Memento Design Pattern

This pattern talks about restoring the original state at any point of time . For example

I start with knowing the mileage of bike around reference value 80KM/h

I store this value in an string instance say : mileage

My reference value / Original value is 80KM/h

Now I start knowing mileage of various bikes and keep updating the the latest known mileage in same string instace :  mileage

current value might change from 80 to 70 to 50 to 40 to 100 and so on  depending upon the mileage of bikes.

So at any point of time If i want to know the reference value , It's no more there is mileage string.

So how do I make sure that I keep on working on current value as well as be able to retrieve the original value at any point of time.

There might be many ways you ca think across , Memento design pattern suggest a standard way to do that.

Pattern revolves around two contributors

Originator
Care Taker

Originator stores the original value in a non-modified way.
Care Taker takes care of returing current or original value at any point of time
Client directly interacts with Care taker to get current or original value


Now lets see how simple is it to implement it in java :


Originator class stores the original value and it can't be modified .







public class Originator {

    private final String mileage ="80";
   
    public String getFinalInstance(){
        return mileage;
    }

}CareTaker provides method to get to current or original mileage at any point of time


public class CareTaker {

    private String mileage;
    public CareTaker(String instance) {
       this.mileage=instance;
    }

   public String saveCurrentInstance(String instance){
       this.mileage=instance;
       return this.mileage;
   }
  
   public String restoreDefaultInstance(){
       Originator finalInstance=new Originator();
      return  finalInstance.getFinalInstance();
}
   public String getCurrentValue(){
       return  mileage;
   }
}

-->


And here is our Client who interacts with CareTaker to play with current value and reach original value at any point of time.


public class MementoClient {

    public MementoClient() {
        // TODO Auto-generated constructor stub
    }
   
    public static void main(String args[]){
       
       System.out.println("Some original reference value is already set .......");
        CareTaker instance =new CareTaker("50");
        System.out.println("Let us create an instance with current value ::: 50");
        System.out.println("Lets find out the  original reference  value ");
       System.out.println("it is :::" + instance.restoreDefaultInstance());
        System.out.println("and the current value is :: ");
       System.out.println(instance.getCurrentValue());
        System.out.println("Lets change the value to 40");
        instance.saveCurrentInstance("40");
        System.out.println("Now current value is :::  " + instance.getCurrentValue());
        System.out.println("Let's again change the value to :::100");
        instance.saveCurrentInstance("100");
        System.out.println("Now current value is : " + instance.getCurrentValue());
        System.out.println("Now I want to get to the original  value");
        System.out.println("it is ::: " + instance.restoreDefaultInstance());
    }

}






Executing this application as java application will print below logs on console :

===============================
Some original reference value is already set .......
Let us create an instance with current value ::: 50
Lets find out the  original reference  value
it is :::80
and the current value is ::
50
Lets change the value to 40
Now current value is :::  40
Let's again change the value to :::100
Now current value is : 100
Now I want to get to the original  value
it is ::: 80
================================

 

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.