Observer design pattern in java

Lets talk about observer design pattern today--

Lets take in consideration a real time scenarion 

There is a News Control Center that controls current news and subscribers . 

Lets say there are two subscribers Ching and Chong

Ching and Chong are registered by News Control center for current news 

If at any point of time current news are updated News needs to be informed to its all subscribers. 

Subscriber will not keep on checking news all the time If any current news is updated. They will register once for all and News

will ensure that they are timely informed about every new news. So how will that hapend





Ching and Chong will act as observers / listener . News will see If any new news is updated it is informed to its subscribers 


Below Code details this and implement the scenario in java code .

NewsControlCenter Class will hold the main control to apply for subscriber registeration and bring in new news to the agency


==================



public class NewsControlCenter {

    /**
     * @param args
     */
    public static void main(String[] args) {

        Subscriber sub1=new Subscriber("ching");
        Subscriber sub2=new Subscriber("chong");
        News news=new TimesNews();
        news.registerSubscriber(sub1);
        news.registerSubscriber(sub2);
        
        System.out.println("==========news changed======");

        news.updateNews("Narayan traced");
        
        System.out.println("==========news changed again ======");

        news.updateNews("Govt declared prize money on informing Narayan's whereabout.");

        
       
    }

}

==================


import java.util.List;

public interface News {
    void registerSubscriber(Subscriber sub);

    void unRegisterSubscriber(Subscriber sub);

    void updateNews(String news);
    void inform(Subscriber sub);
    void inform(List subs);
    void readCurrentNews();
}


==================








import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class TimesNews implements News {

    String currentNews = "Asharam in Jail";

    List subscribers = new ArrayList();

    @Override
    public void registerSubscriber(Subscriber sub) {
        subscribers.add(sub);
    }

    @Override
    public void unRegisterSubscriber(Subscriber sub) {
        subscribers.remove(sub);
    }

    @Override
    public void updateNews(String news) {

        this.currentNews = news;
        inform(subscribers);

    }

    public void inform(Subscriber sub) {

    }

    public void inform(List subs) {

        for (Iterator iterator = subs.iterator(); iterator.hasNext();) {
            Subscriber subscriber = (Subscriber) iterator.next();
            inform(subscriber);
        }

    }

    public void readCurrentNews() {
        System.out.println(this.currentNews);
    }

}


=====================

-->


public class Subscriber {

    String name;

    public Subscriber(String name) {
        this.name = name;
    }

    public void readNews(News news) {

        System.out.println("Mr. " +name+ ": got this news:::");
        news.readCurrentNews();
}
}


=================



On executing this program as java application . Below logs will be written on console

----------------------------------------------------------

opening news for ::: ching
Asharam in Jail
opening news for ::: chong
Asharam in Jail
==========news changed======
Mr. ching: got this news:::
Narayan traced
Mr. chong: got this news:::
Narayan traced
==========news changed again ======
Mr. ching: got this news:::
Govt declared prize money on informing Narayan's whereabout.
Mr. chong: got this news:::
Govt declared prize money on informing Narayan's whereabout.

-------------------------------------------------------------------------------------

Once again on visitor's design pattern

I have two employees in my organization : Manoj and Kumar

I wish to have Manoj's employee  data in text format and Kumar's employee data in HTML format

What I would do to get it done :

1. I will create Employee instance called Manoj
2. I will ask Manoj to accept Textformatter as it's formatter
3. Once Manoj accept it , it will ask it's Formatter to apply text formatter on it's data

Code for this is as below :

------------------------------------------

package com;

public class Employee {

    /**
     * @return the name
     */
    public String getName() {
        return name;
    }

    /**
     * @param name
     *            the name to set
     */
    public void setName(String name) {
        this.name = name;
    }

    String name;

    public Employee() {
        // TODO Auto-generated constructor stub
    }

    public void accept(Formatter visitor) {

        visitor.visit(this);
    }

}

--------------------------------------------

package com;

public interface Formatter {

    

   void visit(Employee emp);

    

   String getResult();

}

------------------------------------------

package com;

public class TextFormat implements Formatter {

    

    String formatting;

    public TextFormat() {

        // TODO Auto-generated constructor stub

    }

    @Override

    public void visit(Employee emp) {

        

        formatting= "formatted the "+ emp.getName()+ " employee data in text format";

