Java Interview Question Answer for Experience

How many types of Operations in Java Stream?

There are mainly 3 types of operations.

1. Intermediate operations

The intermediate operation returns another stream as a result, they can be chained together to form a pipeline of operations.
- filter()
Filter the elements based on a specified condition. details...
- map()
Transforms each of the elements in a stream to another value. details...
- sorted()
Sorts the elements of a stream. details...

2. Terminal operations

The terminal operation means after calling this method on Stream, you cannot call any other method on Stream.
- forEach()
It iterates all the elements in a stream.
- collect()
The stream.collect() method accumulate elements of any Stream into a Collection, it is a part of stream terminal operation. The Collector class provides different methods like toList(), toSet(), toMap(), and toConcurrentMap() to collect the result of Stream into List, Set, Map, and ConcurrentMap in Java. It accepts a Collector in arguments to accumulate elements of Stream into a specified Collection such as stream().collect(Collectors.toList()).
- anyMatch() and allMatch()
The anyMatch operation checks if any element of a stream matches a given condition, while the allMatch operation checks if all elements   satisfy the given condition.
- count()
The count operation returns the number of elements in a stream as a long value
- reduce()
It is used to reduce the elements of a stream to a single value.

3. Short-circuit operations

- findFirst()
The findFirst() method finds the first element in a Stream. we use this method when we specifically want the first element from a   sequence.
- anyMatch()
It checks the stream if it satisfies the given condition.

What is filter() method in Java Stream?

The stream filter() method provides the functionality to filter stream's elements on the basis of given lambda expression or predicate. The stream filter() method expects a lambda expression or predicate in argument and the lambda expression returns a boolean(true/false) value, based on boolean value filter method filters the elements.

1. filter() is an intermediate stream operation.
2. filter method expects a predicate in argument.
3. The Predicate. is a functional interface. So, you can also pass lambda expression here.
4. filter() method does not change/modify the original source/collection.
5. filter() method creates a new collection(list/array) based on the condition and returns the new collection.
6. Stream does not modify the source. It produces a new Stream by filtering or applying any logic.

//Importing required classes
    import java.util.*;
    import java.io.*;
    import java.util.stream.*;
    
    public class FilterExample {
        static void main ( String args[]) {
    
        Integer[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };
        //Filtering the even numbers from a collection
        List<Integer> evenList = 
        Arrays.asList(numbers)
            .stream()
            //passing lambda expression in argument
            .filter(i -> i % 2 == 0)
            .collect(Collectors.toList());            
        System.out.println( "Filtered even numbers : "+ evenList );
     }
    }
    
    Output:
        Filtered even numbers : [2, 4, 6, 8, 10, 12]
    
    

What is map() method in Java Stream?

The stream map() method is an intermediate operation. This method transforms elements of a stream by applying a mapping function to each of the elements and returns a new stream. It accepts a Function mapper object as an argument. This function describes how each element in the original stream is transformed into an element in the new stream.

1. Stream map(Function mapper) is an intermediate operation.
2. Stream map() method accepts a Function mapper object in argument.
3. Stream map() method is used to transform elements of a stream by applying a mapping function to each of the elements.
4. Stream map() method returnes a new stream.

//Importing required classes
import java.util.*;
import java.io.*;
import java.util.stream.*;

public class StreamMapExample {
    public static void main (String args[]){ 
    Integer[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 };
    //Filtering the even numbers from a collection
    List<Integer> evenList = 
    Arrays.asList(numbers).stream() 
        //passing lambda expression in argument
        .filter(i -> i % 2 == 0)
        //multiplying by 2 for each element after filter
        .map(i -> i*2)
        .collect(Collectors.toList());
                
    System.out.println("Filtered even numbers : "+ evenList);
 }
}


Output:
    Filtered even numbers : [4, 8, 12, 16, 20, 24]

What is Docker?

Docker is a container management service. It is an open-source platforms used for developing, packaging, and deploying applications. It is a container management service and it was released in 2013.

You need to install the Docker engine on your device. The Docker engine allows to create and manage docker containers, docker images, docker hub, docker desktop etc.

What is Docker container?

Container is a logical packaging process and runnable instance of an images.

What is Docker Compose?

Docker Compose is used to run multiple containers as a single service. You need to create a YAML file and provide details in the YAML file. Suppose database image name with the port, username, password etc. Docker Compose allows you to define and manage multi-container applications in a single YAML file.

For Example of docker-compose.yml

version: '3.7'
    services:
      app:
        image: openjdk:17-jdk-slim
        ports:
          - 8000:8000
      redis:
        image: redis
        volumes:
        - ./data:/data
        ports:
        - "6379:6379"
      web:
        image: tomcat
        ports:
        - '8082:8080'
      db:
        image: postgres:14.1-alpine
        restart: always
        environment:
          - POSTGRES_USER=username
          - POSTGRES_PASSWORD=*******
        ports:
          - '5432:5432'

How to run Docker Compose?

- Create a docker-compose.yaml file in the root directory in your application.
- Open command prompt and go to the directory where you have created docker-compose.yaml file.
- Run command as docker compose up in the command prompt.

What is Redis?

Redis stands for Remote Dictionary Server. It is an open source NoSQL database. It doesn’t have any tables, rows and columns. Also, it doesn’t allow statements like select statement, insert statements, update statements etc.

It stores objects using key/value and basically used to cache and quick-response in an application to speed or increase performance.

Various ways of Redis

1. In-Memory Database
2. Cache

What is In-Memory Database?

In-Memory Database(IMDS) enables minimal response times. It provides responses in microsecond of times. It is the good choice for the application if an application handles the large numbers of traffic.

Advantages:

1. Low latency - provides real time responses
2. High throughput
3. High scalability

What is Cache

The cache provides data quickly with high latency to increase the data retrieval performance. Redis is the good choice for caching API calls, session states, and database queries etc.

Advantages:

1. High latency - provides data quickly and increase performance.
2. Faster access to cached data.
3. Response time is faster because data is retrieved from memory.

How Redis Works?

When a request comes, the code of logic checks in the Redis cache for the desired data. If appropriate data is available in the redis cache, logic simply returns the data from redis cache.

If the requested data is not available/found in the redis cache, the logic falls back to the database or original data source to retrieve the required information to return required data. Subsequently, the fetched data is stored in the Redis cache.

