Interview Questions of Java for Freshers

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Q.1 What is the difference between abstract class and interfaces.
Ans. a. Abstract class can have both concrete and abstract method but interface can only have abstract methods.
        b.Astract class have instance variable and interface have not instance variable.
        c.In interface all the varibles are implicitly final and static but in abstract class it is not.
        d. Abstract class can have any access public,private proteced or not but interface can have public or none only.
       e.Abstract class can define constructor and interface can not.
       f.A class can implement any number of interfaces but a subclass can only one abtract class.

Q.2 What is Inheritance?
Ans. Inheritance is the process of acquiring properties of another object into another object

Q.3 Is multiple inheritance is possible in java?
Ans. Multiple inheritance is not possible in java directly but by using interface it can be achieved.

Q.4 What is Object class.
Ans. Object class is the super class of every class. It is provided by java.lang package. So it automatically available to enviorment.It can refer to any other object.The method provided by Object class can be used by any other class.wait(), toString(),notify(),notifyAll() ,clone()   provided by Object class.

Q.5 What is the difference between c++ and java?
Ans.Java is  mutithreaded.
        Java has no pointers.
        Java has automatic memory mangement (Garbage collection).
       Java does not have operator overloading.
       Java is platform independent.
       There are no destructor in java.

Q.6 What is javac and java?
Ans. Javac reads class and interface definition written in java programming language and compiles them into byte code(class files).
        Java tool launches a java application.It does this by starting a Java Runtime Environment,loading a specified class and invokes the class”s main method .

Q.7 What is the difference between final and finalize()?
Ans.  Final prevents the class to be inherited,method to be overridden and declare varibale euivalent constant.It also provides performance enhancement.Because the compiler is free to inline calls to them
Finalize() is method form java.lang.Object and called by the garbage collector on an object when GC determines that there are no more references to the object.It can never run more than once on any object.

Q.8 What is the difference between equals() and ==.
Ans.equals() methos compares the character inside a string object.The == operator compares two object references to see whether they refer to same instanecs.

Q.9 What are Class, Constructor and Primitive data types?
Ans.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.

Q.10 What is casting?
Ans.Casting is used to convert the value of one type to another.

Q.11 What are different types of access modifiers?
Ans.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.

Q.12 What is UNICODE?
Ans.Unicode is used for internal representation of characters and strings and it uses 16 bits to represent each other.

Q.13 What is garbage collector?
Ans. The java programming lanugage removes you from the responsibility of de-allocating memory.It provides a system level thread that tracks each memory locations.During idle cycles in JVM the garbage collection thread checks for and frees the memory that can be freed means the object  that has no longer reference by any variable.

Q.14 What is meant by Inheritance and what are its advantages?
Ans.Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses.

Q.15  What is the difference between this() and super()?
Ans. this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.

Q.16 What is an abstract class?
Ans. An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.

Q.17What are Transient and Volatile Modifiers?
Ans.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.

Q.18 What is method overloading and method overriding?
Ans.Method overloading: When a method in a class having the same method name with different arguments is said to be method overloading. Method overriding : When a method in a subclass having the same method name with same arguments as in its superclass is said to be method overriding. 

Q.19 What is difference between overloading and overriding?
Ans.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. 

Q.20 What is meant by Inheritance and what are its advantages?
Ans. Inheritance is the process of inheriting all the features from a class. The advantages of inheritance are reusability of code and accessibility of variables and methods of the super class by subclasses. 

Q.21 What is the difference between superclass and subclass?
Ans. A super class is a class that is inherited whereas sub class is a class that does the inheriting. 

Q.22 What modifiers may be used with top-level class?
Ans. public, abstract  can be used for top-level class.

Q.23 What are inner class and anonymous class?
Ans. 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. 

Q.24 What is a package?
Ans. A package is a collection of classes and interfaces that provides a high-level layer of access protection and name space management. 

Q.25 What is a reflection package?
Ans. java. lang. reflect package has the ability to analyze itself in runtime.

Q.26 What is interface and its use?
Ans. 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.

