JAVA-NETWORKING:

The term network programming refers to writing programs that execute across multiple devices (computers), in which the devices are all connected to each other using a network.

The java.net package of the J2SE APIs contains a collection of classes and interfaces that provide the low-level communication details, allowing you to write programs that focus on solving the problem at hand.

The java.net package provides support for the two common network protocols:

• TCP: TCP stands for Transmission Control Protocol, which allows for reliable communication between two applications. TCP is typically used over the Internet Protocol, which is referred to as TCP/IP.
• UDP: UDP stands for User Datagram Protocol, a connection-less protocol that allows for packets of data to be transmitted between applications.

This tutorial gives good understanding on the following two subjects:

1. Socket Programming: This is most widely used concept in Networking and it has been explained in very detail.
2. URL Processing: This would be covered separately. Click here to learn about URL Processing in Java language.

Socket Programming:

Sockets provide the communication mechanism between two computers using TCP. A client program creates a socket on its end of the communication and attempts to connect that socket to a server.

When the connection is made, the server creates a socket object on its end of the communication. The client and server can now communicate by writing to and reading from the socket.

The java.net.Socket class represents a socket, and the java.net.ServerSocket class provides a mechanism for the server program to listen for clients and establish connections with them.

The following steps occur when establishing a TCP connection between two computers using sockets:

1. The server instantiates a ServerSocket object, denoting which port number communication is to occur on.
2. The server invokes the accept() method of the ServerSocket class. This method waits until a client connects to the server on the given port.
3. After the server is waiting, a client instantiates a Socket object, specifying the server name and port number to connect to.
4. The constructor of the Socket class attempts to connect the client to the specified server and port number. If communication is established, the client now has a Socket object capable of communicating with the server.
5. On the server side, the accept() method returns a reference to a new socket on the server that is connected to the client's socket.

After the connections are established, communication can occur using I/O streams. Each socket has both an OutputStream and an InputStream. The client's OutputStream is connected to the server's InputStream, and the client's InputStream is connected to the server's OutputStream.

TCP is a twoway communication protocol, so data can be sent across both streams at the same time. There are following usefull classes providing complete set of methods to implement sockets.

ServerSocket Class Methods:

The java.net.ServerSocket class is used by server applications to obtain a port and listen for client requests

The ServerSocket class has four constructors:

SN	Methods with Description
1	public ServerSocket(int port) throws IOException Attempts to create a server socket bound to the specified port. An exception occurs if the port is already bound by another application.
2	public ServerSocket(int port, int backlog) throws IOException Similar to the previous constructor, the backlog parameter specifies how many incoming clients to store in a wait queue.
3	public ServerSocket(int port, int backlog, InetAddress address) throws IOException Similar to the previous constructor, the InetAddress parameter specifies the local IP address to bind to. The InetAddress is used for servers that may have multiple IP addresses, allowing the server to specify which of its IP addresses to accept client requests on
4	public ServerSocket() throws IOException Creates an unbound server socket. When using this constructor, use the bind() method when you are ready to bind the server socket

If the ServerSocket constructor does not throw an exception, it means that your application has successfully bound to the specified port and is ready for client requests.

Here are some of the common methods of the ServerSocket class:

SN	Methods with Description
1	public int getLocalPort() Returns the port that the server socket is listening on. This method is useful if you passed in 0 as the port number in a constructor and let the server find a port for you.
2	public Socket accept() throws IOException Waits for an incoming client. This method blocks until either a client connects to the server on the specified port or the socket times out, assuming that the time-out value has been set using the setSoTimeout() method. Otherwise, this method blocks indefinitely
3	public void setSoTimeout(int timeout) Sets the time-out value for how long the server socket waits for a client during the accept().
4	public void bind(SocketAddress host, int backlog) Binds the socket to the specified server and port in the SocketAddress object. Use this method if you instantiated the ServerSocket using the no-argument constructor.

When the ServerSocket invokes accept(), the method does not return until a client connects. After a client does connect, the ServerSocket creates a new Socket on an unspecified port and returns a reference to this new Socket. A TCP connection now exists between the client and server, and communication can begin.