    }

    public String getResult(){

        

        return formatting;

    }

}

--------------------------------------------

package com;

public class Test {

    public Test() {

        // TODO Auto-generated constructor stub

    }

    /**

     * @param args

     */

    public static void main(String[] args) {

        Employee emp =new Employee();

        

        Formatter visitor =new TextFormat();

        

        emp.setName("Manoj");

        emp.accept(visitor);

        

        System.out.println(visitor.getResult());

        

    

        

    }

}

----------------------------------------------

Similarly for Kumar to have it's data formatted in HTML format 

1. I will create Employee instance called Kumar

2. I will ask Kumar to accept HTMLformatter as it's formatter

3. Once KUMAR accept it , it will ask it's Formatter to apply HTML formatter on it's data 

Code for this is : 

---------------------------------------

package com;

public class HTMLFormat implements Formatter {

    String formatting;

    

    public HTMLFormat() {

        // TODO Auto-generated constructor stub

    }

    @Override

    public void visit(Employee emp) {

        formatting= "formatted the "+ emp.getName()+ " empolyee data in HTML format";

    }

    

    public String getResult(){

        return formatting;

    }

}

-----------------------------------

package com;

public class Test {

    public Test() {

        // TODO Auto-generated constructor stub

    }

    /**

     * @param args

     */

    public static void main(String[] args) {

        Employee emp =new Employee();

        

        Formatter visitor =new TextFormat();

        

     

        

        visitor =new HTMLFormat();

        emp.setName("Kumar");

        emp.accept(visitor);

        

       System.out.println(visitor.getResult());

        

    }

}

-----------------------------------------

Test class to collectively apply formatters on Manoj and Kumar below Code could be used in single Test class:

----------------------------------------

package com;

public class Test {

    public Test() {

        // TODO Auto-generated constructor stub

    }

    /**

     * @param args

     */

    public static void main(String[] args) {

        Employee emp =new Employee();

        

        Formatter visitor =new TextFormat();

        

        emp.setName("Manoj");

        emp.accept(visitor);

        

        System.out.println(visitor.getResult());

        

        visitor =new HTMLFormat();

        emp.setName("Kumar");

        emp.accept(visitor);

        

       System.out.println(visitor.getResult());

        

    }

}

-------------------------------

Website Crawler with fork and Join Framework

                Website Crawler with fork and Join Framework 
Here are the classes involved in writing code for this exercise . It can be directly copied and executed using java 7 as fork and Join libraries are available in java only version 1.7 onwards.

Along with these classes you would need HTMLParser jar file , which is used to retrieve links available in a page linked to a particular link. 

Please download htmlparser-1.6.jar file and include in the class path to execute below code


====================================================================

WebsiteCrawler class initiates the logic . It create ForkJoinPool which is used to contain the threads to take up and execute the work stealing algorithm.total work is divided among these threads and is executed is parallel . Thus overall processing is executed faster and multiple processor/core hardware is effectively utilized






import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.concurrent.ForkJoinPool;

/**
 *
 * @author Manoj
  */
public class WebsiteCrawler implements LinkTracker {

    private final Collection linksCrawled = Collections.synchronizedSet(new HashSet());
    private String inputUrl;
    private ForkJoinPool pool;

    public WebsiteCrawler(String inputUrl, int maxThreadCoulnt) {
        this.inputUrl = inputUrl;
        pool = new ForkJoinPool(maxThreadCoulnt);
    }

    private void init() {
        pool.invoke(new LinkSearcher(this.inputUrl, this));
    }

 
   

    public void addVisited(String s) {
        linksCrawled.add(s);
    }


    public boolean visited(String s) {
        return linksCrawled.contains(s);
    }

    public static void main(String[] args) throws Exception {
        new WebsiteCrawler("http://efectivejava.blogspot.in", 50).init();
    }
}

-->


===================================================================

LinkTracker interface provides the basic methods required to execute the link search logic
/**
 *
 * @author Manoj
 */
public interface LinkTracker {

  
    boolean visited(String link);

    void addVisited(String link);
}

import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.RecursiveAction;

import org.htmlparser.Parser;
import org.htmlparser.filters.NodeClassFilter;
import org.htmlparser.tags.LinkTag;
import org.htmlparser.util.NodeList;



==================================================






This is the class where core recursive logic is executed . To divide ,assign and execute the logic recursively this class extends RecursiveAction class and overrides compute() method. compute method is invoked recursively and execute the logic for every link . After visit ,visited link is added to the set and all child URLS found for current URL are added as recursiveAction in the list to be executed by compute() method. 



