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Q901. Is this code legal in java i.e nested class within interface implementing the parent interface ?

public interface MyInterface {
public class MyClass implements MyInterface {
}
}

Core Java

Ans. Yes that's legal in java

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Q902. Can an inner class be subclass of it's parent class ?

Core Java

Ans. Yes that can be done

public class OuterClass {
public class InnerClass extends OuterClass {
}
}

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Q903. What is the difference between inner class and sub class ?

Core Java

Ans. Inner Class is a class that is nested within another class whereas sub class is a class that extends or inherit another class.

Inner class can only be accessed using reference of outer class ( Class name , if inner class is static or object reference, if inner class is non static ) whereas Sub class is accessed directly.

In terms of memory, inner class is stored just like another class having it's own body whereas sub class carries the body of parent class as well as it's own fields in memory.

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Q904. Give an example of how Object Oriented Programming Concepts can be implemented.

Design

Ans. You can implement encapsulation in Java by keeping the fields (class variables) private and providing public getter and setter methods to each of them. Java Beans are examples of fully encapsulated classes. Encapsulation in Java: Restricts direct access to data members (fields) of a class.

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Q905. What is the difference between "Manual Scaling" and "Auto Scaling" ?

Server

Ans. Manual scaling is the process of scaling instances up and down in a an enviornment manually by observing the scaling trigger whereas Auto scaling is the process wherein a Trigger condition specified as a scaling trigger will trigger scaling up and down of instances.

for example - If an organization has a policy of scaling up if requests / sec / instance exceeds 10000, Operations team will have to manually monitor the metric and they can request scaling up and down as the need arise. Manual scaling offers flexibility as team can scale on the basis of combination of factors and can take decision on flyby requires continuous monitoring.

The same trigger can be fed as triggering policy to the environment and can be taken care by the system. This requires availability of such triggering policy within the environment configuration but as it's automatically handled, requires no supervision.

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Q906. How can we get count of all keys within Redis ?

Redis

Ans. We can either do DBSIZE or KEYS *

Keys * will return the list of all keys too so DBSIZE is preferred if we just want the count.

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Q907. How can we check all fields and values for a particular Redis key ?

Redis

Ans. HGETALL <key>

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Q908. How can we know how many instances are currently in service within an Elastic Beanstalk environment ?

Amazon Web Services (AWS)

Ans. We can go to respective environment and check configuration and then scaling. It mentions that as current status like

Current status:

2 instance(s) in service, Min: 1, Max: 4

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Q909. What do you understand by "Scaling Trigger" ?

Server

Ans. It's a configuration for auto scaling environment that specify the attributes / conditions for scaling instances up and down. For example - We may like environment to scale up when the number of requests / sec increases a particular threshold and scale down when it decreases below a threshold.

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Q910. Solve the producer and consumer problem

Operating System

^{This question was recently asked at 'Dell'.This question is still unanswered. Can you please provide an answer.}

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Q911. Design a Coffee machine using Oops concepts.

Design

^{This question was recently asked at 'ECI Telecom'.This question is still unanswered. Can you please provide an answer.}

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Q912. Explain OOPs

or

Explain OOPs Principles

or

Explain OOPs Concepts

or

Explain OOPs features

or

Tell me something about OOPs

Core Java

Ans. OOPs or Object Oriented Programming is a Programming model which is organized around Objects instead of processes. Instead of a process calling series of processes, this model stresses on communication between objects. Objects that all self sustained, provide security by encapsulating it's members and providing abstracted interfaces over the functions it performs. OOP's facilitate the following features

1. Inheritance for Code Reuse
2. Abstraction for modularity, maintenance and agility
3. Encapsulation for security and protection
4. Polymorphism for flexibility and interfacing

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Usually asked in relation to casting and ClassCastException.

Q913. What is instanceOf operator ? Explain it's use ?

Core Java

Ans. The operator instanceOf is used to verify if the specified object is the instance of specified class or interface.

Syntax if(x instanceOf ABC)

where x is an object reference and ABC could be a class name or interface name. The above statement will be true if x holds an object that is an instance of ABC or any of the child class of ABC or if x holds an object that implements ABC.

instanceOf operator is used to verify in case of downcasting. For ex -

DerivedClass extends BaseClass

x is the reference of BaseClass but holds DerivedClass object ( Polymorphism )

There is an operation that is defined in Derived Class, let's say derivedClassMethod()

We cannot call derivedClassMethod() directly using x as x is reference of BaseClass and not DerivedClass and hence can only access methods that are defined in BaseClass and overridden in derived class.

Though we can cast it to DerivedClass as following
((DerivedClass)x).derivedClassMethod();

But it may throw ClassCastException in case x doesn't hold an instance of DerivedClass at that point.