Socket Class Methods:

The java.net.Socket class represents the socket that both the client and server use to communicate with each other. The client obtains a Socket object by instantiating one, whereas the server obtains a Socket object from the return value of the accept() method.

The Socket class has five constructors that a client uses to connect to a server:

SN	Methods with Description
1	public Socket(String host, int port) throws UnknownHostException, IOException. This method attempts to connect to the specified server at the specified port. If this constructor does not throw an exception, the connection is successful and the client is connected to the server.
2	public Socket(InetAddress host, int port) throws IOException This method is identical to the previous constructor, except that the host is denoted by an InetAddress object.
3	public Socket(String host, int port, InetAddress localAddress, int localPort) throws IOException. Connects to the specified host and port, creating a socket on the local host at the specified address and port.
4	public Socket(InetAddress host, int port, InetAddress localAddress, int localPort) throws IOException. This method is identical to the previous constructor, except that the host is denoted by an InetAddress object instead of a String
5	public Socket() Creates an unconnected socket. Use the connect() method to connect this socket to a server.

When the Socket constructor returns, it does not simply instantiate a Socket object but it actually attempts to connect to the specified server and port.

Some methods of interest in the Socket class are listed here. Notice that both the client and server have a Socket object, so these methods can be invoked by both the client and server.

SN	Methods with Description
1	public void connect(SocketAddress host, int timeout) throws IOException This method connects the socket to the specified host. This method is needed only when you instantiated the Socket using the no-argument constructor.
2	public InetAddress getInetAddress() This method returns the address of the other computer that this socket is connected to.
3	public int getPort() Returns the port the socket is bound to on the remote machine.
4	public int getLocalPort() Returns the port the socket is bound to on the local machine.
5	public SocketAddress getRemoteSocketAddress() Returns the address of the remote socket.
6	public InputStream getInputStream() throws IOException Returns the input stream of the socket. The input stream is connected to the output stream of the remote socket.
7	public OutputStream getOutputStream() throws IOException Returns the output stream of the socket. The output stream is connected to the input stream of the remote socket
8	public void close() throws IOException Closes the socket, which makes this Socket object no longer capable of connecting again to any server

InetAddress Class Methods:

This class represents an Internet Protocol (IP) address. Here are following usefull methods which you would need while doing socket programming:

SN	Methods with Description
1	static InetAddress getByAddress(byte[] addr) Returns an InetAddress object given the raw IP address .
2	static InetAddress getByAddress(String host, byte[] addr) Create an InetAddress based on the provided host name and IP address.
3	static InetAddress getByName(String host) Determines the IP address of a host, given the host's name.
4	String getHostAddress() Returns the IP address string in textual presentation.
5	String getHostName() Gets the host name for this IP address.
6	static InetAddress InetAddress getLocalHost() Returns the local host.
7	String toString() Converts this IP address to a String.

Socket Client Example:

The following GreetingClient is a client program that connects to a server by using a socket and sends a greeting, and then waits for a response.

// File Name GreetingClient.java import java.net.*; import java.io.*; public class GreetingClient {    public static void main(String [] args)    {       String serverName = args[0];       int port = Integer.parseInt(args[1]);       try       {          System.out.println("Connecting to " + serverName                              + " on port " + port);          Socket client = new Socket(serverName, port);          System.out.println("Just connected to "                       + client.getRemoteSocketAddress());          OutputStream outToServer = client.getOutputStream();          DataOutputStream out =                        new DataOutputStream(outToServer);          out.writeUTF("Hello from "                       + client.getLocalSocketAddress());          InputStream inFromServer = client.getInputStream();          DataInputStream in =                         new DataInputStream(inFromServer);          System.out.println("Server says " + in.readUTF());          client.close();       }catch(IOException e)       {          e.printStackTrace();       }    } }

Socket Server Example:

The following GreetingServer program is an example of a server application that uses the Socket class to listen for clients on a port number specified by a command-line argument:

// File Name GreetingServer.java import java.net.*; import java.io.*; public class GreetingServer extends Thread {    private ServerSocket serverSocket;       public GreetingServer(int port) throws IOException    {       serverSocket = new ServerSocket(port);       serverSocket.setSoTimeout(10000);    }    public void run()    {       while(true)       {          try          {             System.out.println("Waiting for client on port " +             serverSocket.getLocalPort() + "...");             Socket server = serverSocket.accept();             System.out.println("Just connected to "                   + server.getRemoteSocketAddress());             DataInputStream in =                   new DataInputStream(server.getInputStream());             System.out.println(in.readUTF());             DataOutputStream out =                  new DataOutputStream(server.getOutputStream());             out.writeUTF("Thank you for connecting to "               + server.getLocalSocketAddress() + "\nGoodbye!");             server.close();          }catch(SocketTimeoutException s)          {             System.out.println("Socket timed out!");             break;          }catch(IOException e)          {             e.printStackTrace();             break;          }       }    }    public static void main(String [] args)    {       int port = Integer.parseInt(args[0]);       try       {          Thread t = new GreetingServer(port);          t.start();       }catch(IOException e)       {          e.printStackTrace();       }    } }

Compile client and server and then start server as follows:

$ java GreetingServer 6066 Waiting for client on port 6066...

Check client program as follows:

$ java GreetingClient localhost 6066 Connecting to localhost on port 6066 Just connected to localhost/127.0.0.1:6066 Server says Thank you for connecting to /127.0.0.1:6066 Goodbye!

INTERFACE:

An interface is a collection of abstract methods. Aclass implements an interface, thereby inheriting the abstract methods of the interface.

An interface is not a class. Writing an interface is similar to writing a class, but they are two different concepts. A class describes the attributes and behaviors of an object. An interface contains behaviors that a class implements.

Unless the class that implements the interface is abstract, all the methods of the interface need to be defined in the class.

An interface is similar to a class in the following ways:

• An interface can contain any number of methods.
• An interface is written in a file with a .java extension, with the name of the interface matching the name of the file.
• The bytecode of an interface appears in a .class file.
• Interfaces appear in packages, and their corresponding bytecode file must be in a directory structure that matches the package name.

However, an interface is different from a class in several ways, including:

• You cannot instantiate an interface.
• An interface does not contain any constructors.
• All of the methods in an interface are abstract.
• An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
• An interface is not extended by a class; it is implemented by a class.
• An interface can extend multiple interfaces.

Declaring Interfaces:

The interface keyword is used to declare an interface. Here is a simple example to declare an interface:

Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface.

The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code.

Example:

Let us look at an example that depicts encapsulation:

/* File name : NameOfInterface.java */ import java.lang.*; //Any number of import statements public interface NameOfInterface {    //Any number of final, static fields    //Any number of abstract method declarations\ }

Interfaces have the following properties:

• An interface is implicitly abstract. You do not need to use the abstract keyword when declaring an interface.
• Each method in an interface is also implicitly abstract, so the abstract keyword is not needed.
• Methods in an interface are implicitly public.

Example:

/* File name : Animal.java */ interface Animal {       public void eat();       public void travel(); }

Implementing Interfaces:

When a class implements an interface, you can think of the class as signing a contract, agreeing to perform the specific behaviors of the interface. If a class does not perform all the behaviors of the interface, the class must declare itself as abstract.

Aclass uses the implements keyword to implement an interface. The implements keyword appears in the class declaration following the extends portion of the declaration.

/* File name : MammalInt.java */ public class MammalInt implements Animal{    public void eat(){       System.out.println("Mammal eats");    }    public void travel(){       System.out.println("Mammal travels");    }    public int noOfLegs(){       return 0;    }    public static void main(String args[]){       MammalInt m = new MammalInt();       m.eat();       m.travel();    } }

This would produce following result:

Mammal eats Mammal travels

When overriding methods defined in interfaces there are several rules to be followed:

• Checked exceptions should not be declared on implementation methods other than the ones declared by the interface method or subclasses of those declared by the interface method.
• The signature of the interface method and the same return type or subtype should be maintained when overriding the methods.
• An implementation class itself can be abstract and if so interface methods need not be implemented.

