Core Java - Interview Questions and Answers for 'Basic' | Search Interview Question

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Q121. Explain System.out.println ?

Ans. System is a class within java.lang package that contains several useful class fields and methods. It cannot be instantiated and hence can use only statically.even in this case this has been used statically i.e with class name itself and without creating an instance.

out is the static reference of Printstream declared as following in the System Class -

public final static PrintStream out = null;

println is the method of PrintStream class.

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Q122. Why calculations in float or Double sometimes gives unpredictable results ?

Core Java

Ans. Because the way float values are stored its not precise.

For example - 0.1 is actually stored as
0.1000000000000000055511151231257827021181583404541015625

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Basic and Very Frequently asked.

Q123. What is Polymorphism in Java ?

Core Java

Ans. Polymorphism means the condition of occurring in several different forms.

Polymorphism in Java is achieved in two manners

1. Static polymorphism is the polymorphic resolution identified at compile time and is achieved through function overloading whereas

2. Dynamic polymorphism is the polymorphic resolution identified at runtime and is achieved through method overriding.

Sample Code for overloading

Sample Code for overriding

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Very frequently asked.

Q124. What is the use of synchronized keyword ?

Core Java

Ans. Synchronize is used to achieve mutual exclusion i.e at one time, the segment of the code, method that has been declared synchronized should be executed by single thread only and hence the lock needs to be retrieved before executing the segment and then released.

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Frequently asked.

Q125. When does the finally block gets executed ?

Core Java

Ans. A finally block of code always executes, whether or not an exception has occurred.The only time finally won't be called is if you call System.exit() or if the JVM crashes first.

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Q126. What is an IP address ?

Java EE

Ans. An Internet Protocol address is a numerical label assigned to each device participating in a computer network.

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Very Frequently asked to fresh graduates and less experienced.

Q127. What are the principle concepts of Object Oriented Programming ?

Core Java

Ans. Abstraction Polymorphism Inheritance Encapsulation

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Q128. What is a Database Trigger ?

Database

Ans. A trigger is a special kind of stored procedure that automatically gets executed upon an event in the database server.

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Q129. What is a Database Procedure ?

Database

Ans. Stored procedures are a batch of SQL statements along with programming constructs ( if else, loops etc ) and stored as a single program that can be called by different clients and hence reused.

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Q130. Which data structure would you recommend for ordered and sorted data?

Core Java

Ans. TreeSet and TreeMap are used for maintaining sorted elements.

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Frequently asked to Fresh graduates.

Q131. Write a program to implement Binary search ?

Core Java

Ans. http://algs4.cs.princeton.edu/11model/BinarySearch.java.html

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Q132. How is == operator different for objects and primitive types ?

Core Java

Ans. For objects or references, == operator check if the reference on left and right points to the same object.

For primitive types or variables, == operator check if the variable on left and right holds the same value.

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Q133. What is the difference between = and == in Java ?

Core Java

Ans. = is the assignment operator that assigns the result of the expression on the right to the variable on the left, whereas

== is the operator to check object equality to see if the reference on left and right are pointing to the same object. For primitive types, its used to check if both variables holds the same value.

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Q134. What is a classpath in Java ?

Core Java

Ans. Classpath is the parameter for JVM to look for java classes ( .class files ) that are to be looad by class loader

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Q135. What is meant by size and index of a collection ?

Core Java

Ans. As the name suggest size is the size of collection / data structure i.e number of elements or memory utilized by the collection.

Index is the position of an element in a collection or data structure.

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Q136. Difference between multiple and multi level inheritance ?

Core Java

Ans. Multiple Inheritance refers to the concept of a class inheriting multiple classes. Example - Class C extends Class A ,Class B. This is not allowed in Java.

Multilevel Inheritance refers to the concept of Inheritance in a chain. Example - Class B extends Class A, Class C extends Class B. This is permitted in Java.

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Q137. Can we access private members of the parent class ? i.e Are private members of the class visible in the derived class ?