What is Ehcache?

EhCache is an open source and Java based library, it is used to cache data to increase the performance. It is lightweight and Flexible, we can easyly integrate in the application.

1. On-Heap - Heap memory is used to store cache.
2. Off-Heap - RAM memory is used to store cache.
3. Disk - Disk is used to store cache.
4. Clustered: Remote server is used to store cache.

What is messaging system?

Messaging system is the process of exchanging messages between two or more sources, end-points, servers etc. There are two processes of messaging system.

1. Sender or Producer
The responsibility of sender is to send/write the messages. The sender is also known as Producer who publishes the messages.
2. Receiver or Consumer
The responsibility of receiver is to read that messages. The receiver is also known as Consumer who consumes the messages.

What is collect() method in Java Stream?

The stream.collect() method accumulate elements of any Stream into a Collection, it is a part of stream terminal operation. The Collector class provides different methods like toList(), toSet(), toMap(), and toConcurrentMap() to collect the result of Stream into List, Set, Map, and ConcurrentMap in Java. It accepts a Collector in arguments to accumulate elements of Stream into a specified Collection such as stream().collect( Collectors.toList() ), stream().collect( Collectors.toSet() ), stream().collect (Collectors.toMap(....) ), stream().collect( Collectors.counting()) etc..

It is used to perform different types of reduction operations such as calculating the sum of numeric values, finding the minimum or maximum number, concatenating strings to new string, collecting elements into a new stream.

1. The stream.collect() method is a part of stream terminal operation.
2. The stream.collect() method accumulate elements of any Stream into a Collection.
3. It is used to convert to list using, stream().collect(Collectors.toList()).
4. It is used to convert to map using, stream().collect(Collectors.toMap()).
5. It is used to convert to set using, stream().collect(Collectors.toSet()).
6. It is used for calculating the sum of numeric values.
7. It is used for collecting elements into a new stream.
8. It is used for concatenating strings to new string.
9. It is used for finding the minimum or maximum number.

//Importing required classes
    import java.util.ArrayList;
    import java.util.List;
    import java.util.stream.*;
    
    public class CollectMethodExample {
        public static void main(String args[]){
        //Declaring List
        List<Integer> intList = new ArrayList< Integer >();
        intList.add( 6 );
        intList.add( 11 );
        intList.add( 14 );
        intList.add( 7 );
        intList.add( 3 );
        intList.add( 2 );
        intList.add( 16 );
        intList.add( 15 );
        intList.add( 19 );
    
        List<Integer> evenNumList = 
            intList.stream()
            .filter( i-> i%2==0 )
            //Converting filtered data to a List
            .collect( Collectors.toList() );
    
        System.out.println( "Even Num List" +evenNumList )
    
        }
    }
    
    Output:
        Even Num List [6, 14, 2, 16]
    

What is Streaming In The Messaging system?

The Streaming process allows parallel execution of the data, where one record executes without waiting for the output of the previous record. In a distributed system normally we see the streaming process and parallel execution to simplify the task and improve performance. Its executes the thread/data without waiting for another.

What is Apache Kafka?

Apache Kafka is an open source platform and distributed messaging system with publish and subscribe events. It is a mediator between the producer and consumer systems. The producer send data to the Kafka, where consumer consumes the data from Kafka.

Basically Apache Kafka is a Messaging system to pass messages between applications, the messaging sytem means its simple exchanging of messages between two or more sources, end-points, servers etc. It can handle high volume of data and enables us to send messages from one source to another source.

Kafka was originally developed by LinkedIn and later it was donated to the Apache Software Foundation.
Kafka Complete Tutorial...

What is Kafka Cluster?

The Kafka cluster is a group of Kafka brokers that work together to handle the data or messages for Kafka system. Each broker is a separate process that runs on a different machine and communicates with other brokers through a network.

1. The Kafka cluster is a group of Kafka brokers.

2. Each broker is a separate process that runs on a different machine and communicates with other brokers through a network.

3. Kafka cluster contains ZooKeeper for maintaining their cluster state.

4. Kafka Cluster contains Kafka brokers, brokers contains topics, topic contains partitions and they works together.

What is Kafka Topic?

The kafka topic is a category or a common name used to store and publish of stream of messages/data. Basically, data or messages stores in topics and messages are sent to specific topic and read from specific topic. Each topic has a name that should be unique across the entire Kafka cluster.

A topic can be splited into several parts which are known as the partitions of the topic. We need to specify the number of partitions while creating a topic. The message/data stored into a partition with an incremental id known as its Offset value.

1. The kafka topic is a category used to store and publish of messages/data.

2. Each topic has a name that should be unique across the entire Kafka cluster.

3. A topic can be splited into several parts which are known as the partitions of the topic.

4. We need to specify the number of partitions during creating a topic.

5. The message stored into a partition with an id known as its Offset value.

What is Kafka Broker?

A single Kafka server is called a Kafka Broker. The Kafka Broker is the container of topics, it holds several topics with their multiple partitions. Each broker is a separate process that runs on a different machine and communicates with other brokers through a network.

A cluster can have multiple brockers and each broker is identified by a unique numeric ID and the brocker contains certain topic partitions. All the topic partitions data is Distributed across all brokers(load balanced).

1. A single Kafka server is called Kafka Broker.

2. The Kafka Broker is the container of topics, it holds several topics with their multiple partitions..

3. A cluster can have multiple brockers and each broker is identified by a unique numeric ID.

4. the Brocker contains certain topic partitions.

What is Kafka Consumer?

The main purpose of the Kafka Consumer is to read data from a topic. The Kafka consumer fetches the messages from topic's partitions. The Kafka Consumer required Deserializer to transform bytes to object to identify key and value of messages.

If a consumer reads data from a topic's partition, data/messages comes in order way such as message 0, message 1, message 2, message 3, message 4, message 5 and so on.

If a consumer reads data from a multiple topic's partitions, there is no ordering across partitions.

1. The main purpose of the Kafka Consumer is read data from a topic.
2. The Kafka consumer fetches the messages from topic's partitions.
3. The Kafka Consumer required Deserializer to transform bytes to object to identify key and value of messages.
4. If a consumer reads data from a topic's partition, data/messages comes in order way such as message 0, message 1, message 2, message 3, message 4, message 5 and so on.
5. If a consumer reads data from a multiple topic's partitions, there is no ordering across partitions.
6. Example of KeyDeserializer=IntegerDeserializer.
7. Example of ValueDeserializer=StringDeserializer.