When implementation interfaces there are several rules:

• A class can implement more than one interface at a time.
• A class can extend only one class, but implement many interface.
• An interface itself can extend another interface. An interface cannot extend another interface.

Extending Interfaces:

An interface can extend another interface, similarly to the way that a class can extend another class. The extends keyword is used to extend an interface, and the child interface inherits the methods of the parent interface.

The following Sports interface is extended by Hockey and Football interfaces.

//Filename: Sports.java public interface Sports {    public void setHomeTeam(String name);    public void setVisitingTeam(String name); } //Filename: Football.java public interface Football extends Sports {    public void homeTeamScored(int points);    public void visitingTeamScored(int points);    public void endOfQuarter(int quarter); } //Filename: Hockey.java public interface Hockey extends Sports {    public void homeGoalScored();    public void visitingGoalScored();    public void endOfPeriod(int period);    public void overtimePeriod(int ot); }

The Hockey interface has four methods, but it inherits two from Sports; thus, a class that implements Hockey needs to implement all six methods. Similarly, a class that implements Football needs to define the three methods from Football and the two methods from Sports.

Extending Multiple Interfaces:

A Java class can only extend one parent class. Multiple inheritance is not allowed. Interfaces are not classes, however, and an interface can extend more than one parent interface.

The extends keyword is used once, and the parent interfaces are declared in a comma-separated list.

For example, if the Hockey interface extended both Sports and Event, it would be declared as:

public interface Hockey extends Sports, Event

Tagging Interfaces:

The most common use of extending interfaces occurs when the parent interface does not contain any methods. For example, the MouseListener interface in the java.awt.event package extended java.util.EventListener, which is defined as:

package java.util; public interface EventListener {}

An interface with no methods in it is referred to as a tagging interface. There are two basic design purposes of tagging interfaces:

Creates a common parent: As with the EventListener interface, which is extended by dozens of other interfaces in the Java API, you can use a tagging interface to create a common parent among a group of interfaces. For example, when an interface extends EventListener, the JVM knows that this particular interface is going to be used in an event delegation scenario.

Adds a data type to a class: This situation is where the term tagging comes from. A class that implements a tagging interface does not need to define any methods (since the interface does not have any), but the class becomes an interface type through polymorphism.

ENCAPSULATION:

Encapsulation is one of the four fundamental OOP concepts. The other three are inheritance, polymorphism, and abstraction.

Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. If a field is declared private, it cannot be accessed by anyone outside the class, thereby hiding the fields within the class. For this reason, encapsulation is also referred to as data hiding.

Encapsulation can be described as a protective barrier that prevents the code and data being randomly accessed by other code defined outside the class. Access to the data and code is tightly controlled by an interface.

The main benefit of encapsulation is the ability to modify our implemented code without breaking the code of others who use our code. With this feature Encapsulation gives maintainability, flexibility and extensibility to our code.

Example:

Let us look at an example that depicts encapsulation:

/* File name : EncapTest.java */ public class EncapTest{    private String name;    private String idNum;    private int age;    public int getAge(){       return age;    }    public String getName(){       return name;    }    public String getIdNum(){       return idNum;    }    public void setAge( int newAge){       age = newAge;    }    public void setName(String newName){       name = newName;    }    public void setIdNum( String newId){       idNum = newId;    } }

The public methods are the access points to this class.s fields from the outside java world. Normally these methods are referred as getters and setters. Therefore any class that wants to access the variables should access them through these getters and setters.

The variables of the EncapTest class can be access as below::

/* File name : RunEncap.java */ public class RunEncap{    public static void main(String args[]){       EncapTest encap = new EncapTest();       encap.setName("James");       encap.setAge(20);       encap.setIdNum("12343ms");       System.out.print("Name : " + encap.getName()+                              " Age : "+ encap.getAge());     } }

This would produce following result:

Name : James Age : 20

Benefits of Encapsulation:

• The fields of a class can be made read-only or write-only.
• A class can have total control over what is stored in its fields.
• The users of a class do not know how the class stores its data. A class can change the data type of a field, and users of the class do not need to change any of their code.

