Question

i want to make a game with 2 players .i will use udp server and 2 clients and i dont
know how to connect 2 clients to 1 server at the same time and how they will communicate.
I will use only java.At last how the mouse click will syncronize with server

  public void mousePressed(MouseEvent event) { 
  }  
  public void mouseReleased(MouseEvent event) { 
  }  
  public void mouseEntered(MouseEvent event) { 
  }  
  public void mouseExited(MouseEvent event) { 
  } 

the server

public class Provider {
public ServerSocket providerSocket;
Socket connection = null;
ObjectOutputStream out;
ObjectInputStream in;
String message;
String[] torino={"null","null"};
Provider() {}
void run()
{
    try {
        //1. creating a server socket (8080=port , 2 =number of connections)
        providerSocket = new ServerSocket(8080, 2 );
        //2. Wait for connection
        System.out.println("Waiting for connection");
        connection = providerSocket.accept();
        System.out.println("Connection received from " + connection.getInetAddress().getHostName());
        //3. get Input and Output streams
        out = new ObjectOutputStream(connection.getOutputStream());
        // flush= clean the object out
        out.flush();
        in = new ObjectInputStream(connection.getInputStream());
        sendMessage("Connection successful");
        //4. The two parts communicate via the input and output streams
        try {
            //take the message from client
            message = (String)in.readObject();
            if (torino[0]=="null") 
                torino[0]=message;
            } else if (torino[1]=="null") {
                torino[1]=message;              
            }
        }
        catch(ClassNotFoundException classnot) {
            System.err.println("Data received in unknown format");
        }
    } catch(IOException ioException) {
        ioException.printStackTrace();
    }
    finally {
        //4: Closing connection
        try {
            in.close();
            out.close();
            providerSocket.close();
        } catch(IOException ioException) {
            ioException.printStackTrace();
        }
    }
}
//method to send messages from server to clients
void sendMessage(String msg)
{
    try {
        out.writeObject(msg);
        out.flush();
        System.out.println("server>" + msg);
    }
    catch(IOException ioException) {
        ioException.printStackTrace();
    }
}

//main method
public static void main(String args[])
{
    Provider server = new Provider();
    while(true) {
        server.run();
    }
}

Answer 1

So now you can see in your code that you have a point where the server socket is waiting for a connection. That is the accept() method. At that point, you need to create a new Thread for handling the connection and let the main thread continue waiting for another connection. You may want to keep track of the number of connections made in a variable, say numConnections.

 while(numConnections < 2){
       connection = providerSocket.accept();
       Thread t = new Thread(new ConnectionHandler(connection));
       t.start();
       numConnections++;
 }

Now you ConnectionHandler class will do the work of the connection:

 class ConnectionHandler implements Runnable{
        Socket connection;
        public ConnectionHandler(Socket connection){
             this.connection = connection;
        }

        public void run(){
           //here you do all the code associated with handling the connection
           //such as your Object Streams and so on.
        }
 }

A quick Google search turns up many tutorials and examples of doing this such as this one. There are also some YouTude videos. You may want to start with one of those.