What is Kafka Producer?

The kafka Producer is a client application that write messages and sends events to a Kafka cluster. A Kafka producer sends messages to a topic, where messages are distributed to partitions according key hashing process.

How Kafka Producer Publish Data To The Cluster or Topic?

There are two strategies to publish data to topics.

1. Message Keys

2. Acknowledgment

What is Stream API?

Stream API are designed to be efficient and can support improving the performance of programs. We can avoid unnecessary loops and iterations in our programs by using Stream functionality. Streams can be used for filtering, collecting, printing, and converting data from one structure to another such as one collection to another collection.

1. Java 8 introduces Stream API, and some additional features to allows functional-style operations on the elements.
2. The stream can be used by importing java.util.stream package.
3. The stream API is used to process collections of objects.
4. A Stream is a sequence of components that can be processed sequentially.
5. The elements of a stream are only visited once during the life of a stream.
6. Stream does not modify the source. It produces a new Stream by filtering or applying any logic.

Example of filterig data using stream API

import java.io.*;
import java.util.*;
import java.util.stream.*;

public class StreamFilterExample {
  static void main (String args[]){ 
    int[] numbers = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

    //Filtering the even numbers from a collection
    List<Integer> evenList = 
        Arrays.asList(numbers).stream() 
            .filter(i -> i % 2 == 0)
            .collect(Collectors.toList());
                  
    System.out.println( "Even numbers after stream operation : "+ evenList );         
          
 }
}
 

Output:
    Even numbers after stream operation : [2, 4, 6, 8, 10]

What is Functional Interface In Java?

An Interface that contains exactly one abstract method is known as functional interface. It can have any number of default and static methods but can contain only one abstract method. Functional Interface is the approach of functional programming in Java. It is also known as SAM interfaces Single Abstract Method Interfaces.

Lambda expressions

Rules for a functional interface?

1. A functional interface must have exactly one abstract method.
2. A functional interface can have any number of default, static methods
3. No need to declare the abstract keyword as by default the method defined inside the interface is abstract only.

Example of functional interface with Lambda Expression

/*@FunctionalInterface*/ //It is optional
public interface MultipleInterface {
  public int multiple(int x,int y);
}

public class Calculate{
 public static void main (String[] args) {
   //with lambda
 MultipleInterface m = (x, y) -> {
     return x * y;
   };

   int multipleVal =   m.multiple(10,30);
   System.out.println( "Multiple Value is "+multipleVal);
  }
}

Output:
    Multiple Value is 300

What is @FunctionalInterface Annotation?

@FunctionalInterface annotation is optional but it is used to ensure that the functional interface can’t have more than one abstract method. If you add more than one abstract methods, an "Unexpected @FunctionalInterface annotation" message you can get. It is not mandatory to use this annotation.

Example of functional interface with @FunctionalInterface annotation

//It is optional
@FunctionalInterface
interface Area {
	int calculateArea(int x);
}

class Square {
  public static void main (String[] args) {
	//with lambda
    Area a = (x) -> {			
        return x * x;
    };

    int areaVal =   a.calculateArea(20);
    System.out.println( "Area of Square is: "+areaVal);
  }
}

Output:
    Area of Square is: 400

How many types of functional interfaces?

There are four types of functional interfaces.

1. Consumer -> Bi-Consumer  details..
2. Predicate -> Bi-Predicate     details..
3. Function -> Bi-Function, Unary Operator, Binary Operator  details..
4. Supplier  details..

What is Consumer Interface?

The consumer interface of the functional interface accepts only one single argument without any return value. The consumer interface has no return value. It contains an abstract accept() and a default andThen() method. It can be used as the assignment target for a lambda expression or method reference. Example of Consumer Interface..

Consumer Interface Methods.

1. void accept(T t)
The accept() method takes as input the type T and returns no value. This is the single abstract method of Consumer. It accepts a single generic argument of type T and returns nothing (i.e. void). This is the method that's implemented by a lambda expression or method reference.
2. andThen(Consumer after)
- The andThen() method is a default method. The method accepts a Consumer and returns another Consumer.

What is Predicate Interface?

Predicate accepts an argument and returns a boolean value. Predicate functional interface in Java is a type of function that accepts a single argument and returns a boolean (True/ False). It provides the functionality of filtering, it filters a stream components on the base of a provided predicate. Example of predicate interfaces..

Predicate Interface Methods.

1. boolean test(T t)
The Predicate test() method returns true if the input argument matches the predicate, otherwise false.
2. default Predicate and(Predicate other)
The Predicate and() method is used to compose two expression(lambda) that represents a short-circuiting logical AND of this predicate. It can be uses to perform logical operations by combining two lambda expressions. See the example below. Here, we used and() method to check whether a number lies between two numeric range or not.
3. default Predicate negate()
The Predicate negate() method allows invert the result of a Predicate. It returns a predicate that represents the logical negation of this predicate.
4. or(Predicate other)
The Predicate or() method works similarly to and(), but it returns true if either of the any Predicates return true.

What is Function Interface?

The function type functional interface receives a single argument, processes it, and returns a value. Functional interface is taking the key from the user as input and searching for the value in the map for the given key.

public static void main (String[] args) {  
    private static HashMap<Integer, String> Employee = 
                new HashMap<>();
    Function<Integer, String> addFunc = (Integer ID)-> {
        if(Employee.containsKey(ID)) {
            return Employee.get(ID);
        }else{
            return "Invalid ID";
        }
    };
    // Returns the Employee object if present in the hashmap or return Invalid Id message        
    System.out.println( addFunc.apply( 500 )); 
    };

What is Supplier Interface?

The Supplier functional interface is also a type of functional interface that does not take any input or argument and returns a single output. The Supplier interface takes only one generic type, the type of data it is going to return. get() is the abstract method of the Supplier.

public static void main (String[] args) { 
    Supplier<String> supp = () -> "Hello World";
    System.out.println( supp.get() );        
};

Output:
    Hello World

What is Lambda Expression In Java?

