Master List Of Core Java Interview Questions: post@1

What is an Object?
An object is an entity with certain attributes or qualities and behaviors, for a simple example, a 'Laptop' is an object which has certain attributes like weight,color,screen size,manufacturer etc.It has various behaviors or activities to do or act upon, as play games, browse Internet,write/check emails , watch movies ,listen music etc.
What is a Class?
A class is a collection of attributes and behaviors of objects with certain similarities and an instance of a class is represented by an object.A simple example of class is a 'Car' which represents variety of Car objects with different attribute values and behaviors.The different objects of 'Car' class can be, for example : A Mercedes Car,a Toyota Car, two different objects from same class but different attributes and different behaviors too.
What is OOAD?
Object Oriented Analysis and Design(OOAD) is a methodology to analyze,design and develop application using objects and their relations and message based communication to each other.Everything in OOAD is visualized in terms of objects and classes.OOAD introduced a paradigm shift from thinking and programming procedurally to objects oriented programming.This approach helps in designing complex real time systems with ease.The features like Data Abstraction and Encapsulation, Inheritance and Polymorphism form fundamentals of object oriented programming.
Advantages:
• Enhanced Reusability
• Modular approach towards problem solving which will be
• Better Maintainability
Better Performance if system is designed cautiously using OOAD concepts
What is Data Abstraction?
Data Abstraction is extraction of essential information for a particular purpose and ingnoring the remainder of the information,e.g. a car is consisted of an engine,air filters,a carburetor,a gear box,a steering,a fuel tank,tyres etc.A driver of a car need not to be bothered about several finer points of the car,he/she should know what it requires to drive a car.Take another user, a car mechanic, he will require different set of information in order to repair the car.
What is Data Encapsulation?
Data Encapsulation is wrapping informations(attributes and behaviors) within an object.A suitable example is a class as it wraps methods and data within itself. The attributes of a class corresponds to its data members while behaviour corresponds to member methods of the class.
What is the difference between Data Abstraction and Information Hiding?
Data Abstraction is often confused with information hiding while they altogether are two different technical concepts.Here are few established definitions of Data Abstraction:
What is Inheritance and what are different types of it?
Inheritance is a mechanism by which a specific object acquires attributes and behaviors of more general objects.In OOP terminology ,Inheritance is the mechanism which allows a Class 'A' to inherit properties of Class 'B' and we say 'A inherits from B' or in other words B is a 'Superclass'/'Parent class' while A is a 'Subclass'/'Child class'. A typical example of inheritance is a family tree which consists of son,father,grandfather,great grandfather and so on.The different types of Inheritance are:

1.Single Inheritance
2.Multiple Inheritance
3.Multilevel Inheritance
4.Hierarchical Inheritance
5.Hybrid Inheritance
Why Java uses Singly rooted hierarchy?
All objects in Java are inherited from same base class called 'Object'.In Java all objects have common interface to implement and it makes implementaion of Garbage collector lot easier in Java.The necessary implementaion is provided in base class , and the garbage collector can then send the necessary messages to every objectc in the system.Without singly rooted hierarchy,it would have been difficult to implement garbage collection feature.It enables lot of ease to programmers not to be bothered about memory management while development.It greatly simplifies argument passing amongst object too on the heap. As Java started from scratch and has no backward compatibility issues with any existing language, it was a logical choice to use the singly-rooted hierarchy in common with most other object-oriented programming languages.
Why does Java not support Multiple Inheritance?
Java does not support multiple inheritance atleast not the way it does in case of C++.In designer’s view Multiple Inheritance poses many problems and confusions than it solves.e.g. famous Diamond problem The diamond problem is an ambiguity that can occur when a class multiply inherits from two classes that both descend from a common super class. In such scenarios assuming if Java implements multiple inheritance then it would be difficult to know which method is to be called by an inheriting class object of two of the super classes. In Java, interfaces solve all these ambiguities caused by the diamond problem. Through interfaces, Java allows multiple inheritance of interface but not of implementation. Implementation, which includes instance variables and method implementations, is always singly inherited. As a result, confusion will never arise in Java over which inherited instance variable or method implementation to use.
Why is Java not 100% pure OOP language?
Java takes inspirations from C and C++.The native datatypes like 'char','int','float','double' are straight pick from C, which is not an Object Oriented Language.Resonably enough, Java is not a 100% pure Object Oriented Language.
What is Early Binding?
The assignment of types to variables and expressions at compilation time is known as 'Early Binding',it is also called 'static binding' and 'static typing'.
What is Polymorphism/Late Binding?
When an object is sent a message then it does not know itself what type it is, the runtime environment will decide about function calling over an object. This feature of connecting an object with its associated message at runtime is known as Polymorphism or Late binding or Dynamic binding.
What is method overloading?
A method with changed formal parameters will lead to implementing method overloading.
int calculateSum(int i,int j)
float calculateSum(float i,int j)
double calculateSum(double i,int j)
float calculateSum(int i,float j)
What is method overriding?
The method with same signature but with changed implementation lead to method overriding and that can occur in a parent child relation of classes. A method defined in parent class can be overridden in its child class with different implementation from its base class.
1. Pointers are supported in C++ while not in Java. The memory management is done automatically with help of part of JVM called Garbage Collector.
2. Multiple inheritance is not supported in Java but supported in C++.
3. There are no structures and unions in Java.
4. There is no scope resolution operator in Java (::).
5. There are no destructors in Java like C++.
6. There is no virtual keyword in Java because all non-static method use dynamic binding.
How is Java different from C++?
Java is a platform independent, object oriented language while C++ is having some of its features from C, which is a procedural language so it is not pure object oriented. Even Java is not 100% pure object oriented.