INHERITANCE

Inheritance can be defined as the process where one object acquires the properties of another. With the use of inheritance the information is made manageable in a hierarchical order.
When we talk about inheritance the most commonly used keyword would be extends and implements. These words would determine whether one object IS-A type of another. By using these keywords we can make one object acquire the properties of another object.
IS-A Relationship:
IS-A is a way of saying : This object is a type of that object. Let us see how the extends keyword is used to achieve inheritance.
public class Animal{
}

public class Mammal extends Animal{
}

public class Reptile extends Animal{
}

public class Dog extends Mammal{
}
Now based on the above example, In Object Oriented terms following are true:
Animal is the superclass of Mammal class.
Animal is the superclass of Reptile class.
Mammal and Reptile are sub classes of Animal class.
Dog is the subclass of both Mammal and Animal classes.
Now if we consider the IS-A relationship we can say:
Mammal IS-A Animal
Reptile IS-A Animal
Dog IS-A Mammal
Hence : Dog IS-A Animal as well
With use of the extends keyword the subclasses will be able to inherit all the properties of the superclass except for the private properties of the superclass.
We can assure that Mammal is actually an Animal with the use of the instance operator.




Example:
public class Dog extends Mammal{
   public static void main(String args[]){

      Animal a = new Animal();
      Mammal m = new Mammal();
      Dog d = new Dog();

      System.out.println(m instanceof Animal);
      System.out.println(d instanceof Mammal);
      System.out.println(d instanceof Animal);
   }
}
This would produce following result:
true
true
true
Since we have a good understanding of the extends keyword let us look into how the implements keyword is used to get the IS-A relationship.The implements keyword is used by classes by inherit from interfaces. Interfaces can never be extended.
Example:
public interface Animal {}

public class Mammal implements Animal{
}

public class Dog extends Mammal{
}
The instanceof Keyword:Let us use the instanceof operator to check determine whether Mammal is actually an Animal, and dog is actually an Animal
interface Animal{}

class Mammal implements Animal{}

class Dog extends Mammal{
   public static void main(String args[]){

      Mammal m = new Mammal();
      Dog d = new Dog();

      System.out.println(m instanceof Animal);
      System.out.println(d instanceof Mammal);
      System.out.println(d instanceof Animal);
   }
}
This would produce following result:
true
true
true
HAS-A relationship:
These relationships are mainly based on the usage. This determines whether a certain class HAS-A certain thing. This relationship helps to reduce duplication of code as well as bugs.
Lets us look into an example:
public class Vehicle{}
public class Speed{}
public class Van extends Vehicle{
        private Speed sp;
}
This shows that class Van HAS-A Speed. By having a separate class for Speed we do not have to put the entire code that belongs to speed inside the Van class., which makes it possible to reuse the Speed class in multiple applications.
In Object Oriented feature the users do not need to bother about which object is doing the real work. To achieve this, the Van class hides the implementation details from the users of the Van class. SO basically what happens is the users would ask the Van class to do a certain action and the Vann class will either do the work by itself or ask another class to perform the action.
A very important fact to remember is that Java only supports only single inheritance. This means that a class cannot extend more than one class. Therefore following is illegal:
public class extends Animal, Mammal{}
However a class can implement one or more interfaces. This has made Java get rid of the impossibility of multiple inheritance

The Bytecode:

The key that allows Java to solve both the security and the portability problems just described is that the output of a Java compiler is not executable code. Rather, it is bytecode. Bytecode is a highly optimized set of instructions designed to be executed by the Java run-time system, which is called the Java Virtual Machine (JVM). That is, in its standard form, the JVM is an interpreter for bytecode. This may come as a bit of a surprise. As you know, C++ is compiled to executable code. In fact, most modern languages are designed to be compiled, not interpreted—mostly because of

performance concerns. However, the fact that a Java program is executed by the

JVM helps solve the major problems associated with downloading programs over

the Internet.