Lambda expression is similar to java method, but lambda expression does not need name and it can be implemented in the body of a method. It is an instance of functional interface. An interface with a single abstract method is called functional interface.
The lambda expression is a short block of code which takes parameters and returns a value

1. Lambda expression is similar to java method but without name.
2. Lambda expression can be created without belonging to any class, it can be implemented in the body of a method.
3. Lambda expression is an instance of functional interface.
4. Lambda expression is a short block of code which takes parameters and returns a value.

Example of lambda expression

/*@FunctionalInterface*/   //It is optional
public interface MultipleInterface {
    public int multiple(int x,int y);
}

public class Calculate{
  public static void main (String[] args) { 
    //with lambda
        MultipleInterface m = (x, y) -> {
        return x * y;
      }

    int multipleVal =   m.multiple(10,30);
    System.out.println( "Multiple Value is "+multipleVal);
}

Output:
    Multiple Value is 300

What is difference between accept() and andThen() method in Java?

The accept() and andThen()methods are methods of Consumer Interfaces.

Consumer Interface Methods.

1. void accept(T t)
The accept() method takes as input the type T and returns no value. This is the single abstract method of Consumer. It accepts a single generic argument of type T and returns nothing (i.e. void). This is the method that's implemented by a lambda expression or method reference.
andThen(Consumer after)
The andThen() is a default method. In other words, it has an implementation and is thus non-abstract. The method accepts a Consumer and returns another Consumer.

What is sorted() method in Java?

Stream interface provides a sorted() method to sort a list. The sorted() method returns a sorted stream according to the natural order. If the elements are not comparable, it throws java.lang.ClassCastException.

1. Stream sorted() method is an intermediate operation.
2. Stream interface provides a sorted() method to sort a list.
3. Stream sorted() method returns a sorted stream according to the natural order.
4. If the elements are not comparable, it throws java.lang.ClassCastException.

Example of stream sorted method in Java.

//Importing required classes
import java.util.*;
import java.io.*;
import java.util.stream.*;

public class StreamSortedExample {
public static void main ( String args[]){ 
Integer[] numbers = { 100, 20, 13, 440, 5, 16, 7, 28, 2, 11, 1 };
ListInteger> sortedList = 
Arrays.asList(numbers).stream() 
.sorted()
.collect( Collectors.toList() );
        
System.out.println( ""Sorted list : "+ sortedList ); 
}
}


Output:
Sorted list : [1, 2, 5, 7, 11, 13, 16, 20, 28, 100, 440]

What is reduce() method in Java?

The reduce() method is a part of terminal operations in java stream. It performs operations to produce a single value as result by reducing, such as average(), sum(), min(), max(), and count() etc. The reduce() method applies the binary operator to each of the elements to reduce it to a single value. The return type of the reduce() method is type T or it can be an Optional.

1. We can use Stream.reduce()
2. Syntex -> Optional<Integer> max = intList.stream().reduce(( i, j ) -> i > j ? i : j);
3. The return type of the reduce() method is type T or it can be an Optional.
4. The reduce method is a part of terminal operations in java stream.
5. It performs operations to produce a single value as result by reducing, such as average(), sum(), min(), max(), and count() etc.
6. The reduce() method applies the binary operator to each element to reduce it to a single value.

Example of Java Reduce Method

//Importing required classes
import java.util.ArrayList;
import java.util.List;
import java.util.stream.*;

public class ReduceMethodExample {
  public static void main(String args[]){
    //Declaring List
    List<Integer> intList = new ArrayList<Integer>();
    intList.add( 6 );
    intList.add( 11 );
    intList.add( 14 );
    intList.add( 7 );
    intList.add( 3 );
    intList.add( 2 );
    intList.add( 16 );
    intList.add( 15 );
    intList.add( 19 );

    Optional<Integer> max = 
        intList.stream()
        .reduce(( i, j ) -> i > j ? i : j);

    System.out.println( max )
    System.out.println( max.get() )
}
}

Output:
Optional[19]
19

What is Predicate test() method in Java Stream?

The Predicate test() method returns true if the input argument matches the predicate, otherwise false.

Example of Predicate test() method.

public static void main( String[] args){ 
    Predicate<Integer> pr = a -> ( a > 10 );
    
    // Calling Predicate method  
    System.out.println( pr.test(20) );       
    } 
    
    Output:
        true

What is Predicate isEqual() method in Java Stream?

The Predicate isEqual() method is a static method of the Predicate interface that is used to test the equality of two objects. The object can be a string, integer, or a class object.

Example of Predicate isEqual() method.

public static void mai( String[] args) {  
Predicate<String> pr = 
        Predicate.isEqual( "MyAllText" );

// Calling Predicate method  
System.out.println(
    pr.test( "AllText" )
); 
// Calling Predicate method  
System.out.println(
    pr.test( "MyAllText" )
);
// Calling Predicate method  
System.out.println(
    pr.test( "MyAll" )
);        
}

Output:
    false
    true
    false

What is Predicate and() method in Java Stream?

The Predicate and() method is used to compose two expression(lambda) that represents a short-circuiting logical AND of this predicate. It can be uses to perform logical operations by combining two lambda expressions. See the example below. Here, we used and() method to check whether a number lies between two numeric range or not.

Example of Predicate and() method

public static void main( String[] args) {   
    Predicate<Integer> pr1 = 
        x -> (x > 20);
    
    Predicate<Integer> pr2 = 
        x -> (x < 80);
    
    // Calling Predicate method  
    System.out.println(
        pr1.and( pr2 ).test(100)
    ); 
    // Calling Predicate method  
    System.out.println(
        pr1.and( pr2 ).test(50)
    ); 
    // Calling Predicate method  
    System.out.println(
        pr1.and( pr2 ).test(10)
    );
    }
    
    Output:
        false
        true
        false
    

What is Predicate negate() method in Java Stream?

The Predicate negate() method allows invert the result of a Predicate. It returns a predicate that represents the logical negation of this predicate.

public static void main( String[] args) {
    Predicate<String> startsWithCharJ = 
        s -> s.startsWith( "H" );
    Predicate<String> hasLengthOfInt5 = 
        s -> s.length() == 5;
        System.out.println("**Before Negating**")
        // Calling without negate method 
        System.out.println(
        startsWithCharJ.test( "Horse" )
        ); 
        // Calling without negate method 
        System.out.println(
        hasLengthOfInt5.test( "India" )
        ); 
    
    // Here we are negating 
    Predicate<String> negateStartsWithCharJ = 
        startsWithCharJ.negate();
    Predicate<String> negatehasLengthOfInt5 = 
        hasLengthOfInt5.negate();
    
        System.out.println("**After Negating**")
        
        // Calling after negate predicate 
        System.out.println(
            negateStartsWithCharJ.test( "Horse" )
        ); 
        // Calling after negate predicate 
        System.out.println(
        negatehasLengthOfInt5.test( "India" )
        );     
    } 
    
    Output:
        **Before Negating**
        true
        true
        **After Negating**
        false
        false
    
    

What is Predicate or() method in Java Stream?

