and buffer.
 Taglib: it is used to declare a custom tag library which is used in the page.
111. What are JSP actions ? JSP actions use constructs in XML syntax to control the behavior of the
servlet engine. JSP actions are executed when a JSP page is requested. They can be dynamically inserted
into a file, re-use JavaBeans components, forward the user to another page, or generate HTML for the
Java plugin.Some of the available actions are listed below:
 jsp:include – includes a file, when the JSP page is requested.

 jsp:useBean – finds or instantiates a JavaBean.
 jsp:setProperty – sets the property of a JavaBean.
 jsp:getProperty – gets the property of a JavaBean.
 jsp:forward – forwards the requester to a new page.
 jsp:plugin – generates browser-specific code.
112. What are Scriptlets ? In Java Server Pages (JSP) technology, a scriptlet is a piece of Java-code
embedded in a JSP page. The scriptlet is everything inside the tags. Between these tags, a user can add
any valid scriplet.
113. What are Decalarations ? Declarations are similar to variable declarations in Java. Declarations
are used to declare variables for subsequent use in expressions or scriptlets. To add a declaration, you
must use the sequences to enclose your declarations.
114. What are Expressions ? A JSP expression is used to insert the value of a scripting language
expression, converted into a string, into the data stream returned to the client, by the web server.
Expressions are defined between <% = and %>tags.
JAVA QUESTION ANSWERS
1. 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.
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. 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.
5. What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default behavior. An Interface can only declare
constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract.
An interface has all public members and no implementation. An abstract class is a class which may have the usual
flavors of class members (private, protected, etc.), but has some abstract methods.
6. 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.
7. What is an Iterator?
Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface
allows you to walk through a collection of objects, operating on each object in turn.
Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained;
generally it is not advisable to modify the collection itself while traversing an Iterator.
8. 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.
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.
9. 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.

10. What is final class?
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).
11. What if the main() method is declared as private?
The program compiles properly but at runtime it will give "main() method not public." message.
12. What if the static modifier is removed from the signature of the main() method?
Program compiles. But at runtime throws an error "NoSuchMethodError".
13. What if I write static public void instead of public static void?
Program compiles and runs properly.
14. What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".
15. 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.
16. If I do not provide any arguments on the command line, then the String array of main() method will
be empty or null?
It is empty. But not null.
17. How can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown
aNullPointerException on attempting to print args.length.
18. 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.
19. 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.
20. 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.

21. Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.
22. 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 about it. And the
JVM will internally load the class only once no matter how many times you import the same class.
23. 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.
Example: IOException thrown by java.io.FileInputStream's read() method·
Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are
unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the
exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be
thrown.
Example: 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.
24. 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.
25. Are the imports checked for validity at compile time? Example: 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;
26. Does importing a package imports the subpackages as well? Example: 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
packageMyTest only. It will not import any class in any of it's subpackage.
27. 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.
Example: String s; is just a declaration while String s = new String ("abcd"); Or String s =
"abcd";are both definitions.
28. What is the default value of an object reference declared as an instance variable?
The default value will be null unless we define it explicitly.
29. Can a top level class be private or protected?
No. A top level class cannot be private or protected. It can have either "public" or no modifier. If it does not have a
modifier it is supposed to have a default access.
If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This
means that a top level class can not be private. Same is the case with protected.
30. What type of parameter passing does Java support?
In Java the arguments are always passed by value.
31. Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.
32. 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.
33. What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
34. 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.
35. 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.
36. 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.
37. 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.
38. 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.
39. When you serialize an object, 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.
40. 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.
41. What happens to the static fields of a class during serialization?
There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are
1. Serialization ignores static fields, because they are not part of ay particular state state.
2. Base class fields are only hendled if the base class itself is serializable.
3. Transient fields.
42. Does Java provide any construct to find out the size of an object?
No, there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java.
43. What are wrapper classes?
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes.
They are example: Integer, Character, Double etc.
44. Why do we need wrapper classes?
It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not
primitive data types. And also the wrapper classes provide many utility methods also.