Q.27 What is an abstract class?
Ans. An abstract class is a class designed with implementation gaps for subclasses to fill in and is deliberately incomplete.

Q.28 What is the difference between Integer and int?
Ans. 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. 

Q.29 What is a cloneable interface and how many methods does it contain?
Ans. It is not having any method because it is a TAGGED or MARKER interface. 

Q.30 What is the difference between abstract class and interface?
Ans. 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.

Q.31 Can you have an inner class inside a method and what variables can you access?
Ans. Yes, we can have an inner class inside a method and final variables can be accessed. 

Q.32 What is the difference between Array and vector?
Ans. Array is a set of related data type and static whereas vector is a growable array of objects and dynamic. 

Q.33 What is the difference between exception and error?
Ans. 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. 

Q.34 What is the difference between process and thread?
Ans. Process is a program in execution whereas thread is a separate path of execution in a program. 

Q.35 What is multithreading and what are the methods for inter-thread communication and what is the class in which these methods are defined?
Ans. 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. 

Q.36 What is the class and interface in java to create thread and which is the most advantageous method?
Ans. Thread class and Runnable interface can be used to create threads and using Runnable interface is the most advantageous method to create threads because we need not extend thread class here. 

Q.37 What are the states associated in the thread?
Ans.Thread contains ready, running, waiting and dead states. 

Q.38 What is synchronization?
Ans. Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time. 

Q.39 When you will synchronize a piece of your code?
Ans. When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption. 

Q.40 What is deadlock?
Ans. When two threads are waiting each other and can’t precede the program is said to be deadlock. 

Q.41 What is daemon thread and which method is used to create the daemon thread?
Ans. 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. 

Q.42 Are there any global variables in Java, which can be accessed by other part of your program?
Ans. No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here

Q.43 What is an applet?
Ans. Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.

Q.44 What is the difference between applications and applets?
Ans. a)Application must be run on local machine whereas applet needs no explicit installation on local machine. b)Application must be run explicitly within a java-compatible virtual machine whereas applet loads and runs itself automatically in a java-enabled browser. d)Application starts execution with its main method whereas applet starts execution with its init method. e)Application can run with or without graphical user interface whereas applet must run within a graphical user interface. 

Q.45 How does applet recognize the height and width?– Using getParameters() method. 

Q.46 When do you use codebase in applet?
Ans. When the applet class file is not in the same directory, codebase is used.

Q.47 What is the lifecycle of an applet?
Ans. init() method – Can be called when an applet is first loaded start() method – Can be called each time an applet is started. paint() method – Can be called when the applet is minimized or maximized. stop() method – Can be used when the browser moves off the applet’s page. destroy() method – Can be called when the browser is finished with the applet. 

Q.48 How do you set security in applets?
Ans. using setSecurityManager() method.

Q.49 What is an event and what are the models available for event handling?
Ans. An event is an event object that describes a state of change in a source. In other words, event occurs when an action is generated, like pressing button, clicking mouse, selecting a list, etc. There are two types of models for handling events and they are: a) event-inheritance model and b) event-delegation model 

Q.50 What are the advantages of the event delegation model over the event-inheritance model?
Ans. The event-delegation model has two advantages over the event-inheritance model. They are: a)It enables event handling by objects other than the ones that generate the events. This allows a clean separation between a component’s design and its use. b)It performs much better in applications where many events are generated. This performance improvement is due to the fact that the event-delegation model does not have to be repeatedly process unhandled events as is the case of the event-inheritance. 

Q.51 What is source and listener?
Ans. source : A source is an object that generates an event. This occurs when the internal state of that object changes in some way. listener : A listener is an object that is notified when an event occurs. It has two major requirements. First, it must have been registered with one or more sources to receive notifications about specific types of events. Second, it must implement methods to receive and process these notifications.

