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

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Q181. What should a class do if its extending an abstract class ?

Core Java

Ans. It should either implement the abstract methods or re-declare them abstract.

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Q182. What are different types of variables in Java ?

Ans. 1. Function Local Variables
2. Instance Variables
3. Class variables or static variables

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Frequently asked to Fresh graduates.

Q183. Write a program to implement Binary search ?

Core Java

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

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Q184. 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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Q185. 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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Very Frequently asked to fresh graduates and less experienced.

Q186. What are the principle concepts of Object Oriented Programming ?

Core Java

Ans. Abstraction Polymorphism Inheritance Encapsulation

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Q187. 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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All these acronyms JDK,JRE,JVM etc are very frequently asked.

Q188. 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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Q189. What is CRUD ?

Database

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

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Q190. 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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Q191. 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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Q192. 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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Q193. 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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Q194. 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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Q195. 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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Q196. 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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Q197. 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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Q198. 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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Q199. 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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Q200. 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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Q201. 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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Q202. 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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Q203. Can there be a Thread without a Process ?

Core Java

Ans. No.

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Very frequently asked.

Q204. 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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Q205. 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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Q206. What is a web service ?

Java EE

Ans. A Web service is a service offered by one system to another, for communication over web through http. XML are JSON are usually used for sending across information from one system to another.

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Q207. What is the difference between http and https ?

Java EE

Ans. https encrypts the data using SSL whereas http sends it as plain text, So https is secure protocol whereas http is not.

Moreover https connects on port 443, while HTTP is on port 80

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Q208. Difference between Array and ArrayList ?

Core Java

Ans. <a href="http://javahungry.blogspot.com/2015/03/difference-between-array-and-arraylist-in-java-example.html" rel="nofollow">http://javahungry.blogspot.com/2015/03/difference-between-array-and-arraylist-in-java-example.html</a>

Sample Code for ArrayList

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Q209. Difference between Error and Exception ?

Core Java

Ans. An Error indicates serious problems that a reasonable application should not try to catch whereas

An Exception indicates conditions that a reasonable application might want to catch.

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Q210. Which method of String class is used to convert Boolean to String ?

Core Java

Ans. toString() is an overloaded method of String class that is used to convert many data types to String, Boolean being one of them.

toString(Boolean bool)

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