Because of these resons we need wrapper classes. And since we create instances of these classes we can store them
in any of the collection classes and pass them around as a collection. Also we can pass them around as method
parameters where a method expects an object.
45. What are checked exceptions?
Checked exception are those which the Java compiler forces you to catch.
Example: IOException are checked exceptions.
46. What are runtime exceptions?
Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of
wrong business logic etc. These are not checked by the compiler at compile time.
47. What is the difference between error and an exception?
An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error.
These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad
input etc. Example: FileNotFoundException will be thrown if the specified file does not exist. Or
aNullPointerException will take place if you try using a null reference.
In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering
proper values etc.).
48. How to create custom exceptions?
Your class should extend class Exception, or some more specific type thereof.
9. If I want an object of my class to be thrown as an exception object, what should I do?
The class should extend from Exception class. Or you can extend your class from some more precise exception type
also.
50. If my class already extends from some other class what should I do if I want an instance of my class
to be thrown as an exception object?
One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any
exception interface as well.
51. How does an exception permeate through the code?
An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code
which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a
matching type is found then that block will be invoked. If a matching type is not found then the exception moves up
the method stack and reaches the caller method.
Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch
block handling the appropriate type of exception is found. If it does not find such a block then finally the program
terminates.

52. What are the different ways to handle exceptions?
There are two ways to handle exceptions,
1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and
2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those
exceptions.
53. Is it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch
block or a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of
the method.
54. If I write return at the end of the try block, will the finally block still execute?
Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will
execute. The finally block will execute and then the control return.
55. If I write System.exit(0); at the end of the try block, will the finally block still execute?
No. In this case the finally block will not execute because when you say System.exit(0); the control immediately
goes out of the program, and thus finally never executes.
56. 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.
57. 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.
58. 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.
59. 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.
60. 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.
61. 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.
62. 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.
63. 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.
64. 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.
65. 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.
66. 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.
67. What is daemon thread and which method is used to create the daemon thread?
Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection
operation for the java runtime system.setDaemon method is used to create a daemon thread.
68. Can applets communicate with each other?
At this point in time applets may communicate with other applets running in the same virtual machine. If the applets
are of the same class, they can communicate via shared static variables. If the applets are of different classes, then
each will need a reference to the same class with static variables. In any case the basic idea is to pass the information
back and forth through a static variable.
An applet can also get references to all other applets on the same page using the getApplets() method
ofjava.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its
public members.

It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and
store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this
same server. Implementing this is non-trivial.
69. What are the steps in the JDBC connection?
While making a JDBC connection we go through the following steps :
Step 1 : Register the database driver by using :
Class.forName(" driver classs for that specific database" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement("select * from TABLE NAME");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
70. How does a try statement determine which catch clause should be used to handle an exception?
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 exceptionis
executed. The remaining catch clauses are ignored.
71. Can an unreachable object become reachable again?
An unreachable object may become reachable again. This can happen when the object's finalize() method is
invoked and the object performs an operation which causes it to become accessible to reachable objects.
72. What method must be implemented by all threads?
All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable
interface.
73. What are synchronized methods and synchronized statements?
Synchronized methods are methods that are used to control access to an object. A thread only executes a
synchronized method after it has acquired the lock for the method's object or class.
Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a
thread has acquired the lock for the object or class referenced in the synchronized statement.
74. 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).

75. What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
76. 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).
77. 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
thejava.lang.System class.
78. 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.
79. 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.
80. Is Empty .java file a valid source file?
Yes. An empty .java file is a perfectly valid source file.
81. Can a .java file contain more than one java classes?
Yes. A .java file contain more than one java classes, provided at the most one of them is a public class.
82. Is String a primitive data type in Java?
No. String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in
Java are instances of String class defined in java.lang package.
83. Is main a keyword in Java?

No. main is not a keyword in Java.
84. Is next a keyword in Java?
No. next is not a keyword.
85. Is delete a keyword in Java?
No. delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.
86. Is exit a keyword in Java?
No. To exit a program explicitly you use exit method in System object.
87. What happens if you dont initialize an instance variable of any of the primitive types in Java?
Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0(zero), a
boolean will be initialized to false.
88. What will be the initial value of an object reference which is defined as an instance variable?
The object references are all initialized to null in Java. However in order to do anything useful with these references,
you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such
default initialized references.
89. What are the different scopes for Java variables?
The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can
have one of the three scopes at any given point in time.
1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of
object, and remain accessible as long as the object accessible.
2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of
method excecution. When the method finishes execution, these variables fall out of scope.
3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time
and remain there as long as the class remains loaded. They are not tied to any particular object instance.
90. What is the default value of the local variables?
The local variables are not initialized to any default value, neither primitives nor object references. If you try to use
these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the
local varaible not being initilized.
91. How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();
Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.

92. Can a public class MyClass be defined in a source file named YourClass.java?
No. The source file name, if it contains a public class, must be the same as the public class name itself with
a .javaextension.
93. Can main() method be declared final?
Yes, the main() method can be declared final, in addition to being public static.
94. What is HashMap and Map?
Map is an Interface and Hashmap is the class that implements Map.
95. 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 unsynchronized
and Hashtable is synchronized.
96. Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.

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