The Predicate or() method works similarly to and(), but it returns true if either of the any Predicates return true.

Example of Predicate or() method

public static void main( String[] args) { 
Predicate<String> startsWithCharH = 
    s -> s.startsWith( "H" );
Predicate<String> hasLengthOfInt5 = 
    s -> s.length() == 5;

Predicate<String> startsWithCharHOrHasLengthOf5 = 
    startsWithCharH.or( hasLengthOfInt5 );

    System.out.println( 
        startsWithCharHOrHasLengthOf5
        .test( "Hero" )
    ); 
    System.out.println(
        startsWithCharHOrHasLengthOf5
        .test( "India" )
    ); 
    System.out.println(
        startsWithCharHOrHasLengthOf5
        .test( "Method" )
    );
} 

Output:
    true
    true
    false

What is forEach() method in Java Stream?

The forEach() method is a default method defined in the Iterable and Stream interface. It is a terminal operation in the stream API. The purpose of the forEach() method is iterating the elements one by one of a collection to create a result. The forEach() method accept a lambda expression in argument.

1. The forEach() method is a default method in the Iterable and Stream interface.
2. The forEach() method is a terminal operation.
3. The forEach() method iterates the elements one by one of a collection to produce a result.
4. The forEach() method accept accept a lambda expression in argument.

//Importing required classes
import java.util.ArrayList;
import java.util.List;

public class ForEachExample {
    public static void main(String args[]){
    //Declaring List
    List<String> countryList = new ArrayList< String >();
    countryList.add( "Afghanistan" );
    countryList.add( "Albania" );
    countryList.add( "Algeria" );
    countryList.add( "Australia" );
    countryList.add( "India" );
    countryList.add( "Brazil" );
    countryList.add( "Japan" );
    countryList.add( "Italy" );

    //forEach method to printing country names
    countryList.forEach( country -> System.out.println( country ) );
    
     }
}

Output:
    Afghanistan
    Albania
    Algeria
    India
    Australia
    Brazil
    Japan
    Italy

What is count() method in Java Stream?

The stream count() method is a part of terminal operations in java stream. The count() method returns the count of elements in the stream. The return type of count() method is long. It is the reduction process, it may traverse the stream to produce a result.

1. The count() method is a part of terminal operations in java stream.
2. The count() method returns the count of elements in the stream.
3. The return type of count() method is long.
4. It is the reduction process, it may traverse the stream to produce a result.
5. You can use like list.stream().count().

//Importing required classes
import java.util.ArrayList;
import java.util.List;
import java.util.stream.*;

public class CountMethodExample {
    public static void main(String args[]){
    //Declaring List
    List<Integer> intList = new ArrayList< Integer >();
    intList.add( 6 );
    intList.add( 11 );
    intList.add( 14 );
    intList.add( 7 );
    intList.add( 3 );
    intList.add( 2 );
    intList.add( 16 );
    intList.add( 15 );
    intList.add( 19 );

    //calling count method
    long count = intList.stream().count();

    System.out.println( count )
    
    }
}

Output:
    9

What is findFirst() method in Java Stream?

The findFirst() method is a part of Short circuit operation in Java Stream. It returns an Optional 1st element of the stream or empty if the stream is empty.

1. The findFirst() method is a part of stream short circuit operation.
2. It returns an Optional 1st element of the stream or empty if the stream is empty.
3. Syntex is Optional<T> findFirst().

//Importing required classes
import java.util.ArrayList;
import java.util.List;
import java.util.Optional;

public class FindFirstExample {

    public static void main(String args[]) {
    List<String> strList = new ArrayList<String>();
    strList.add("One");
    strList.add("Two");
    strList.add("Three");
    strList.add("Four");
    strList.add("Five");

    Optional<String> firstElement = strList.stream().findFirst();

    System.out.println(firstElement);
    System.out.println(firstElement.get());
 
    }
}

Output:
Optional[One]
One
    

What is ConcurrentHashMap in Java?

ConcurrentHashMap is an enhancement of HashMap. It helps to improve the performance with threads in an application. ConcurrentHashMap is a thread-safe, multiple threads can access it simultaneously without any synchronization issues. ConcurrentHashMap introduced in JDK 1.5 and available in the java.util.concurrent package.

1. The structure for ConcurrentHashMap is Hashtable.
2. ConcurrentHashMap class is thread-safe.
3. Default size of the ConcurrentHashMap is 16.
4. You can not put the null object as key or value in the ConcurrentHashMap.
5. Any number of threads can access at a time for read read operation.
6. ConcurrentHashMap divides object into a number of segments and lock the particular segment in the case of updating.

Example of ConcurrentHashMap in Java

//Importing required classes
import java.util.concurrent.ConcurrentHashMap;
public class ConcurrentHashMapExample {
    public static void main(String args[]){
        ConcurrentHashMap map = new ConcurrentHashMap<String, Integer>();
        map.put("A1", 100);
        map.put("A2", 200);
        map.put("A3", 300);

        System.out.println("Map size is " + map.size());

        int valueA1 = map.get("A1");
        System.out.println("Value of A1 is " + valueA1);

        map.remove("A3");
        System.out.println("Map size " + map.size());
     
    }
}

Output:
Map size is 3
Value of A1 is 100
Map size 2

Explain Internal Working of HashMap in Java?

HashMap is introduced in 1.2. HashMap in Java stores the object in Key - Value pairs, the objects are accessible by key or an index. If you try to insert the duplicate key, it will replace the value of old with new value of that key.

HashMap contains an array of Nodes and Node is a class with below attributes.

1. int hash
2. K key
3. V value
4. Node next

Node Class

Hashmap uses the Hashing process to store the objects in a Bucket. First, you need to understand about the Hashing process and Bucket.

