Core Java

1.What is OOPs?

ANSWER
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

2.is the difference between Procedural and OOPs?

ANSWER
: a) In procedural program, programming logic follows certain procedures and the instructions are executed one after another. In OOPs 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.

3.are Encapsulation, Inheritance and Polymorphism?

ANSWER
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.

4.What is the difference between Assignment and Initialization?

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

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

ANSWER
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?

ANSWER
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?

ANSWER
Constructor will be automatically invoked when an object is created whereas method has to be called explicitly.

8.What is the use of bin and lib in JDK?

ANSWER
Bin contains all tools such as javac, appletviewer, awt tool, etc., whereas lib contains API and all packages.

9.What is casting?

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

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

ANSWER
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.

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

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

12.What is UNICODE?

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

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

ANSWER
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.

14.What is finalize() method ?

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

15.What is method overloading and method overriding?

ANSWER
: 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 class having the same method name with same arguments is said to be method overriding.

16.What is difference between overloading and overriding?

ANSWER
: 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 meant by Inheritance and what are its advantages?

ANSWER
: 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.

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

ANSWER
: this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.

19.What is the difference between superclass and subclass?

ANSWER
A super class is a class that is inherited whereas sub class is a class that does the inheriting.

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

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

21.What are inner class and anonymous class?

ANSWER
: 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.

22.What is a package?

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

23.What is a reflection package?

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

24.What is an abstract class?

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

25.What is the difference between Integer and int?

ANSWER
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.

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

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

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

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

27. Can you have an inner class inside a method and what variables can you access?

ANSWER
Yes, we can have an inner class inside a method and final variables can be accessed.

28.What is the difference between String and String Buffer?

ANSWER
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.

29.What is the difference between Array and vector?

ANSWER
Array is a set of related data type and static whereas vector is a growable array of objects and dynamic.

30.What is the difference between exception and error?

ANSWER
The exception class defines mild error conditions that your program encounters. Ex: Arithmetic Exception, FilenotFound exception Exceptions can occur when -- try 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. Ex: Running out of memory error, Stack overflow error.

31.What is the difference between process and thread?

ANSWER
Process is a program in execution whereas thread is a separate path of execution in a program.

32.What is multithreading and what are the methods for inter-thread communication and what is the classin which these methods are defined?

ANSWER
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.

33.What is the class and interface in java to create thread and which is the most advantageous method?

ANSWER
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.

34.What are the states associated in the thread?

ANSWER
Thread contains ready, running, waiting and dead states.

35.What is synchronization?

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

36.When you will synchronize a piece of your code?

ANSWER
When you expect your code will be accessed by different threads and these threads may change a particular data causing data corruption.

37.What is daemon thread and which method is used to create the daemon thread?

ANSWER
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.

38.Are there any global variables in Java, which can be accessed by other part of your program?

ANSWER
No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.

39.What is an applet?

ANSWER
Applet is a dynamic and interactive program that runs inside a web page displayed by a java capable browser.

40.What is the difference between applications and applets?

ANSWER
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.

41.What is the lifecycle of an applet?

ANSWER
• init( ) - Can be called when an applet is first loaded

41.What is an event and what are the models available for event handling?

ANSWER
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

42.What is source and listener ?

ANSWER
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.

43.What is adapter class?

ANSWER
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( ).

44.What is meant by controls and what are different types of controls in AWT?

ANSWER
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.

45. What is the difference between choice and list?

ANSWER
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.

46.What is a layout manager and what are different types of layout managers available in java.awt?

ANSWER
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.

47.What is the difference between set and list?

ANSWER
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.

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

ANSWER
A Stream is an abstraction that either produces or consumes information. There are two types of Streams 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.

49.What is an I/O filter?

ANSWER
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.

50.What is serialization and deserialization?

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