Sunday, 27 August 2017

Internal implementation of ArrayList in Java

    There are two List interface implementation classes in Collection , i.e ArrayList and LinkedList. The ArrayList is most commonly used Collection class in java. We can discuss about how ArrayList internally works and what are the methods .

     ArrayList internally uses array object to add the elements. In other words, ArrayList is backed by Array data-structure. The array of ArrayList is resizable i.e dynamic in nature.

     Below code is internal implementation or source code of add(), remove(), contains() and size() methods of ArrayList.


package com.pr;

import java.util.Arrays;

public class CustomArrayList {

       private static int DEFAULT_CAPACITY = 10;
       private int size = 0;
       private transient Object[] element = {};
       
       public CustomArrayList () {
             element = new Object[DEFAULT_CAPACITY];
       } 

       public boolean add(Object obj) {
            if (size == element.length) {
                 increaseCapacity();
             }
             element[size++] = obj;
             return true;
       }
 
       public Object get(int index) {
             if (index >= size || index<0) {
                     throw new ArrayIndexOutOfBoundsException();
             }
             return element[index];
       }
 
       public Object remove(int index) {
            if (index < size) {
                   Object obj = element[index];
                   element[index] = null;
                   int temp = index;
                   while (temp < size) {
                         element[temp] = element[temp + 1];
                         element[temp+1] = null;
                         temp++;
                    }
                    size--;
                    return obj;
             } else {
                  throw new ArrayIndexOutOfBoundsException();
             }
       }
 
       public boolean contains(Object obj) {
              boolean flag = false;
              if (obj == null) {
                   for (int i = 0; i<size; i++) {
                        if (element[i] == null) {
                             flag = true;
                        }
                    }
                } else {
                      for (int i = 0; i<size; i++) {
                           if (obj.equals(element[i])) {
                                flag = true;
                           }
                      }
                }
                return flag;
        } 
 
        private void increaseCapacity() {
              int increasedSize = element.length * 2;
              Arrays.copyOf(element, increasedSize);
        }

        public int size() {
             return size;
        }
}
 

The main program :--

package com.pr;

public class MainClass {
      public static void main(String[] args) {
            CustomArrayList list = new CustomArrayList();
            list.add("AB");
            list.add("BC");
            System.out.println(list.size());
            System.out.println(list.contains("AB")); 
      }
}
  

  Output : - 2
                  true


Related Post :
Collection Related Interview Questions and Answers in Java(List,Map & Set) 

Friday, 25 August 2017

What are different Spring Bean Scopes?

       The core of the Spring framework is a Bean Factory and it's mechanism to create and manage the beans inside the Spring containers.  In bean, attribute Scope is referred to what kind of object has to create and returned to the container.
       In Spring, there are five bean scopes are available, three of which are available only if you use web-aware ApplicationContext.  Bean scopes are singleton,  prototype, request, session and global session.

Spring Bean Scopes
Spring Bean Scopes

singleton

The Spring IOC container can create only one instance per container irrespective of how many times you request for this instance. This is the default bean scope. This singleton behavior is maintained by bean factory itself.

Example:- EmployeeService.java,

package com.adnblog.employee;

public class EmployeeService { 
 
     String message;

     public String getMessage() {
           return message;
     }

     public void setMessage(String message) {
          this.message = message;
     }
}  

Spring bean configuration file : spring-employee.xml (if scope is not declared, default it should be singleton)

<beans xmlns="http://www.springframework.org/schema/beans"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://www.springframework.org/schema/beans
 http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">

       <bean id="employeeService"
            class="com.adnblog.employee.EmployeeService" />

</beans>

Spring code - Main Program
 
package com.adnblog;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.adnblog.employee.EmployeeService;

public class MainClass { 
 
       public static void main( String[] args ) { 
 
              ApplicationContext context =
                               new ClassPathXmlApplicationContext(new String[] {"spring-employee.xml"});

              EmployeeService empService = (EmployeeService)context.getBean("employeeService");
              empService.setMessage("empService Message");
              System.out.println("Message : " + empService.getMessage());