Q.52 What is adapter class?
Ans. An adapter class provides an empty implementation of all methods in an event listener interface. Adapter classes are useful when you want to receive and process only some of the events that are handled by a particular event listener interface. You can define a new class to act listener by extending one of the adapter classes and implementing only those events in which you are interested. For example, the MouseMotionAdapter class has two methods, mouseDragged()and mouseMoved(). The signatures of these empty are exactly as defined in the MouseMotionListener interface. If you are interested in only mouse drag events, then you could simply extend MouseMotionAdapter and implement mouseDragged() . 

Q.53 What is meant by controls and what are different types of controls in AWT?
Ans.Controls are components that allow a user to interact with your application and the AWT supports the following types of controls: Labels, Push Buttons, Check Boxes, Choice Lists, Lists, Scrollbars, Text Components. These controls are subclasses of Component

Q.54 What is the difference between choice and list?
Ans. A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices and only one item may be selected from a choice. A List may be displayed in such a way that several list items are visible and it supports the selection of one or more list items. 

Q.55 What is the difference between scrollbar and scrollpane?
Ans. A Scrollbar is a Component, but not a Container whereas Scrollpane is a Conatiner and handles its own events and perform its own scrolling. 

Q.56 What is a layout manager and what are different types of layout managers available in java AWT?
Ans. A layout manager is an object that is used to organize components in a container. The different layouts are available are FlowLayout, BorderLayout, CardLayout, GridLayout and GridBagLayout.

Q.57 How are the elements of different layouts organized?
Ans. FlowLayout: The elements of a FlowLayout are organized in a top to bottom, left to right fashion. BorderLayout: The elements of a BorderLayout are organized at the borders (North, South, East and West) and the center of a container. CardLayout: The elements of a CardLayout are stacked, on top of the other, like a deck of cards. GridLayout: The elements of a GridLayout are of equal size and are laid out using the square of a grid. GridBagLayout: The elements of a GridBagLayout are organized according to a grid. However, the elements are of different size and may occupy more than one row or column of the grid. In addition, the rows and columns may have different sizes.

Q.58 Which containers use a Border layout as their default layout?
Ans. Window, Frame and Dialog classes use a BorderLayout as their layout. 

Q.59 Which containers use a Flow layout as their default layout?
Ans. Panel and Applet classes use the FlowLayout as their default layout. 

Q.60 What are wrapper classes?
Ans. Wrapper classes are classes that allow primitive types to be accessed as objects. 

Q.61 What are Vector, Hashtable, LinkedList and Enumeration?
Ans. 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.

Q.62 What is the difference between set and list?
Ans. 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. 

Q.63 What is a stream and what are the types of Streams and classes of the Streams?
Ans. A Stream is an abstraction that either produces or consumes information. There are two types of Streams and they are: Byte Streams: Provide a convenient means for handling input and output of bytes. 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. 

Q.64 What is the difference between Reader/Writer and InputStream/Output Stream?
Ans.The Reader/Writer class is character-oriented and the InputStream/OutputStream class is byte-oriented. 

Q.65 What is an I/O filter?
Ans. 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. 

Q.66 What is serialization and deserialization?
Ans.Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects. 

Q.67 What is JDBC?
Ans. 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. 

Q.68 What are drivers available?
Ans. a) JDBC-ODBC Bridge driver b) Native API Partly-Java driver c) JDBC-Net Pure Java driver d) Native-Protocol Pure Java driver 

Q.69 What is the difference between JDBC and ODBC?
Ans. 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. 

Q.70 What are the types of JDBC Driver Models and explain them?
Ans.There are two types of JDBC Driver Models and they are: a) Two tier model and b) Three tier model Two tier model: In this model, Java applications interact directly with the database. A JDBC driver is required to communicate with the particular database management system that is being accessed. SQL statements are sent to the database and the results are given to user. This model is referred to as client/server configuration where user is the client and the machine that has the database is called as the server. Three tier model: A middle tier is introduced in this model. The functions of this model are: a) Collection of SQL statements from the client and handing it over to the database, b) Receiving results from database to the client and c) Maintaining control over accessing and updating of the above. 

Q.71 What are the steps involved for making a connection with a database or how do you connect to a database?
Ans.  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”);
        

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