1. Pointers are supported in C++ while not in Java. The memory management is done automatically with help of part of JVM called Garbage Collector.
2. Multiple inheritance is not supported in Java but supported in C++.
3. There are no structures and unions in Java.
4. There is no scope resolution operator in Java (::).
5. There are no destructors in Java like C++.
6. There is no virtual keyword in Java because all non-static method use dynamic binding.
What is UML and how is it useful in designing large systems?
Unified Modelling Language(UML) is a notational language which comprises of several tools and techniques to support object oriented development.UML captures scenarios(use case diagram),object interactions(sequence diagram),class interactions(class diagrams) and object states(state diagrams).

UML helps in designing large and complex systems.It starts with analysis of business requirement and coming up with basic business flow chart and static diagrams i.e. use case diagrams which gives pictorial view of business requirements and captures scenarios. The next step is Interaction diagrams,which mainly consists of Sequence diagrams.A Sequence diagram tells how objects interact with each other through message passing in most importantly in what sequence.Then classes are identified of the system with various class identification approaches like 'Noun Phrase Approach','CRC Approach',this exercise results in UML class diagrams.

A modular approach helps in breaking down the complex system where each module can further be divided into components like classes and objects.Once the whole system is refined in terms of reusability of objects,omitting unnecessary objects and classes.The building of skeleton code on best practices of coding, like pattern based approach, helps in foundation of efficient code.
Is UML useful for procedural programming ?
Procedural programming is an unstructured way of programming which consists of set of procedures/method calls/instructions to be executed sequentially in such a way to attain the objective of a program.UML can help here in a very basic way in laying out the sequence of executions of instructions.
What are different notations used in UML ?
UML predominantly includes static and dynamic model diagrams and each diagram has their own set of notations.
In brief, the classification of these diagrams is given as below: #

Structure Diagrams include the Class Diagram, Object Diagram, Component Diagram, Composite Structure Diagram, Package Diagram, and Deployment Diagram.

Behavior Diagrams include the Use Case Diagram (used by some methodologies during requirements gathering); Activity Diagram, and State Machine Diagram.

Interaction Diagrams, all derived from the more general Behavior Diagram, include the Sequence Diagram, Communication Diagram, Timing Diagram, and Interaction Overview Diagram.

What is a Use case and an Actor?
A Usecase represents a particular scenario which corresponds to functional requirement(s) of a system to be designed and developed.An Actor is a user/external program or a system(anyone or anything), which interacts with a system.An Actor may input/receive or both(input and receive) information from the system.
In the diagram shown below, a scenario of buying bus/train/tram ticket from a vending machine is captured through a use case diagram.Here actor is a 'Customer' as shown by a stickman and in ovals all usecases have been documented.
How to identify an Actor?
An Actor can be identified by finding an answer for following points: Who is:
-interacting
-benefited
-maintaining
-supplying information
-using information
-removing information Does system use an external resource?
A good/refined set of actors of system will be arrived iteratively.
What is Generalization?
In UML , a generalization relationship is a relationship in which one model element (the child) is based on another model element (the parent). Generalization relationships are used in class, component, deployment, and use case diagrams.
Generalization corresponds to inheritance implemenation amongst classes.
What is Association and how it maps into a Java class?
An Association specifies how objects are related to one another.To identify associations,look for verb and prepositional phrases like 'part of','next to','works for' or 'contained in'.While identifying implicit associations,a lot common sense and general knowledge is required.It is very important to eleminate redundant associations while desiging a system.The most important aspects of associations are:
Cardinality - a cardinality of one on a given relationship end generates a Java reference, for example
public class Customer
{
Purchase purchase; ...
}
A cardinality of many (depicted as a number or *) generates a Java container:

public class Customer
{
List purchases; ...
}

Navigability - For given an instance of an object on one side of a association you can access an instance on the other side. If a association can only be traversed in one direction then this will be indicated with arrows. If there are no arrows then the association is bi-directional.

