General

Many times we need to declare multiple variables of the same type to hold related values.For example list of students in a class.As there are lot of students in a class so to store the names of students in  variables we can declare the variables as:

String Student1="John";
String Student2="Priya";
String Student3="Amit";
...

So using variables to store students names means we need to declare lot of variables.This approach has the following disadvantages:

• In case of big collection of data ,the number of variables can increase.
• It is difficult to manage lot of variables specially when assigning or fetching these values.
• The code is difficult to read and understand.

Because of disadvantages arrays are prefered way to store large collection of related data.

For example if we have to assign and read the list of students in a class using variables we will do it as:

InputStreamReader in=new InputStreamReader(System.in);
BufferedReader br=new BufferedReader(in);
String Student1=br.readLine();
String Student2=br.readLine();
...

System.out.println(Student1);
System.out.println(Student2);

As you can see there are 2 problems in this approach.

• We have to know in advance the exact number of variables we are going to assign
• Assigning and reading the values makes the code difficult to understand

We can easily assign and read values using array as:

String[] student=new String[2];
InputStreamReader in=new InputStreamReader(System.in);
BufferedReader br=new BufferedReader(in);
for(int count=0;count<student.length;count++)
{
student[count]=br.readLine();
}

for(int count=0;count<student.length;count++)
{
System.out.println(student[count]);
}

Arrays can be assigned and retrieved using a for loop.This makes arrays easier to work with a group of data unlike ordinary variables.

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Arrays are useful for storing a collection of data.For example if we have a group of student data related to students in a School then we can define an array as:

class Student
{
public int Id;
public String Name;
public String Address;
}

String[] students=new String[10000];
students=getStudents();

The method getStudents() may fetch the list of students from webservice or database.

So arrays are useful for storing a set of data.But the main problem with arrays is that the size of an array is fixed and it can not be changed later on.For example if the number of Students in school increases then we can not increase the size of the array after we have declared the above array.

One solution is to declare the arrays with suffixiet size to accomodate the future requirments.So in the above case we can declare the array of size 15000 ,since we assume the number of students can not be more than that.

String[] students=new String[15000];

But the problem now is that we have declared the size of the array which is much more than the actual number of Students.This means that extra memory is bing allocated for Students.Though that memory will never be used.

For these types of scenarios Arraylist is a better solution.Arraylist differs from arrays since the size of an Arraylist can be altered at runtime.So if we don’t know how many elements an array will contain we can use an ArrayList.

Following are the steps to declare and add elements to an ArrayList

1. Import the required namespaces.
ArrayList class implements the List interface.It extends the AbstractList class.To work with ArrayList<T> in our code we need to import the following package from java.util library:

import java.util.ArrayList;

2. Create an ArrayList object
Once we have imported the required package we can create an object of the generic Arraylist as:
//create a list of Students

public class Student {
public int Id;
public String Name;
public String Address;
}
ArrayList<Student> students = new ArrayList<Student>();

3. Add elements to an ArrayList
For adding elements to arraylist we use the add() method

//add item to list
Student student=new Student(); 
student.Id=1; student.Name="John";
student.Address="Autralia"; 
students.add(student);

For adding element at a specific index we use an overload of add() method as:

student=new Student();
student.Id=2;
student.Name="Amit";
student.Address="India";

students.add(1,student);

Note that we have provided the index as the first parameter.

4. Reset elements at a specific location
We can reset element at a specific index using the set() method as:

student=new Student();
student.Id=2;
student.Name="Marc";
student.Address="UK";
students.set(1,student);

In the above code we are replacing the Student at second position ,”Amit” with Student “Marc”

5.Retrieve elements from the arraylist.
We can retrieve an item from the arraylist using the get() method as:

student=students.get(1);

6.Get the number of elements in ArrayList
There is a size() method which returns the number of elements in an ArrayList at any given time.
So if we execute the above code in sequence then size() will return the value 2.

System.out.println(students.size());

output: 2

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In a typical web application communication between the browser and the web server follows the request and reply pattern.Browser makes the request to the web server which sends the response in return.So the communication needs to be initiated by the web browser.This works well for most of the web applications.

Sometimes we need to implement a scenario in which the web server has some information that it needs to return back to the browser.This scenario is difficult to implement using the normal request reply pattern that the HTTP protocol supports.

Common example of such an application is a chat application in which the server can anytime send the response to the browser.

In long polling the web browser keeps the web server asking if it has the information ready.The problem with this approach is that network bandwidth is wasted as the browser has to make multiple requests until the information is returned by the web server.Also the resources are not efficiently utilized as the browser needs to make multiple requests.

WebSockets in HTML5 provides an ideal way to implement such kind of scenario in which the server can return the response back to the browser ,once the response is ready.So websockets provides a means to implement bidirectional communication in which the either the web browser or the web server can start the communication.Web browser receives notifications when the server sends response back to the browser.

“WebSockets is a two way communication technology over TCP protocol.It is an implementation of push technology concept in which the server informs the browser when it has some data ready to be sent to the browser.”

Implementing WebSocket

1.Create a WebSocket object.First argument is the url of the server we want to connect to.Second argument is a string or array of subprotocols to be used for connection.Example of one such subprotocol is SOAP.

var websocket= new WebSocket(url, [SubProtocol] );

2. Send message to the server   After we have created a WebSocket object we can send a message to the server by using the send() method.

websocket.onopen = function () {
websocket.send('Test Message');
};

onopen is an event handler which is executed when the connection to the server is established.

3. Retrieve the data returned from the server  The data returned by the server is available in the  onmessage eventhandler.This event handler is called when the server returns the data.

websocket.onmessage = function(e) {
alert(e.data);
};

4.Implement the onclose event handler  The onclose even handler is called when the connection closed.

websocket.onclose = function()
{ 
alert("Connection closed"); 
};

Server implementation of WebSocket

Server can be implemented in different technologies such as:

• .NET
• Java
• Node.js
• Ruby
• C++

Some of the advantages of websockets are:

• Websockets uses a protocol defined by IETF for communication.
• Unlike HTTP protocol Websockets allows full duplex or bidirectional   communication.Communication between the web browser and web server can happen simultaneously
• Websockets communicate on the standard TCP port 80 ,so communication using websockets is allowed by most of the firewalls

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Internet was used mainly by humans for information interchange and interaction.This can be called the internet of the people.Internet of things on the other hand is a collection of things.

Things can be a vast array of things such as smart phones,embedded devices,smart watches etc.

Internet of things means that the devices are communicate with each other using wireless network (or internet).sensors are embedded in the devices or things.These sensors collect data and the different devices communicate with each other.In short embedded devices are connected to the internet ,hence the name internet of things.

Internet Of Things is a concept which means interconnection of devices over internet where each device is assigned a unique address.

This is such a an revolutionary concept because instead of different devices existing in isolation they can now be a part of a large interconnected network and can access different devices on the network. IoT devices usually have sensors to collect data and can communicate to other IoT devices.

IoT devices could be useful in different areas from healthcare to real time applications.

The  Thing here  can refer to different variety of devices.These devices can vary from washing machines to automobiles with sensors.Since these devices utilize internet to connect to each other so unique IP address needs to be assigned to each device.

IPIv6 which is the latest version of Internet Protocol has increased address space that can be used to assign unique address’ to devices .This means that practically every possible device can be a part of  Internet of Things

Since devices uses internet to connect with each other,they use standard technologies instead of proprietary ones.The Internet Engineering Task Force (IETF) is one such organization that develops internet standards and is working on developing common standards for IoT .So every device can communicate using common standards.

IoT is expected to be the next big transformation since the arrival of internet.

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