To understand the code further Please execute this code in debug mode and walk through the flow.


 public class LinkSearcher extends RecursiveAction {

    private String url;
    private LinkTracker tracker;

  
    public LinkSearcher(String url, LinkTracker tracker) {
        this.url = url;
        this.tracker = tracker;
    }

    @Override
    public void compute() {
        if (!tracker.visited(url)) {
            try {
                List actions = new ArrayList();
                URL uriLink = new URL(url);
                Parser parser = new Parser(uriLink.openConnection());
                NodeList list = parser.extractAllNodesThatMatch(new NodeClassFilter(LinkTag.class));

                for (int i = 0; i < list.size(); i++) {
                    LinkTag extracted = (LinkTag) list.elementAt(i);

                    if (!extracted.extractLink().isEmpty() && !tracker.visited(extracted.extractLink())) {

                        actions.add(new LinkSearcher(extracted.extractLink(), tracker));
                    }
                }
                tracker.addVisited(url);
                System.out.println(url);

                invokeAll(actions);
            } catch (Exception e) {
            }
        }
    }
}

Why should override hashcode method while overriding equals method

 Why should override hashcode method while overriding equals method

How do we compare two instances of a class in java 

Lets say there is a class 

public class Employee {

    int age;

    Employee(int age) {

        this.age = age;

    }

    public static void main(String args[]) {

        Employee emp1 = new Employee(10);
        Employee emp2 = new Employee(10);
      
        System.out.println("Are two instances equal?  :" +emp1.equals(emp2));

    }

}

On executing above program , It prints :
Are two instances equal?  :false

Why equals method says that two instances are different although they are instances of same class and have same age?

Reason : While invoking equals method first hashcode() method is executed . If hashcode() method returns different hashcode value for instances being compared equals() method is not called. Different hashcode declares that instances are different and no further comparison takes place.

So Is hashcode values for above tow instances are different ?

Lets see that 

public static void main(String args[]) {

        Employee emp1 = new Employee(10);
        Employee emp2 = new Employee(10);
       
        System.out.println("hashcode value for emp1  " + emp1.hashCode());
        System.out.println("hashcode value for emp2  "+emp2.hashCode());


    }

-->


Executing above method prints 

hashcode value for emp1  327325694
hashcode value for emp2  1657319091

 
So hashcode value returned is different for two instances . Actually hashcode() method works on its inherent algorithm to generate hashcode value of instances . Executing above program again might generate different hashcode values 

Let me execute the above program again and see what values it prints on console 

It prints 

hashcode value for emp1  1025601370
hashcode value for emp2  1578474768
 

So it has it's internal algorithm to generate hashcode value which dynamically generates the hashcode value and we have no control on that..

So how do we make equals method return true flag when comparing two instances of Employee class

let's override equals method as below 

  public  boolean equals(Object obj){
       
       return this.age==((Employee)obj).age  ;  
    }
 
Here Object class equals method is overridden . Now equals method is customized so it will declare two instances equal if their ages are equal . So emp1.equals(emp2) should return true 







But bigger barrier is hashcode() method . This method does not let the call go to equals() method . It returns false based of different hashcode value for two instances . 

So what do we do to make the call reach equals() method?

 Equals() method can be invoked only if hashcode value for two instances being compared are equal . So can we customize hashcode() method to make it happen . Let's try doing that 

     public int hashCode() {
         return this.age;
     }

Now executing hashcode value will print 

hashcode value for emp1  10
hashcode value for emp2  10

So hashcode values are equal. Hashcode method itsself can't decide in this case If 

two instance are equal and to decide that equals() method is invoked. 

Thus It is must to override hashcode() method to make equals() method invoked. 

So we can conclude 

If equals() method is customized and overridden then it make it work as we expect It is 

must to override hashcode method in a way to make call to equals() possible. 