Hashing Process

Hashing is a process of converting an object into integer form by using the method hashCode().

1. Buckets

Array of the node is called buckets. Each node has a data structure like a LinkedList. More than one can be in the same bucket. The every object refer to a node to store in the HashMap and each nodes refer to the buckets. So, basically buckets is the container of nodes.

2. hashCode() method -

This is the method of the object class. It returns an integer value of memory reference of the object. The hashCode() returns an integer value of memory reference of the object and that is used to store object in a bucket in the Hashmap.

3. equals() method -

When we put an object into HashMap, suppose hascode() method returned a value that is already exist inthe bucket in the HashMap then equals() method comes in the process to compares the Key with existing object's key, whether they are equal or not. If equal then simply HashMap replace the value of old with new value with the same key. Otherwise, both objects will be stored in the same node sing LinkedList data structure and next will be a pointer to another node.

4. Calculating Index -

Index = hashcode(Key) & (n-1). Interally uses bitwise AND operator to convert binary to decemal.
Example:
Suppose, binary value of hashcode(Key) & (n-1) is 1010
1. length of 1010=4
2. decimal = 1*(2*2*2) + 0*(2*2) + 1*(2) + 0;
3. decimal = 8+0+2+0
4. decemial = 10
Object will be stored in the 10th bucket.

What is HashTable in Java?

A Hashtable is an array of a list. Each list is known as a bucket. Hashtable class contains unique elements, It doesn't allow null key or null value Hashtable class is synchronized. The default capacity of Hashtable class is 11 and loadFactor is 0.75.

What is HashTable in Java?

A Hashtable is an array of a list. Each list is known as a bucket. Hashtable class contains unique elements, It doesn't allow null key or null value Hashtable class is synchronized. The default capacity of Hashtable class is 11 and loadFactor is 0.75.

Difference between HashMap and Hashtable?

1. HashMap is non synchronized and Hashtable is synchronized.
   * HashMap is not-thread safe so its no a good approach to shared between many threads without synchronization.
   * Hashtable is thread-safe and you can share with many threads.
2. You can add one null key and multiple null values in the HashMap but Hashtable doesn't allow any null key or value.
3. HashMap is fast as it is non synchronized whereas, Hashtable is slow as it is synchronized.
4. Iterator is used to iterate a HashMap whereas, Enumerator and Iterator both are used to iterate a HashTable.

What is Enum?

An enum is a special class that represents a group of constants.

import java.util.*;
    enum months {  
        JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE, JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER
        } 

How to Synchronise a HashMap?

Map m = Collections.synchronizedMap(hashMap);

What is Java EnumSet?

EnumSetprovides the functionality of Set implementation for use with enum types.

import java.util.*;
    enum months {  
        JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE, JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER
      } 
      
      public static void main(String[] args) {  
        Set<months> set = EnumSet.of(months.JANUARY, months.FEBRUARY, months.MARCH);  
        
        Iterator<months> iter = set.iterator();  
        while (iter.hasNext())  
          System.out.println(iter.next());  
      }
Output:
    JANUARY
    FEBRUARY
    MARCH

What is Java EnumMap?

EnumMap provides the functionality of Map implementation with enum type of keys.

import java.util.*;
enum Months {  
    JANUARY, FEBRUARY, MARCH, APRIL, MAY, JUNE, JULY, AUGUST, SEPTEMBER, OCTOBER, NOVEMBER, DECEMBER
} 
      
public static void main(String[] args) {  
    
    EnumMap<Months, String> map = new EnumMap<Months, String>(Days.class);  
    map.put(Months.JANUARY, "1");  
    map.put(Months.FEBRUARY, "2");  
    map.put(Months.MARCH, "3");  
    map.put(Months.APRIL, "4");  
    map.put(Months.MAY, "5");  

}

How to create Immutable class in Java?

Immutable class in java means, once an object is created we cannot change its value.
The wrapper classes such as Integer, Boolean, Byte, Short and String classes are immutable.

Rules for the creating custom Immutable class in Java.

1. The class must be declared as final, so that child classes can not be created.
2. All attributes in the class must be declared as private so that direct access is not allowed from outside of the class.
3. All attributes in the class must be declared as final so that value can not be changed object creation.
4. Parameterized constructor must be created with all the fields.
5. Need to implement deep copy inside constructor so that data members can not be modified with an object reference.

Example of custom Immutable class in Java.

import java.util.Date;
import java.util.HashMap;
import java.util.Map;

final class BookAuthor {
    private final Integer authorId;
    private final String authorName;
    private final Date dob;
    private final Map<Integer, String> bookMap = new HashMap<Integer, String>();


    BookAuthor(Integer authorId, String authorName, Map<Integer, String> bookMap, Date dob) {
        this.authorId = authorId;
        this.authorName = authorName;
        //Cloning Copy
        this.dob = (Date) dob.clone();
        //Deep Copy
        for (var entry : bookMap.entrySet()) {
            this.bookMap.put(entry.getKey(), entry.getValue());
        }
    }

    public Integer getAuthorId() {
        return authorId;
    }

    public String getAuthorName() {
        return authorName;
    }

    public Date getDob() {
        return dob;
    }

    public Map<>Integer, String> getBookMap() {

        Map<Integer, String> tempMap = new HashMap<Integer, String>();
        for (var entry : this.bookMap.entrySet()) {
            tempMap.put(entry.getKey(), entry.getValue());
        }
        return tempMap;
    }
}

What is difference between Comparable and Comparator Interface in Java?

Comparable

1. The Comparable compares itself with another object.
2. The class itself must implements the java.lang.Comparable interface to compare its instances.
3. Need to override the compareTo() method in the class itself.

Comparator

1. The Comparator provides multiple sorting sequences.
2. You can sort the collection on the basis of multiple elements such as id, name, age and city etc..
3. Comparator doesn't affect the original class. The actual class is not modified.
4. Comparator has compare() method to sort elements.

