Creational Patterns

• Abstract Factory
• Builder
• Factory
• Prototype
• Singleton

Structural Patterns

• Adapter
• Bridge
• Composite
• Decorator
• Facade
• Flyweight
• Proxy

Behavioral Patterns

• Chain of Responsibility
• Command
• Interpreter
• Iterator
• Mediator
• Memento
• Observer
• State
• Strategy
• Template Method
• Visitor

Process:

• an architectural construct : can affect the architecture of an application.
• processes are independend execution units
• they have their own state, address space
• use interprocess communication to communicate between threads

Threads:

• coding construct : doesn’t affect the architecture.
• a single process might contain multiple threds
• all threds within a process share process’s address space and state
• threads can communicate with each other directly

• System = final System class
• out          =  static PrintStream object
• println = method of a printstrem class

Q. What is difference between abstract class and interfaces.

• abstract class can provide implementation, interface has no implementation at all
• used using extends, used using implements
• both can not be instantiated

Q. What will be the output of following code snippet?

public class MyTests
{
	public static void main(String args[])
	{
		String s=null;
		System.out.println(s.hashCode());
	}
}

The program will be compiled without any errors, at run time it will throw NullPointerException.

Q. What will be the output of following code snippet.

public class MyTests
{
	private static void main(String args[])
	{
	}
}

The program will be compiled without any errors, at run time it will give an Error saying “Main Method not public”

Q. What is difference between sleep() and wait()?

When we say sleep(2000), then the thread sleeps for exact 2 seconds, but when we say wait(2000), the  thread might wake up on receiving notify() OR notifyall() call.

Q. What are serializable, externalizable interfaces?

These are interfaces used while serialization process. Serialization is a process in which object is converted in byte stream to transfer over network. Serializable acts as marker interface and externalizable is specialization of serializable interface with methods like readExternal, writerExternal, which gives you more control over serialization process.

Q. How threds are created?

In java threads can be created either by extending Thread class Or implementng Runnable Interface.We need to override run() method.

Q. Differenes between Vector and ArrayLists

• ArrayLists are not synchronized while vectors are synchronized.
• By default ArrayList grows by 50%  and Vector grows by 100%.

Q. What is difference between HashMap and HashTable

• HashTable is synchronized and HashMaps are unsynchronizd
• HashTable do not allow null as a key, HashMap allow null as key

A: - Inheritance : When a child class acquires all the methods and properties of parent class it is called inheritance. Java supports inheritance either using classes or interfaces.

Encapsulation: In above class, we have declared id as private and defined two methods that will act as mutator and accessors for this id propert. Thus we are encapsulateing objects property and  methods with in an object, keeping it safe from outside world.

Polymorphism: In below scenario, we have added 2 declaration of a function add() to child class, one is for adding  integers and another is to add float values. This kind of behavior when same name is used to perform different tasks as per the parameters passed to it is called polymorphism.

class Parent
{
	private int id;
	public Parent()
	{
	   aMethod();
	}
 
	public void aMethod()
	{
	   System.out.println("I am in Parent");
	}
 
	public int getId()
	{
	  return this.id;
	}
 
	public void setId(int id)
	{
	  this.id = id;
	}
}
 
class Child extends Parent
{
	public Child()
	{
	  bMethod();
	}     	
 
	public void bMethod()
	{
	  System.out.println("I am in Child");
	}
 
	public void add(int a, int b)
	{
	}
 
	public void add(float a, float b)
	{
	}
}

In java polymorphism is achieved by function overloading. It is also called Compile time Polymorphism. Compile decides at compile time that which version of function is called depending up on the parameters passed.

function overriding is also type of polymorphism, some times it is called Run time polymorphism, because the methods version is decided depending on the reference type of object at run time.

Important Notes:

public void aMethod(int a, long b)
{
   System.out.println("int a, long b");
}
public void aMethod(long a, int b)
{
   System.out.println("long a, int b");
}

For above methods to be polymorphic you need to use them as -

someobject.aMethod(1,2L);
someobject.aMethod(1L,2);
//someobject.aMethod(1,2); //compile time error
//someobject.aMethod(2,1); //compile time error

SortTest remains the main class that will perform creating and filling objects for us. This time employee class is plain old java classs that holds some data about each employee we create.

