Introduction to Computing
Concepts
Note Set 23
Networking
IP: 192.168.0.123
Network
IP: 97.99.88.77
IP: 111.222.121.99
ClientServer Model
Data Transfer
Between client and server
ClientActively makes a
Connection to the server
ServerWaits Passively
for a connection
Each computer has many “ports” that allow it to communicate with
More than one other computer.
In Java - Server
Let’s say this server is running on computer w/ IP 192.168.0.101
ServerSocket listener = new ServerSocket(3333);
Socket connection;
//Blocks and waits for someone to attempt to connect
//Will accept the connection only on the port listed above
connection = listener.accept();
//More code to come
So a client on another computer would attempt to connect to IP
192.168.0.101 on port 3333.
In Java - Client
Let’s say this client is running on computer w/ IP 192.168.0.202
//attempt to connect to server running on
Socket client = new Socket(“192.168.0.101”, 3333);
Communication
• Two computer connected to the same socket can
send and receive information to/from each other
//attempt to connect to server running on
Socket client = new Socket(“192.168.0.101”, 3333);
//Write to socket with a printwriter
PrintWriter output =
new PrintWriter(client.getOutputStream());
//Read from socket with a Scanner – just like from
//the keyboard
Scanner input =
new Scanner (client.getInputStream());
Exception Handling
• Helps recover from runtime errors
▫ Try to connect to a server but can’t get a connection
• Some methods throw exceptions that must be
caught in the code you write.
try {
//attempt to connect to server running on
Socket client = new Socket(“192.168.0.101”, 3333);
} catch (Exception e) {
System.out.println(“Error”);
System.out.println(e.getMessage());
}
Breakout IN Groups
• Modify Simple Server
▫ Have server generate a random number between 1
and 10, then wait for a connection
▫ Upon connection, read a number from the input
stream and check.
▫ Send back to client a message of correct guess or
incorrect guess
• Modify Simple Client
▫ Get a number between 1 and 10 from the user
▫ Connect to server and then send number
▫ Read back response and display to the user