How to make a website SEO friendly , Search engine optimization

                         How to make a website SEO friendly , Search engine optimization

Page Title

Your title should be from 20 to 70 characters length. However up to 120 characters can be considered as correct.
Make sure that title is correct and includes most important keyword.
Each page should have it's own, unique title.

Meta Description

Meta description should be from 100 to 160 characters length. However up to 200 characters can be considered as correct.
Use all characters to describe your page using most important keyword.
Each page should have it’s own, unique description.
Meta description usually shows in search engine results as a description of page and it should encourage to visit your site.

Meta Keywords

Meta keywords has no influence on website positioning.
You can remove all meta keywords or limit them to only few describing your content.

Meta Robots

Meta tag robots has crucial impact on page indexing by search engine robots.
Value "noindex" prevents from indexing of page. Value "nofollow" prevents robots from following all links at page.
Most common and default (if empty) values are index, 

Encoding

 Encoding has an impact on the correct display of special characters.
Popular encodings are UTF-8, Latin1, ISO-8859-2 etc.
Encoding has no influence on website positioning, but incorrect encoding can cause problems with display special characters.

Body Content

Words and Chars

 Make sure that the text on page was not less than 250 words.

 

Text / HTML Ratio

The TEXT / HTML ratio is the ratio of the number of characters of plain text to the page's HTML code, expressed as a percentage.
The higher the number, the greater amount of content is on your site, and lesser HTML code.
A value below 10% means that there is too little text or that the HTML code is "littered" or negligent.
Make sure that the TEXT / HTML ratio is not below 10%.

Headers

H1

H2

H3

H4

H5

H6

Headers are very important factor of on-page optimization.
Correct document should contain most important

and second-level

.
Remember to insert important keyword in header.

Bolds

 The and bold type indicate the important keywords on the page.

Search engines take into account emphasized keywords that are in the text.

-->

Images

 Images enrich the content of the website.
No images on the page may be a warning signal against spam, or the page is of low quality.
Publish interesting pictures, photos or diagrams related to the content of the page.
Using the ALT attribute describe accurately each picture.

Frames

 Frames can cause problems to search engines and make your website works slow.
Avoid use of frames on websites.

Internal and External Links

External Links

1                    External Links

Outbound links (external links) are all links leading outside your website.
It is worth to link to other sites that contain high quality content related to the theme of your site.
Do not link to sites which you do not trust, in particular the spam pages.
Limit the number of outbound links to a maximum of 15 links.

Internal Links

2                    Internal Links

Internal links are any links pointing to the other pages of your website.
By placing links to other pages you are reinforcing internal linking and strengthening the usability of your website.
Always link to the most important pages of your website.

Additional Files

Robots.txt

The robots.txt file instructs search engine robots by allowing or restricting access to selected folders and pages of your website.
The robots.txt file has a huge impact on the correct page indexing in search engine results.
Make sure your website has a valid robots.txt file.

Sitemap.xml

The sitemap.xml file should contain a list of all pages of your website that need to be indexed by search engines.
Sitemap.xml file informs the search engine about relevant pages on your site, their latest update and significance in the context of the entire site.
Ensure the correctness of sitemap.xml

Social Media Signals

Facebook Likes

The number of gained "Likes" can have a positive impact on the position of your site in search results.
Install the Facebook Like button on your website and encourage your users to like your site.

Facebook Shares

By sharing your site with friends on Facebook you have a chance to gain new visitors. A large number of "shares" may have a positive impact on the position of your site in search engines.

Google +1

The number of gained Google +1 has a positive impact on the position of your site in search results.
Install Google +1 button on your website and encourage your users to click on +1.

HTTP Headers

HTTP headers is the server return information, not visible on the website.
SEO audit can be performed only on sites that have an HTTP 200 OK header, which indicates proper functioning of the page.
Any other value of the HTTP code prevents from page positioning.

Domain and Server

IP Address

IP Address has no influence on website positioning.
On one IP address can work hundreds of different websites.
Avoid sharing IP address with spam websites.

Name Servers

DNS servers has no influence on website positioning.
Like IP address, many websites can use the same DNS servers.
Avoid sharing DNS names with spam websites.

Server Geolocation

 Geographical localization of server has no direct influence on website positioning.
Use hosting companies located in regions where you are making your business.