Here I have created scenarios or you can say strategies OR classes for sorting as SortById,  SortByName, SortBySalary. Each of above class implements comparator interface and overrides compare method, which takes two objects to compare and return eithre -1, 1 or 0.

Note: I have added few helper functions that really helps while coding like  print - this takes array list of objects and prints in format I want instead of class id and all that. toString -  overridden this function so I can return the class string as required

Here goes the code -

import java.util.*;
 
public class SortTest
{
	public static void main(String args[])
	{
		new SortTest();
	}
 
	public SortTest()
	{
		//fill some employee objects
		ArrayList list = new ArrayList();
		list.add(new Employee(500,"Shifoo",150000));
		list.add(new Employee(504,"Oogway",120000));
		list.add(new Employee(503,"Tigress",100000));
		list.add(new Employee(730,"Mantis",45000));
 
		System.out.println("Initial List :");
		print(list);
		System.out.println("\n");
 
		Collections.sort(list,new SortyById());
		System.out.println("Sorted List By Id:");
		print(list);
		System.out.println("\n");
 
		Collections.sort(list,new SortyByName());
		System.out.println("Sorted List By Name:");
		print(list);
		System.out.println("\n");
 
		Collections.sort(list,new SortyBySalary());
		System.out.println("Sorted List By Salary:");
		print(list);
		System.out.println("\n");
 
	}
 
	public void print(ArrayList list)
	{
		Iterator it = list.iterator();
		while(it.hasNext())
		{
			Employee emp = (Employee) it.next();
			System.out.println(emp);
		}
	}
}
 
class Employee
{
	public int id;
	public String name;
	public double salary;
 
	public Employee(int id, String name,double salary )
	{
		this.id = id;
		this.name = name;
		this.salary = salary;
	}
 
	public String toString()
	{
		return this.id +", "+this.name+", "+this.salary;
	}
}
 
//scenario | strategy - I
class SortyById implements Comparator
{
	public int compare(Object object1, Object object2)
	{
		int value=0;
 
		Employee emp1 = (Employee) object1;
		Employee emp2 = (Employee) object2;
 
		if(emp1.id > emp2.id)
			value = 1;
		else if(emp1.id < emp2.id)
			value = -1;
		else if(emp1.id == emp2.id)
			value = 0;
 
		return value;
	}
}
 
//scenario | strategy - II
class SortyByName implements Comparator
{
	public int compare(Object object1, Object object2)
	{
		Employee emp1 = (Employee) object1;
		Employee emp2 = (Employee) object2;
 
		return emp1.name.compareTo(emp2.name);
	}
}
 
//scenario | strategy - III
class SortyBySalary implements Comparator
{
	public int compare(Object object1, Object object2)
	{
		int value=0;
 
		Employee emp1 = (Employee) object1;
		Employee emp2 = (Employee) object2;
 
		if(emp1.salary > emp2.salary)
			value = 1;
		else if(emp1.salary < emp2.salary)
			value = -1;
		else if(emp1.salary == emp2.salary)
			value = 0;
 
		return value;
	}
}

Output:

As you can see -

• we can create multiple sort strategy for sorting
• we can compare heterogeneous objects

Here we will see sorting example using Comparable Interface. As an example we will be sorting objects of Employee class.

When using comaprable interface make sure the object class
which you are sorting implements comaprable interface and override compareTo method correctly.

As I was more used to Java 1.4, I didnt followed any Java Generics implemetation.

SortTest is main class that will actually creaate an array list and fill it with some random Employee objects.

Employee is class that will implement compareTo method. I have added simple if-else control structure that will decide what to return on the basis of employee id.
less : -1           equal : 0           greater : 1

Here is the code for classes:

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import java.util.*;
 
public class SortTest
{
	public static void main(String args[])
	{
		new SortTest();
	}
 
	public SortTest()
	{
		//fill some employee objects
		ArrayList list = new ArrayList();
		list.add(new Employee(500,"Shifoo",150000));
		list.add(new Employee(504,"Oogway",120000));
		list.add(new Employee(503,"Tigress",100000));
		list.add(new Employee(130,"Mantis",100000));
 
		System.out.println("Initial List :");
		print(list);
		System.out.println("\n");
 
		Collections.sort(list);
 
		System.out.println("Sorted List :");
		print(list);
		System.out.println("\n");
 