              //trying to create second instance
             EmployeeService empService1 = (EmployeeService)context.getBean("employeeService");
             System.out.println("Message : " + empService1.getMessage());
      }
}

Output :  Message : empService Message
                Message : empService Message

prototype :  

The Spring IOC container creates new bean instance of the object at every time request for that specific bean is made. 
   To achieve prototype scope, declare scope attribute as prototype in the bean configuration file as below,

<beans xmlns="http://www.springframework.org/schema/beans"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://www.springframework.org/schema/beans
 http://www.springframework.org/schema/beans/spring-beans-2.5.xsd">

       <bean id="employeeService"
            class="com.adnblog.employee.EmployeeService"  scope="prototype"/>

</beans>

Run the main program, you will get output  

Output : Message : empService Message
                Message : null


request :

   In this bean scope, a new bean instance will be created for each web request made by client. As soon as request completes, bean will be out of scope and garbage collected.  This scope only available web aware ApplicationContext i.e WebApplicationContext.

session :

    It is also like request scope, this ensures one instance of bean per user session. As soon as user ends its session, bean is out of scope.  This scope only available web aware ApplicationContext i.e WebApplicationContext.

 global-session :

   This is something which is connected to Portlet applications. When your application works in Portlet container it is built of some amount of portlets. Each portlet has its own session, but if your want to store variables global for all portlets in your application than you should store them in global-session. This scope doesn’t have any special effect different from session scope in Servlet based applications.

Saturday, 19 August 2017

Producer Consumer Problem - Solution using BlockingQueue in Java

           In previous post, discussed the Producer Consumer problem-solution using wait() and notify() methods. In this post, we will learn how to solve the Producer Consumer problem using BlockingQueue. 
   
          BlockingQueue is a queue(added in java.util.concurrent package) which is thread safe to insert or retrieve elements from it. And also it provides a mechanism which blocks request for inserting new elements when the queue is full and also blocks request for removing elements when the queue is empty. The classes that implementing BlockingQueue are ArrayBlockingQueue, DelayQueue, LinkedBlockingQueue and PriorityBlockingQueue and so on.

Program to Solve the Producer-Consumer Problem using BlockingQueue :--


package com.pr;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;

public class ProducerConsumer {
     public static void main (String[] args) {
    
          BlockingQueue<Integer> sharedQueue = new LinkedBlockingQueue<Integer>();
          Producer producer = new Producer(sharedQueue);
          Consumer consumer = new Consumer(sharedQueue);
          Thread p = new Thread(producer, "Producer Thread");
          Thread c = new Thread(consumer, "Consumer Thread");
          p.start();
          c.start();
      }
}

class Producer implements Runnable {

      BlockingQueue<Integer> sharedQueue = new LinkedBlockingQueue<Integer>();
 
      public Producer(BlockingQueue<Integer> sharedQueue) {
            this.sharedQueue = sharedQueue;
      }

      public void run() {
            for (int i = 1; i<=10; i++) {
                try {
                      System.out.println("Produced - "+i);
                      sharedQueue.put(i);
                      Thread.sleep(1000);
                } catch(InterruptedException e) {
                      e.printStackTrace();
                }
            }   
       }
}

class Consumer implements Runnable {

       BlockingQueue<Integer> sharedQueue = new LinkedBlockingQueue<Integer>();

       public Consumer(BlockingQueue<Integer> sharedQueue) {
           this.sharedQueue = sharedQueue;
       }

       @Override
       public void run() {
             while (true) {
                 try {
                      System.out.println("Consumed - "+sharedQueue.take());
                 } catch(InterruptedException e) {
                      e.printStackTrace();
                 }
             }
       } 
}
 
OUTPUT : -  
Produced - 1  
Consumed - 1  
Produced - 2  
Consumed - 2  
Produced - 3  
Consumed - 3  
Produced - 4  
Consumed - 4  
Produced - 5  
Consumed - 5  
Produced - 6  
Consumed - 6  
Produced - 7  
Consumed - 7  
Produced - 8  
Consumed - 8  
Produced - 9  
Consumed - 9  
Produced - 10 
Consumed - 10  
 


Related Posts :
Producer Consumer Problem - Solution using wait and notify In Java
Thread(or Multithread) interview questions & answers

Thursday, 10 August 2017

Producer Consumer Problem - Solution using wait and notify In Java

        This is very important question for the experienced interview perspective. This question will ask mostly 4+ years of experienced peoples. In this problem, again two approaches first one using wait and notify another one using BlockingQueue . In this post, we can discuss or solution using wait and notify.
        Before going through solution,  first need to understand the synchronized block and methods. In this producer and consumer problem solution, we are using synchronization concept.