So before casting it to DerivedClass we may like to make sure that it is an instance of DerivedClass and hence won't throw ClassCastException.

So we make a check for it

if(x instanceOf DerivedClass) {
((DerivedClass)x).derivedClassMethod();
}

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Very frequently asked. Usually among very first few questions.

Q914. Define encapsulation in Java ?

Core Java

Ans. Encapsulation is a feature of OOP's that binds the data and it's associated methods together as a single unit and facilitate protection and data hiding by providing minimal interface to outside. For example - member variables are declared private and are accessed through public methods. Moreover we have private methods that can only be used internally and hence providing minimal interface to outside class through use of public methods.

Sample Code for encapsulation

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Q915. Write a Program for insertion sort.

Core Java

^{This question was recently asked at 'Ola Cabs'.This question is still unanswered. Can you please provide an answer.}

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Q916. What is the quickest way to find count of duplicate elements in an arraylist, without using iteration or loops ?

Core Java

Ans. We can copy the elements to a Set and then find the difference of count between ArrayList and Set. As Set don't allow duplicates , they will be removed in the set.

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Q917. What problem we could have with ArrayList which aren't possible with Vectors ?

Core Java

Ans. ArrayLists aren't synchronized and hence doesn't allow synchronized access. As multiple threads can access an arraylist in parallel, it may result in an inconsistent state.

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Q918. What is meant by "Vector is synchronized and ArrayList isn't " ?

Core Java

Ans. It means that only 1 thread can access have access to Vector at a time and no parallel access is allowed whereas Array List allows parallel access by multiple threads.

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Q919. Why iterators of an array list are fail fast ?

Core Java

Ans. Because it's access isn't synchronized and hence access / modification by multiple threads may lead to inconsistent state.

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Q920. How would you explain the statement "Java Programs are collection of objects" ?

Core Java

Ans. Java Programs are collection of objects that communicates with each other to get a task accomplished. To add to those objects, there are common spaces ( static i.e common for objects belonging to a class ) that are used too.

We can visualize objects as departments of an organization in real world. Just like Task gets initiated in one department and then files are moved across different departments to get work done. In a similar fashion, a task is initiated in one object ( having main method ) and then information ( through POJOs / DTOs ) is moved across objects to accomplish a task.

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Q921. What is the use of final keyword ?

Core Java

Ans. final keyword is a modifier that means differently when applied to variables, methods and classes.

1. Final variable cannot be changed once initialized
2. Final method cannot be overridden
3. Final class cannot be sub classed

Sample Code for final variable

Sample Code for final method

Sample Code for final class

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Q922. How can we hide a class in Java ?

Core Java

Ans. By encapsulating it within another class and declaring it private. In such a case, it will only be accessible through parent class or parent class object.

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Q923. Why can't we declare on outer class as private ?

Core Java

Ans. Making an outer class as private will make it inaccessible from anywhere and would make it useless.

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Q924. Can we declare a class private ?

Core Java

Ans. We can declare an inner class as private and hence would be restricting it's access from outside. We cannot make outer class as private.

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Q925. What happens to private variables during inheritance ?

Core Java

Ans. Private variables are not inherited. They are simply ignored and not made part of the derived object body.

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Q926. What is the difference between single instance Web environment and Load Balanced Auto Scaling ?

Amazon Web Services (AWS)

Ans. Single instance means there is only 1 instance that will bear all the traffic load whereas Load balanced server means that there will be a cluster of servers that will host the application and load will be balanced distributed among them. Auto Scaling means that the number of instances will be expanded / shrunken based on the rule. Rule could be the traffic count , response time etc.

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Q927. Difference between Load Balancing and Failover Clustering ?

Server

Ans. Load Balancing is a provision to improve performance by reducing load on a single machine / server. Traffic or load is distributed among different machines and hence resulting in better performance / response time by leveraging more resources.

Failover is a provision to achieve better availability by switching to a backup server in case of failure.

Load Balancing aims as improving performance whereas failover aims at improving availability.

We can achieve both together by using a Load Balancing system which will isolate an instance in case of it's failure.

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Q928. What is Boxing conversion ?

Core Java

Ans. Boxing conversion converts expressions of primitive type to corresponding expressions of reference type.

For example

boolean to Boolean
long to Long
double to Double

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Q929. What is AWS CloudWatch ?

Amazon Web Services (AWS)

Ans. Amazon CloudWatch is a monitoring service for AWS cloud resources like Beanstalk and DB instances. We can use CloudWatch to monitor the state of resources, collect metrices, set alarms and appropriate response to the alarm state.

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Q930. What are the types of applications we can create using Elastic Beanstalk ?

Amazon Web Services (AWS)

Ans. Web Server and Worker

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