INTERVIEW QUESTION ANSWER

JAVA  DOT NET


JAVA INTERVIEW QUESTION ANSWER


1.       What is the difference between procedural and object-oriented programs?-

      a) In procedural program, programming logic follows certain procedures and the instructions are executed            one after another. In OOP program, unit of program is object, which is nothing but combination of data and code.

     b) In procedural program, data is exposed to the whole program whereas in OOPs program, it is accessible with in the object and which in turn assures the security of the code.

2.      What are Encapsulation, Inheritance and Polymorphism?-

 Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse.

 Inheritance is the process by which one object acquires the properties of another object.

 Polymorphism is the feature that allows one interface to be used for general class actions.

3.      What is the difference between Assignment and Initialization?-

 Assignment can be done as many times as desired whereas initialization can be done only once.

4.      What is OOPs?-

 Object oriented programming organizes a program around its data, i. e. , objects and a set of well defined interfaces to that data. An object-oriented program can be characterized as data controlling access to code.

5.       What are Class, Constructor and Primitive data types?-

Class is a template for multiple objects with similar features and it is a blue print for objects.

 It defines a type of object according to the data the object can hold and the operations the object can perform.

Constructor is a special kind of method that determines how an object is initialized when created.

 Primitive data types are 8 types and they are: byte, short, int, long, float, double, boolean, char.

6.      What is an Object and how do you allocate memory to it?-

 Object is an instance of a class and it is a software unit that combines a structured set of data with a set of operations for inspecting and manipulating that data.

 When an object is created using new operator, memory is allocated to it.

7.      What is the difference between constructor and method?-

 Constructor will be automatically invoked when an object is created

 whereas method has to be called explicitly.

8.      What is casting?-

 Casting is used to convert the value of one type to another.

9.      How many ways can an argument be passed to a subroutine and explain them?-

An argument can be passed in two ways:

 They are passing by value and passing by reference.

 Passing by value: This method copies the value of an argument into the formal parameter of the subroutine. Passing by reference: In this method, a reference to an argument (not the value of the argument) is passed to the parameter.

10.   What is the difference between an argument and a parameter?-

 While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.

11.   What are different types of access modifiers?-

public: Any thing declared as public can be accessed from anywhere.

 private: Any thing declared as private can’t be seen outside of its class.

 protected: Any thing declared as protected can be accessed by classes in the same package and subclasses in the other packages.

default modifier : Can be accessed only to classes in the same package.

12.   What is final, finalize() and finally?-

 final : final keyword can be used for class, method and variables.

        A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods.

       A final method can’t be overridden.

        A final variable can’t change from its initialized value.

 finalize() : finalize() method is used just before an     object is destroyed and can be called just prior to garbage collection.

 finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block. The finally block will execute whether or not an exception is thrown. For example, if a method opens a file upon exit, then you will not want the code that closes the file to be bypassed by the exception-handling mechanism. This finally keyword is designed to address this contingency.

13.   What is UNICODE?-

 Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.

14.   What is Garbage Collection and how to call it explicitly?-

 When an object is no longer referred to by any variable, java automatically reclaims memory used by that object. This is known as garbage collection.

 System. gc() method may be used to call it explicitly.

15.   What are Transient and Volatile Modifiers?-

 Transient: The transient modifier applies to variables only and it is not stored as part of its object’s Persistent state. Transient variables are not serialized.

Volatile: Volatile modifier applies to variables only and it tells the compiler that the variable modified by volatile can be changed unexpectedly by other parts of the program.

16.   What is difference between overloading and overriding?-

 a) In overloading, there is a relationship between methods available in the same class whereas in overriding, there is relationship between a superclass method and subclass method.

 b) Overloading does not block inheritance from the superclass whereas overriding blocks inheritance from the superclass.

 c) In overloading, separate methods share the same name whereas in overriding, subclass method replaces the superclass.

 d) Overloading must have different method signatures whereas overriding must have same signature.

17.   What is the difference between this() and super()?-

this() can be used to invoke a constructor of the same class

whereas super() can be used to invoke a super class constructor.

18.   What modifiers may be used with top-level class?-

 public, abstract and final can be used for top-level class.

19.   What are inner class and anonymous class?-

 Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private.

 Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.

20.  What is a package?-

A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management.

21.   What is a reflection package?-

 java. lang. reflect package has the ability to analyze itself in runtime.

22.  What is interface and its use?-

 Interface is similar to a class which may contain method’s signature only but not bodies and it is a formal set of method and constant declarations that must be defined by the class that implements it.

Interfaces are useful for:

 a)Declaring methods that one or more classes are expected to implement

 b)Capturing similarities between unrelated classes without forcing a class relationship.

 c)Determining an object’s programming interface without revealing the actual body of the class.

23.  What is an abstract class?-

An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.

24.  What is the difference between Integer and int?-

a) Integer is a class defined in the java. lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other.

 b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.

25.   What is a cloneable interface and how many methods does it contain?-

It is not having any method because it is a TAGGED or MARKER interface.