Core Java

Ans. No, If both Parent and Derived are outer classes.

public class Vehicle {
private static String manufacturingDate = "2016";
}

public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // error - The field Vehicle.manufacturingDate is not visible
}
}

Yes, If derived is the inner class of Parent.

public class Vehicle {
private static String manufacturingDate = "2016";
static public class Car extends Vehicle{
public static void main(String args[]){
System.out.println(manufacturingDate); // no problem
}
}
}

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Q138. Can we declare the main method as private ?

Core Java

Ans. Yes, compiler won't complain but at runtime it will give an error saying "Error: Main method not found in class". Even though we can use this method as any other private method, it cannot be invocate by executing the class.

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Q139. Difference between GET and POST Requests ?

Web Service

Ans. GET is supposed to get information from the server. Client sends the minimal information so that Server can respond with the response body on basis of request. For example - You want to get complete employment record for employee id 123

POST is supposed to send the information for submission. Payload or a Body is usually sent so that it can be persisted on the server. For example - Sending the complete information of an employee ( id, name , dept etc ) to the server for persisting it.

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Q140. What are the different ways in which information can be sent to server in web service call ?

Web Service

Ans. 1. As Request Param
2. As Path Param
3. Part of Payload / Body in case of PUT and POST Request

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Q141. What is a static variable in Java ?

Core Java

Ans. It is a class level variable that is shared among the objects of that class.

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Q142. What is the purpose of Commit and Rollback in Databases ?

Database

Ans. When a DML is executed, the changes only stays in session and still not pushed to DB Tables, Commit is used to push those changes to the Tables.

In case we realize that we don't want to commit those changes and would like to ignore them, we can use rollback.

For example - You may like that for a banking transaction you would like to update the account balance only if the debit or credit record was correctly inserted, so you may like to encapsulate both DML's - insert for transaction and update for balance in a single transaction and would only commit if both succeeds else rollback.

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Q143. What is a self Join and give an example of a self Join ?

or

What is self Join and What is it's purpose ?

Database

Ans. When a Table Join itself , it's a Self Join. For example - we like to know the pair of department names where first dept has lesser employees than the later.

Select D1.name , D2.name from Dept D1, Dept D2 where D1.employee_count < D2.employee_count

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Q144. What is multilevel inheritance ?

Core Java

Ans. Multi Level Inheritance is multi level hierarchy of classes. Here is the example - http://www.buggybread.com/2015/09/java-se-class-diagram-classes-that_603.html

Class RoleList extends ArrayList which in turn extends AbstractList which in turn extends AbstractCollection.

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Q145. What is DAO ?

Ans. DAO is Data Access Object which is used within Persistence Layer to make Database Transaction. For example , the class holding the Database connection and CRUD methods.

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Q146. What is DTO ?

Ans. DTO is Data Transfer Object i.e Pojo which is supposed to move between different layers of software architecture for transferring information. Example could be the object passed from client in case of web service call or the object passed to Persister for Database Persistence.

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Q147. What is CRUD ?

Database

Ans. CRUD stands for "Create, read, update and delete" are the basic functions of persistence.

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All these acronyms JDK,JRE,JVM etc are very frequently asked.

Q148. Difference between JDK and JRE ?

Core Java

Ans. JRE or Java Runtime Environment is the Java Virtual Machine on which class files are executed. It includes borwser plugins that facilitates execution of class files in browsers.

JDK or Java Development Kit includes JRE , compiler and development tools.

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Frequently asked. Among first few questions in the J2EE segment of interview.

Q149. What is deployment descriptor ?

or

What is the use of deployment descriptor ?

Java EE

Ans. Deployment Descriptor which is usually web.xml is used to specify the classes, resources and configuration of the application and how the web server uses them to serve web requests.This file is usually added to WEB-INF folder and contains following

* Servlet entries and url mapping
* Plugins
* Some info regarding authentication / filters
* Landing Page
* Event Handlers

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Q150. What is the difference between declaration, instantiation and initialization ?

Core Java

Ans. Declaration is intimation to the compiler about the nature of Data a reference is going to hold.

For example - List myList;

Instantiation is reservation of memory.

For example

myList = new ArrayList();

Initialization or construction is setting the default values for member elements.

For example

myList = new ArrayList(mySet);

** Example 2nd is both for instantiation as well as initialization. The only difference is that 2nd will initialized the member elements to their default values whereas 3rd will initialized it with the elements from set.

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