	}
 
	public void print(ArrayList list)
	{
		Iterator it = list.iterator();
		while(it.hasNext())
		{
			Employee emp = (Employee) it.next();
			System.out.println(emp);
		}
	}
}
 
class Employee implements Comparable
{
	public int id;
	public String name;
	public double salary;
 
	public Employee(int id, String name,double salary )
	{
		this.id = id;
		this.name = name;
		this.salary = salary;
	}
 
	public int compareTo(Object object)
	{
		int value=0;
 
		Employee emp = (Employee) object;
 
		if(this.id > emp.id)
			value = 1;
		else if(this.id < emp.id)
			value = -1;
		else if(this.id == emp.id)
			value = 0;
 
		return value;
	}
 
	public String toString()
	{
		return this.id +", "+this.name+", "+this.salary;
	}
}

Output:

Limitations:

1. Objects should be mutally comparable. i.e you can not compare objects of different classes.
2. Using comparable binds sorting to one perticular strategy.
   e.g. In above example we have sorted using employee id to compare. now if we need to sort on employee name it will be difficult.

Next, I will be putting similar example using Comparator Interface.

Class defines 2 constructors.

1. One by default returns you the vector containing only text/html url objects from page.
2. For the other you can specify the type of urls you want from a page. This is helpful when you want to get all images, videos or any other media urls.

The class also considers relative urls. It returns relative urls with http and host name prefixed.
E.g. If you have urls like “/about.php”, then class will return “http://hostname.domain/about.php”

The URLFinder

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import java.util.*;
import java.net.*;
import java.io.*;
 
public class URLFinder 
{
String content;
Vector urlVector;
String type;
URL tUrl;
URLConnectionReader ucreader;
String host;
 
public URLFinder(String uri)
{
 ucreader = new URLConnectionReader(uri);		
 this.content = ucreader.content.toLowerCase();
 this.host = ucreader.host;
 
 urlVector = new Vector(30,10);
 int index = 0;
 while ((index = this.content.indexOf("<a", index)) != -1)
 {
	if ((index = this.content.indexOf("href", index)) == -1) break;
	if ((index = this.content.indexOf("=", index)) == -1) break;
	index++;
	String remaining = this.content.substring(index);
	StringTokenizer st = new StringTokenizer(remaining, "\t\n\r\"\'>#");
	String strLink = st.nextToken();
 
	tUrl = isValidUrl(strLink);
	try 
	{ 
		URLConnection urlConnection = tUrl.openConnection();				
		String type = (String)urlConnection.getContentType();				
		if (type == null)
		{
			System.out.println("ERROR: invalid type "
			+strLink+ "=" + type);
			continue;
		}	
		if (type.indexOf("ext/html")==0)
		{
			System.out.println("ERROR: invalid type "
			+strLink+ "=" + type);
			continue;
		}
		urlVector.addElement(tUrl);
	} 
	catch(Exception e){
		System.out.println("ERROR: invalid URL " 
			+ strLink);
		continue;
	}			
 }
}
 
public URLFinder(String data, String type)
{
 this.content = data.toLowerCase();
 urlVector = new Vector(30,10);
 int index = 0;
 while ((index = this.content.indexOf("<a", index)) != -1)
 {
	if ((index = this.content.indexOf("href", index)) == -1) break;
	if ((index = this.content.indexOf("=", index)) == -1) break;
	index++;
	String remaining = this.content.substring(index);
	StringTokenizer st = new StringTokenizer(remaining, "\t\n\r\"\'>#");
	String strLink = st.nextToken();		
 
	tUrl = isValidUrl(strLink);		    
	try 
	{ 
		URLConnection urlConnection = tUrl.openConnection();				
		String utype = (String)urlConnection.getContentType();
 
		if (utype == null)
		{
			System.out.println("ERROR: invalid type "
			+strLink+ "=" + utype);
			continue;
		}	
		if (utype.indexOf(type)==0)
		{
			System.out.println("ERROR: invalid type "
			+strLink+ "=" + utype);
			continue;
		}
		urlVector.addElement(tUrl.toURI());
	} 
	catch(Exception e){
		System.out.println("ERROR: invalid URL " 
			+ strLink);
		continue;
	}			
 }
}
 
public URL isValidUrl(String strLink)
{
	try	
	{
		tUrl = new URL(strLink);
	}
	catch(MalformedURLException e)
	{
		strLink = "http://"+this.host+strLink;
		tUrl = isValidUrl(strLink);		
	}
	return tUrl;
}
}

Usage

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finder = new URLFinder("http://www.mydomain.com/");
int cap = finder.urlVector.size();			 
String buf = "";			 
for(int j=0;j<cap;j++)
{				 
 URL tmp = (URL)finder.urlVector.get(j);
 System.out.println(tmp.toURI());
}

This will get you all the urls from any web page.