26.  What is the difference between abstract class and interface?-

a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract.

 b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods.

 c) Abstract class must have subclasses whereas interface can’t have subclasses.

27.  What is the difference between String and String Buffer?-

 a) String objects are constants and immutable whereas StringBuffer objects are not.

 b) String class supports constant strings whereas StringBuffer class supports growable and modifiable strings.

28.  What is the difference between exception and error?-

The exception class defines mild error conditions that your program encounters. Exceptions can occur when trying to open the file, which does not exist, the network connection is disrupted, operands being manipulated are out of prescribed ranges, the class file you are interested in loading is missing.

 The error class defines serious error conditions that you should not attempt to recover from. In most cases it is advisable to let the program terminate when such an error is encountered.

29.  What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?-

 Multithreading is the mechanism in which more than one thread run independent of each other within the process.

wait (), notify () and notifyAll() methods can be used for inter-thread communication and these methods are in Object class.

wait() : When a thread executes a call to wait() method, it surrenders the object lock and enters into a waiting state.

 notify() or notifyAll() : To remove a thread from the waiting state, some other thread must make a call to notify() or notifyAll() method on the same object.

30.  What is synchronization?-

 Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.

31.   Are there any global variables in Java, which can be accessed by other part of your program?- No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.

32.  What are wrapper classes?-

 Wrapper classes are classes that allow primitive types to be accessed as objects.

33.  What are Vector, Hashtable, LinkedList and Enumeration?-

 Vector : The Vector class provides the capability to implement a growable array of objects.

Hashtable : The Hashtable class implements a Hashtable data structure. A Hashtable indexes and stores objects in a dictionary using hash codes as the object’s keys. Hash codes are integer values that identify objects. LinkedList: Removing or inserting elements in the middle of an array can be done using LinkedList. A LinkedList stores each object in a separate link whereas an array stores object references in consecutive locations.

 Enumeration: An object that implements the Enumeration interface generates a series of elements, one at a time. It has two methods, namely hasMoreElements() and nextElement(). HasMoreElemnts() tests if this enumeration has more elements and nextElement method returns successive elements of the series.

34.  What is the difference between set and list?-

Set stores elements in an unordered way but does not contain duplicate elements, whereas list stores elements in an ordered way but may contain duplicate elements.

35.   What is a stream and what are the types of Streams and classes of the Streams?-

 A Stream is an abstraction that either produces or consumes information.

There are two types of Streams and they are:

a.        Byte Streams: Provide a convenient means for handling input and output of bytes.

b.        Character Streams: Provide a convenient means for handling input & output of characters.

 Byte Streams classes: Are defined by using two abstract classes, namely InputStream and OutputStream. Character Streams classes: Are defined by using two abstract classes, namely Reader and Writer.

36.  What is the difference between Reader/Writer and InputStream/Output Stream?-

The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented.

37.  What is 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.

38.  What is serialization and deserialization?-

Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

39.  What is JDBC?-

 JDBC is a set of Java API for executing SQL statements. This API consists of a set of classes and interfaces to enable programs to write pure Java Database applications.

40.  What is the difference between JDBC and ODBC?-

 a) OBDC is for Microsoft and JDBC is for Java applications.

 b) ODBC can’t be directly used with Java because it uses a C interface.

 c) ODBC makes use of pointers which have been removed totally from Java.

 d) ODBC mixes simple and advanced features together and has complex options for simple queries. But JDBC is designed to keep things simple while allowing advanced capabilities when required.

 e) ODBC requires manual installation of the ODBC driver manager and driver on all client machines. JDBC drivers are written in Java and JDBC code is automatically installable, secure, and portable on all platforms. f) JDBC API is a natural Java interface and is built on ODBC. JDBC retains some of the basic features of ODBC.

41.   What are the steps involved for making a connection with a database or how do you connect to a database?

a) Loading the driver : To load the driver, Class. forName() method is used. Class. forName(”sun. jdbc. odbc. JdbcOdbcDriver”); When the driver is loaded, it registers itself with the java. sql. DriverManager class as an available database driver.

 b) Making a connection with database: To open a connection to a given database, DriverManager. getConnection() method is used. Connection con = DriverManager. getConnection (”jdbc:odbc:somedb”, “user”, “password”);

 c) Executing SQL statements : To execute a SQL query, java. sql. statements class is used. createStatement() method of Connection to obtain a new Statement object. Statement stmt = con. createStatement(); A query that returns data can be executed using the executeQuery() method of Statement. This method executes the statement and returns a java. sql. ResultSet that encapsulates the retrieved data: ResultSet rs = stmt. executeQuery(”SELECT * FROM some table”);

 d) Process the results : ResultSet returns one row at a time. Next() method of ResultSet object can be called to move to the next row. The getString() and getObject() methods are used for retrieving column values: while(rs. next()) { String event = rs. getString(”event”); Object count = (Integer) rs. getObject(”count”);