Association end - A given class only sees the association through the attributes set on the association end. In other words that simple line actually represents two independent sets of data, one for each of the two classes involved. Besides the cardinality and navigability the most important attribute is the association end name. This name is used to generate the getter and setter methods and in persistent classes database column names.

The different types of associations can be Aggregation and Composition.I will describe them in more details in next blogs to come.
What is Aggregation and how it maps into a Java class?
An Aggregation is an Association which denotes an "is part of" relationship.

Take a 'Car', for example, it is consisted of an engine,a steering wheel, four tires,seats,gear box,fuel tank,engine oil tank, air filters etc.So all constituents of car are parts of it.

If a car is destroyed/smashed, its parts can still be used separately as spares in other cars,so these parts have individual use even when their conatiner entity is destroyed.

In a Java class, an aggregation can be represented from above example as :

class Car
{
List getTires();
List getSeats();
List getAllParts();
}
What is Composition and how it maps into a Java class
A Composition is a tight Association and denotes "whole-part" relationship.So when an object is destroyed then all its constituents are also destroyed, these 'parts' have no meaning/sense in their lone existence from their 'whole'.

The best example of Composition is a 'Human body' which is composed of two legs,two hands,two eyes,two ears and so on.During the lifetime of a human being,all organs make sense being part of whole,but once a human being is dead most of these parts are also dead,unless some of his body parts are not medically reused.

Now come to map composition to Java world, the best example is garbage collection feature of the language.While garbage collecting objects, whole has the responsibility of preventing all its parts being garbage collected by holding some references to them.

It is the responsibility of whole to protect references to its parts not being exposed to outside world.The only way to have true composition in Java is to never let references to internal objects escape their parent's scope.

An example of Inner class as shown in the following code snippet may give you an idea how to implement Composition in Java.

public class Human
{

public Human()
{
Brain brain = new Brain();
}

private class Brain
{
....
....
}

}
What is Dependency and how it maps into a Java class
A Dependency relationship means when a class consumes/uses methods or variables from other class(supplier).So a change to supplier class affects the consumer class as well.Here supplier is indepenedent of any changes being made to consumer classs.

In UML class diagrams, a dependency relationship connector appears as a dashed line with an open arrow that points from the consumer class to the supplier class. A dependency relationship means an "import" statement.
What is the purpose of State machine diagrams?
Objects have both attributes and behaviors. Attribute is also know as state. When objects are incredibly complicated then to have better understanding during different state changes one should develop one or more state machine diagrams, formerly called state chart diagrams in UML 1.x, describing how their instances work.
What are different kinds of Structure diagrams?
Structure Diagrams as part of UML2.1:

Class diagrams
Object diagrams
Composite structure diagrams
Component diagrams
Deployment diagrams
Package diagrams
What are different kinds of Interaction diagrams?
The Interaction diagrams represent how objects interact with one another through message passing.

There are two kinds of Interaction Diagrams :

Sequence Diagram
Collaboartion Diagram

If you want to study these diagrams in detail then go through given by OMG.
What are different kinds of Behavior diagrams?
Behavior Diagrams include :

- Use Case Diagram (used by some methodologies during requirements gathering)
- Activity Diagram
- State Machine Diagram.
What is a Java Virtual Machine (JVM)?
A Java Virtual Machine is a runtime environment required for execution of a Java application.Every Java application runs inside a runtime instance of some concrete implementation of abstract specifications of JVM.It is JVM which is crux of 'platform independent' nature of the language.
What is a JVM consisted of?
Each time a Java Application is executed then an instance of JVM ,responsible for its running,is created.A JVM instance is described in terms of subsystems, memory areas, data types, and instructions.
What is a class loader and what is its responsibilities?
The Class loader is a subsystem of a JVM which is responsible,predominantly for loading classes and interfaces in the system.Apart from this,a class loader is responsible for the following activities:

-Verification of imported types(classes and interfaces)

-Allocating memory for class variables and initializing them to default values.Static fields for a class are created and these are set to standard default values but they are not explicitly initialized.The method tables are constructed for the class.
What is heap and stack?
The heap is the part of memory of JVM where all objects reside.

The stack is consisted of stack frames.When a thread invokes a method,the JVM pushes a new frame onto that thread's Java stack.Each stack frame is consisted of operand stack and the local variable array.All arguments,local variables,intermediate computations and return values if any are kept in these stack corresponding to the method invoked.The stack frame on the top of the stack is called the active stack frame,which is the current place of execution.When the method completes, the virtual machine pops and discards the frame for that method.