The Config

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import java.util.*;
import java.util.Properties;
 
public class Config 
{
   Properties configFile;
   public Config()
   {
	configFile = new java.util.Properties();
	try {			
	  configFile.load(this.getClass().getClassLoader().
	  getResourceAsStream("myapp/config.cfg"));			
	}catch(Exception eta){
	    eta.printStackTrace();
	}
   }
 
   public String getProperty(String key)
   {
	String value = this.configFile.getProperty(key);		
	return value;
   }
}

You can get any property/settings from config with method ‘getProperty’

Here is my config file

#This is comment
mDbUser = myuser
mDbHost = myserver
mDbPwds = mypwd
mDbName = mydb

Usage:

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cfg = new Config();
dbname   = cfg.getProperty("mDbUser");

Hope you find this useful..

1. Specifying user,pwd and database name to connect
2. Specifying connection ID and class will load rest of the variables from CONFIG file

Here I will post the Database class only, class to read config file and loading properties will be posted in different post.

The Database

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import java.sql.*;
 
public class Database 
{	
	Connection conn;
	Statement stmt;
	PreparedStatement pstmt;
	ResultSet rs;
	Config cfg;
	int affectedRows;
 
	/*Reading From CFG*/
	String dbuser,dbpwd,dbname,dbserver;
 
	public Database(){		
	}
 
	public Database(String database, String user,String pwd)
	{
		this.affectedRows = 0;
		try
		{
			Class.forName("com.mysql.jdbc.Driver").newInstance();
			conn = DriverManager.getConnection(
			"jdbc:mysql://localhost:3306/"+database, user
			, pwd);
			stmt = conn.createStatement();			
		}
		catch (Exception e){
			e.printStackTrace();
		}
	}
 
	public Database(String dbid)
	{
		cfg = new Config();
		this.dbname   = cfg.getProperty("mStrName"+dbid);
		this.dbuser   = cfg.getProperty("mStrUser"+dbid);
		this.dbpwd    = cfg.getProperty("mStrPwds"+dbid);
		this.dbserver = cfg.getProperty("mStrHost"+dbid);
 
		try
		{
			Class.forName("com.mysql.jdbc.Driver").newInstance();
			conn = DriverManager.getConnection( 
			"jdbc:mysql://localhost:3306/"+this.dbname, this.dbuser
			, this.dbpwd);
			stmt = conn.createStatement();			
		}
		catch (Exception e){
			e.printStackTrace();
		}
	}
 
	public ResultSet query(String qry)
	{
		try{		
			rs = this.stmt.executeQuery(qry);
		}catch(SQLException se){
			System.out.println("Error : " + se.getMessage());
		}
		return rs;
	}
 
	public int update(String qry)
	{
		int count = 0;
		try{
			count = this.stmt.executeUpdate(qry); 
		}catch(Exception se){
			System.out.println("Error : " + se.getMessage());
		}
		return count;
	}
}

And here is how we are going to use this class:

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Database db = new Database("mydb","myuser","mypwd");
//simple query
ResultSet rs = db.query("SELECT * FROM mytable");

Now you can traverse record rs set in a way you want…

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//update query
int affected = db.update("DELETE FROM mytable");

We will shortly see how we are going to use Config class and its usage..

Then I came to one more solution that I tried an bingooo!, it worked like charm. Just to keep note and save some time of other developers , I am putting all steps here . First 2, 3 steps are commonly suggested.

• Open your apache conf file “httpd.conf” in any editor of your choice.

• Search “mod_rewrite.so”, you may encounter line as follows -
  #LoadModule rewrite_module modules/mod_rewrite.so

• Remove the comment “#”.

• Find “AccessFileName” , you may see a line like -
  “AccessFileName ht.acl” change it to “AccessFileName htaccess.txt”.

• Create “htaccess.txt” in sites root directory.

• Restart apache

Thats it, you are done!