Example of Comparable in Java?

public class ComparableExample implements Comparable<ComparableExample>{
 private Integer id;
 private String name;
 private String email;
 private Integer age;


 @Override
 //Override compareTo method
 public int compareTo(ComparableExample obj) {
    if(age==obj.age)
        return 0;
    else if(age>obj.age)
        return 1;
    else
        return -1;
  }
 }  

Example of Comparator in Java?

import java.util.Comparator;
public class Employee {
    private Integer id;
    private String name;
    private String email;
    private Integer age;
    
    public Employee(Integer id, String name, String email, Integer age) {
        this.id = id;
        this.name = name;
        this.email = email;
        this.age = age;
    }
    
    public Integer getId() {
        return id;
    }
    public String getName() {
        return name;
    }
    public String getEmail() {
        return email;
    }
    public Integer getAge() {
        return age;
    }
}
//Seperate class need to create
class DemoComtarorExample implements Comparator<Employee> {
    @Override
    //Override the compare method
    public int compare(Employee obj1, Employee obj2) {
        if(obj1.getAge() == obj2.getAge())
            return 0;
        else if(obj1.getAge() > obj2.getAge())
            return 1;
        else
            return -1;
    }
} 

What is LinkedList in Java?

LinkedList is a linear data structure where the elements are not stored in contiguous locations and every element is a separate object with a pointer of address of another object. LinkedList class uses doubly linked list to store the elements.

1. LinkedList is a dynamic data structure whose size increases as you add the elements and decreases as you remove the elements from the list.
2. LinkedList class uses doubly linked list to store the elements.
3. LinkedList class is non synchronized.
4. LinkedList maintains insertion order and provides a linked-list data structure.
5. The elements in the linked list are stored in the containers.
6. The object holds the address of the previous container as well as for the next container.
7. Whenever you add an element to the list using add() operation, a new container is created, and this container is linked to the other containers in the list.
8. Manipulation is fast in the LinkedList because no shifting needs to occur.

Example of LinkedList

import java.io.IOException;
import java.util.LinkedList; 
public class LinkedListExample {
  public static void main(String[] args) throws IOException {

    LinkedList ll = new LinkedList();

    ll.add("A1");
    ll.add("A2");
    ll.addLast("A3");
    ll.addFirst("A4");
    ll.add(1, "A5");

    System.out.println(ll);
    //ll.pop();pop the first element
    //ll.poll(); poll and remove the head element
    ll.add("A6");
    ll.remove("A2");
    //ll.remove(2);
    ll.removeFirst();
    ll.removeLast();

    System.out.println(ll);
  }
}
Output:
    [A4, A5, A1, A2, A3]
    [A5, A1, A3]

Spring Boot and what are its Benefits?

Java Spring Boot is an open source lightweight framework that helps to develop application easyly as it provides minimum configuration, annotation base configuaration, no need to add much configuration such as if we need to create datasource just need to add few values in the properties file.

It has embeded HTTP server, we dont need to create a seperate server to rn the application.

Advantages of Spring Boot?

1. Fast deployment.
2. High scalability.
3. Container compatibility.
4. Minimal configuration.
5. Lower production time.
6. Increased productivity.
7. Reduced development time.
8. Easy monitoring and management of applications.

Features Spring Boot?

1. Actuator - enables users to manage and monitor applications.
2. Spring Cloud - enables to create microservices.
3. Embedded servers - such as Jetty and Tomcat.
4. Run the application without deploying.
5. Auto-Configuration functionality - configure Spring applications automatically.

what are the main libraries of Spring Boot?

1. spring-boot-starter-parent
2. spring-boot-maven-plugin
3. spring-boot-starter-test
4. spring-boot-starter-security
5. spring-boot-starter-actuator
6. Spring-boot-starter-web

Spring Boot Annotations?

1. - @SpringBootApplication
2. - @EnableAutoConfiguration
3. - @Autowired -
4. - @Configuration
5. - @ComponentScan
6. - @Component
7. - @Controlle
8. - @Service
9. - @Repository
10. - @RequestMapping

What is Spring Boot Actuator?

Spring Boot Actuator is a module that provides production-ready features to monitor and manage the Spring Boot application. It provides many endpoints that used for monitoring, health checks, auditing, and managing the application.

What is Spring Boot Actuator?

Swagger is used for the detailing and documenting of RESTful APIs.

What is the Java Persistence API?

JPA stands for Java Persistence API. JPA is used to persist data between Java object and relational database. The JPA is only a specification, needs ORM tools like Hibernate, iBatis, and TopLink implements the JPA specification and perform these type of tasks.

Java Persistence API is a collection of classes and methods to persist or store a vast amount of data in a database using ORM.

What is Spring Data JPA?

Spring Data JPA is a part of JPA and it reduces the amount of boilerplate code needed for common database operations like GET, PUT, POST, etc.

What is Spring Repository?

Spring Repository is an extension of Spring Repository which contains APIs for basic CRUD operations, pagination, and Sorting.

What is HQL?

HQL stands for Hibernate Query Language it allows to expression of database queries using entity and property names.

How to create HQL Queries?

Session session = sessionFactory.openSession();
try {
    String hql = "FROM Employee WHERE department.id = :deptID";
    Query<Employee> query = session.createQuery(hql, Employee.class);
    query.setParameter("deptID", 1);
    List<Employee> empList = query.list();
    for (Employee employee : empList) {
        System.out.println("Employee Name: " + employee.getName());
    }
} catch (Exception e) {
    e.printStackTrace();
} finally {
    session.close();
}

How can we add criteria Query?

Session session = sessionFactory.openSession();
try {
    Criteria criteria = session.createCriteria(Employee.class);
    criteria.add(Restrictions.eq("city", "City Name"));
    criteria.add(Restrictions.between("age", 30, 40));
   
    List<Employee> empList = criteria.list();
    for (Employee emp : empList) {
        System.out.println("Employee Name: " + emp.getName());
    }
} catch (Exception e) {
    e.printStackTrace();
} finally {
    session.close();
}
sessionFactory.close();

What is Hibernate?

Hibernate is open source lightweight object relational mapping tool. It provides the functionalty to map between Entity object with database table. It makes automatic database operations like create, read, update, delete operations.

Difference between session and sessionfactory in hibernate?

Hibernate Session is an interface between application and database, session is required to create session factory.

SessionFactory in Hibernate is to create and manage the session instances, database connection,

What is the difference between the get and load method?

1. If object not found for the given identifier then it will return null object but load will throw object not found exception.
2. get returns fully initialized object so this method eager load the object but load always returns proxy object so this method is lazy load the object.
3. get is slow as it loads eager but load is faster than get as it loads lazy.
4. - @Configuration
5. - @ComponentScan
6. - @Component
7. - @Controlle
8. - @Service
9. - @Repository
10. - @RequestMapping

What is database indexing?