Below example,
       --->  Producer will produce total of 10 products and can not produce more than 1 item at a time until products are being consumed by the consumer
In code, when sharedQueue size is 1, wait for producer till consumer consume the product.

     ---> Consumer can consume products only when products are available.
In code, when sharedQueue size is 0, wait for consumer till producer produce the product.

Program to Solve the Producer-Consumer Problem using wait and notify



    package com.pr;  
  import java.util.ArrayList;  
  import java.util.List;  
  public class ProducerConsumer {  
       public static void main (String[] args) {  
            List<Integer> sharedQueue = new ArrayList<Integer>();  
            Producer producer = new Producer(sharedQueue);  
            Consumer consumer = new Consumer(sharedQueue);  
            Thread p = new Thread(producer, "Producer Thread");  
            Thread c = new Thread(consumer, "Consumer Thread");  
            p.start();  
            c.start();  
       }  
  }
  
  class Producer implements Runnable {  
       List<Integer> sharedQueue = new ArrayList<Integer>();  
       public Producer(List<Integer> sharedQueue) {  
            this.sharedQueue = sharedQueue;  
       }  
       public void run() {  
            for (int i = 1; i<=10; i++) {  
                 try {  
                      produce(i);  
                 } catch(InterruptedException e) {  
                      e.printStackTrace();  
                 }  
            }  
       }  
       private void produce(int i) throws InterruptedException{  
            synchronized (sharedQueue) {  
                 if (sharedQueue.size() == 1) {  
                      System.out.println("Queue is full");  
                      sharedQueue.wait();  
                 }  
            }  
            synchronized (sharedQueue) {  
                 System.out.println("Produced : "+i);  
                 sharedQueue.add(i);  
                 Thread.sleep(1000);  
                 sharedQueue.notify();  
            }  
       }  
  }
  
  class Consumer implements Runnable {  
       List<Integer> sharedQueue = new ArrayList<Integer>();  
       public Consumer(List<Integer> sharedQueue) {  
            this.sharedQueue = sharedQueue;  
       }  
       @Override  
       public void run() {  
            while (true) {  
                 try {  
                      consume();  
                      Thread.sleep(1000);  
                 } catch(InterruptedException e) {  
                      e.printStackTrace();  
                 }  
            }  
       }       
       private void consume() throws InterruptedException{  
            synchronized (sharedQueue) {  
                 while (sharedQueue.size() == 0) {  
                      System.out.println("Queue is empty");  
                      sharedQueue.wait();  
                 }  
            }  
            synchronized (sharedQueue) {  
                 Thread.sleep(1000);  
                 System.out.println("Consumed :" +sharedQueue.remove(0));  
                 sharedQueue.notify();  
            }  
       }  
  }   

Output : - Produced : 1
Queue is full
Consumed : 1
Produced : 2
Queue is full
Consumed : 2
Produced : 3
Queue is full
Consumed : 3
Produced : 4
Queue is full
Consumed : 4
Produced : 5
Consumed : 5
Produced : 6
Queue is full
Consumed : 6
Produced : 7
Consumed : 7
Produced : 8
Consumed : 8
Produced : 9
Queue is full
Consumed : 9
Produced : 10
Consumed : 10
Queue is empty

Another way solving the producer consumer problem is using BlockingQueue.  BlockingQueue can reduce the code complexity no need to write wait and notify.  In next post we can discuss about BlockingQueue.

Related Post:-- 
Producer Consumer Problem - Solution using BlockingQueue in Java