FAQ : General Java Interview Questions

Why do we need public static void main(String args[]) method in Java
We need

• public: The method can be accessed outside the class /package

• static: You need not have an instance of the class toaccess the method

• void: Your application need not return a value, as the JVMlauncher would return the value when it exits

• main(): This is the entry point for the application

If the main() was not static, you would require an instance of theclass in order to execute the method.
If this is the case, what would create the instance of the class? Whatif your class did not have a public constructor?

Whatis the difference between an Interface and an Abstract class
In abstract class you can defineas well as declare methods, the methods which are declared are to bemarked as abstract.
In interface all we just declare methods and the definition is providedby the class which is implementing it

Explainserialization
Serialization means storing astate of a java object by coverting it to byte stream

Whatare the rules of serialization
Rules:
1. Static fileds are not serialized because they are not part of anyone particular object
2. Fileds from the base class are handled only if hose are serializable
3. Transient fileds are not serialized

Whatis difference between error and exception
Error occurs at runtime andcannot be recovered, Outofmemory is one such example. Exceptions on the other hand are due conditions which the application encounters such asFileNotFound exception or IO exceptions

Whatdo you mean by object oreiented programming
In object oreinted programmingthe emphasis is more on data than on the procedure and the program isdivided into objects.
The data fields are hidden and they cant be accessed by externalfunctions.
The design approach is bottom up.
The functions operate on data that is tied together in data structure

What are 4 pillars of objectoreinted programming
1. Abstraction
It means hiding the details and only exposing the essentioal parts

2. Polymorphism
Polymorphism means having many forms. In java you can see polymorphismwhen you have multiple methods with the same name

3. Inheritance
Inheritance means the child class inherits the non private propertiesof the parent class

4. Encapsulation
It means data hiding. In java with encapsulate the data by making itprivate and even we want some other class to work on that data then thesetter and getter methods are provided

Differencebetween procedural and object oreinted language
In procedural programming theinstructions are executed one after another and the data is exposed tothe whole program
In OOPs programming the unit of program is an object which is nothingbut combination of data and code and the data is not exposed outsidethe object

Whatis the difference between constructor and method
Constructor will beautomatically invoked when an object is created whereas method has tobe called explicitly.

Whatis the difference between parameters and arguments
While defining method, variablespassed in the method are called parameters. While using those methods,values passed to those variables are called arguments.

Whatis reflection in java
Reflection allows Java code todiscover information about the fields, methods and constructors ofloaded classes and to dynamically invoke them

Whatis a cloneable interface and how many methods does it contain
It is not having any methodbecause it is a TAGGED or MARKER interface

What'sthe difference between a queue and a stack
Stacks works bylast-in-first-out rule (LIFO), while queues use the FIFO rule

Canyou make an instance of abstract class
No you cannot create an instanceof abstract class

Whatare parsers
Parsers are used for processingXML documents. There are 2 types of parsers DOM parser and SAX Parser

Differencebetween SAX and DOM parser
DOM parsers are Object based andSAX parsers are event based
DOM parsers creates Tree in the memory whereas SAX parser does not andhence it is faster than DOM
DOM parser are useful when we have to modify the XML, with SAX parseryou cannot modify the xml, it is read only

Whatis the difference between Java Bean and Java Class
Basically a Bean is a java classbut it has getter and setter method and it does not have any logic init, it is used for holding data.
On the other hand the Java class can have what a java bean has and alsohas some logic inside it

Whatare null or Marker interfaces in Java
The null interfaces are markerinterfaces, they do not have function declarations in them, they areempty interfaces, this is to convey the compiler that they have to betreated differently

Doesjava Support multiple inheritance
Java does not support multipleinheritance directly like C++, because then it is prone to ambiguity,example if a class extends 2 other classes and these 2 parent classeshave same method names then there is ambiguity. Hence in Java Multipleinheritance is supported using Interfaces

Whatare virtual function
In OOP when a derived classinherits from a base class, an object of the derived class may bereferred to (or cast) as either being the base class type or thederived class type. If there are base class functions overridden by thederived class, a problem then arises when a derived object has beencast as the base class type. When a derived object is referred to asbeing of the base's type, the desired function call behavior isambiguous.

The distinction between virtual and not virtual is provided to solvethis issue. If the function in question is designated "virtual" thenthe derived class's function would be called (if it exists). If it isnot virtual, the base class's function would be called.

Doesjava support virtual functions
No java does not support virtualfunctions direclty like in C++, but it supports using Abstract classand interfaces

Describewhat happens when an object is created in Java
Several things happen in aparticular order to ensure the object is constructed properly:

1. Memory is allocated from heap to hold all instance variables andimplementation-specific data of the
object and its superclasses. Implemenation-specific data includespointers to class and method data.

2. The instance variables of the objects are initialized to theirdefault values.

3. The constructor for the most derived class is invoked. The firstthing a constructor does is call the
consctructor for its superclasses. This process continues until theconstrcutor for java.lang.Object is called,
as java.lang.Object is the base class for all objects in java.

4. Before the body of the constructor is executed, all instancevariable initializers and initialization blocks
are executed. Then the body of the constructor is executed. Thus, theconstructor for the base class
completes first and constructor for the most derived class completeslast.

Whatis the purpose of System Class
The purpose of the system classis to provide the access to the System reources

Whatis instanceOf operator usedfor
It is used to if an object canbe cast into a specific type without throwing Class cast exception

Whywe should not have instance variable in an interface
Since all data fields andmethods in an Interface are public by default, then we implement thatinterface in our class then we have public members in our class andthis class will expose these data members and this is violation ofencapsulation as now the data is not secure

Whatis JVM
When we install a java package.It contains 2 things
* The Java Runtime Environment (JRE)
* The Java Development Kit (JDK)

The JRE provides runtime support for Java applications. The JDKprovides the Java compiler and other development tools. The JDKincludes the JRE.

Both the JRE and the JDK include a Java Virtual Machine (JVM). This isthe application that executes a Java program. A Java program requires aJVM to run on a particular platform

Canabstract class be final
No, abstract class cannot befinal

Whena new object of derived Class is created, whose constructor will becalled first, childs or parents
Even when the new object ofchild class is created, first the Base class constructor gets executedand then the child classes constructor

Whatis a singleton class
A singleton is an object thatcannot be instantiated. The restriction on the singleton is that therecan be only one instance of a singleton created by the Java VirtualMachine (JVM) - by prevent direct instantiation we can ensure thatdevelopers don't create a second copy.

Canan abstract class have final method
Yes

Cana final class have an abstract method
No, the compiler will give anerror

Whatis the difference between Authentication and Authorization
Authentication is a process forverifying that an individual is who they say they are. Authorization isan additional level of security, and it means that a particular user(usually authenticated), may have access to a particular resource sayrecord, file, directory or script.