The main purpose of indexing is to improves database performance. An index is a database structure that you can use to improve the performance of database activity. A database table can have one or more indexes.

A database driver can use indexes to find records quickly. While an index speeds up the performance of data retrieval queries (SELECT statement), it slows down the performance of data input queries (UPDATE and INSERT statements).

Types of Indexes?

1. Unique Index
2. Single-Column Index
3. Composite Index
4. Implicit Index

Unique Index

Unique indexes are used for data integrity as well as for for performance.
Example of Unique Index.

CREATE UNIQUE INDEX index_name
on table_name (column_name);

Single-Column Index

A single-column index is created only on one table column.
Example of Single-Column Index.

CREATE INDEX index_name
ON table_name (column_name);

Composite Indexes

A composite index is an index that can be created on two or more columns of a table.
Example of Composite Index.

CREATE INDEX index_name
on table_name (column1, column2);

Implicit Index

Implicit indexes are indexes that are automatically created by the database server when an object is created such as primary key and unique constraints.

What is the view in SQL?

Views in SQL are a kind of virtual table. A view also has rows and columns as they are on a real table in the database. We can create a view by selecting fields from one or more tables present in the database. A View can either have all the rows of a table or specific rows based on certain conditions.

What is normalization?

Normalization reduce data redundancy and enhance data integrity in the table. It also helps to organize the data in the database by removing duplicated data from the relational tables.

Types of normalization?

1. First Normal Form (1NF)

Column should contain only a single value, and each column should have a unique name.

2. Second Normal Form (2NF)

Each row should be associated with a primary key.

3. Third Normal Form (3NF)

Non-key columns should be directly related to the primary key.
4. Boyce-Codd Normal Form (BCNF)

3NF rule is followed in the BCNF.

5. Fourth Normal Form (4NF)

6. Fifth Normal Form (5NF)

What is Denormalization?

Denormalization is is used to optimization or increase performance. In Denormalization, need to add redundant data to one or more tables.

What is Constraint?

Constraint in SQL is a set of rules we are applying the restrictions on the database. The rule checks before inserting data into the database such as NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY etc.

What is Data Integrity?

Data integrity is the and process of ensure the accuracy, completeness, consistency, and validity of an organization's data.

Write an SQL query to find the names of employees starting with "M".

SELECT * FROM Employee WHERE name like 'M%';

Write an SQL query to find the names of employees starting with "M".

SELECT * FROM Employee WHERE name like 'M%';

Write SQL query to fetch “NAME” in upper case with alias.

SELECT upper(NAME) as EMPLOYEE_NAME from employee;

Write a SQL query to fetch unique values of CITY.

SELECT distinct city from EMPLOYEE;
OR
SELECT CITY FROM EMPLOYEE GROUP BY(CITY);

Write a SQL query to print the first two characters of NAME

SELECT SUBSTRING(NAME, 1, 2) from EMPLOYEE;

Write a SQL query to fetch employee details from Employee table order by NAME Ascending and CITY descending.

SELECT * FROM EMPLOYEE ORDER BY NAME , CITY DESC;

Write a SQL query to fetch the EMPLOYEE details whose NAME ends with 'v'.

SELECT * FROM EMPLOYEE where name like '%v';

Write SQL query to fetch the count of Employee having City of 'Mumbai'.

SELECT CITY, COUNT(*) AS COUNT_CITY FROM EMPLOYEE where CITY='Mumbai';

Write a SQL CONCAT query of first_name and last_name.

SELECT CONCAT(first_name, ' ', last_name) as FULL_NAME FROM EMPLOYEE;

Write SQL query to fetch the count of EMPLOYEE for each CITY in the descending order.

SELECT CITY, COUNT(city) FROM EMPLOYEE group by CITY order by COUNT(city);

You have two tables EMPLOYEE and SALARY. Write SQL query to NAME of Employee from and SALARY_AMOUT, SALARY_DATE from Salary table.

SELECT EMPLOYEE.FIRST_NAME, EMPLOYEE.LAST_NAME 
    SALARY.SALARY_AMOUNT, SALARY.SALARY_DATE
    FROM EMPLOYEE
    INNER JOIN 
    SALARY ON EMPLOYEE.ID=SALARY.ID;

Write SQL query to fetch to fetch 2nd highest salary.

SELECT * FROM Employee ORDER BY salary DESC LIMIT 2, 1;

Write SQL query to clone another table with data.

CREATE TABLE new_table_name AS SELECT * FROM Employee;

Write SQL query to find the average of salary.

SELECT AVG(salary) FROM employee;

Write SQL query to find the average salary of each city.

SELECT CITY, AVG(salary) FROM employee GROUP BY CITY;

Write SQL query to find the city name where city name count is one.

SELECT CITY FROM employee GROUP BY CITY HAVING COUNT(CITY) = 1;

What is Singleton Design Pattern?

The Singleton Design Pattern provides only one instance of a class. Application can access the instance of Singleton class from multiple places.

Create Singleton design pattern

1. Static member
2. Private constructor
3. Static factory method

Example of Singleton class in Java.

public class SingleToneDesignPatternExample {
  private static SingleToneDesignPatternExample obj;

  private SingleToneDesignPatternExample(){}

  private static SingleToneDesignPatternExample getSInstance(){
    if(obj==null){
        synchronized (SingleToneDesignPatternExample.class){
            obj = new SingleToneDesignPatternExample();
        }
    }
    return obj;
  }
}

What is SOLID Design Pattern?

The SOLID design pattern principles are below.

1. Single Responsibility

   - A class should only have one responsibility.

2. Open/Closed

   - Class should be open for extension but closed for modification.

3. Liskov Substitution

   - If class A is a subtype of class B, we should be able to replace B with A without disrupting the behavior of our program.

4. Interface Segregation

   - larger interfaces should be split into smaller ones.

5. Dependency Inversion

What is Jenkins?

Jenkins is a CICD (continuous integration and continuous delivery) tool that helps for the delivery of projects. Jenkins install on a server where we acn do the central build procees.

1. Developer push code in a repository.
2. Jenkins takes the source code from repository.
3. Jenkins starts the build process after taking the changes from source code.
4. Jenkins deploy the build version into aserver after completed the build.