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Server.java

+import java.io.BufferedReader;
+import java.io.InputStreamReader;
+import java.io.PrintWriter;
+import java.net.InetAddress;
+import java.net.ServerSocket;
+import java.net.Socket;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+
+public class Server {
+    private static HashMap<SocketDetails, String[]> sessions;
+    private static HashMap<String, ArrayList<SocketDetails>> chats;
+    public static void main(String[] args) throws Exception{
+        ServerSocket serverSocket = new ServerSocket(2001); //initialise server socket, sessions hash map and chats hash map
+        sessions = new HashMap<SocketDetails, String[]>();
+        chats = new HashMap<String, ArrayList<SocketDetails>>();
+        while (true) { //continuously listen for socket connections
+            Socket socket = serverSocket.accept(); //accept any socket connection
+            BufferedReader listener = new BufferedReader(new InputStreamReader(socket.getInputStream()));
+            String[] message = listener.readLine().split("/"); //reads in sent message
+            SocketDetails sd = new SocketDetails(socket, Integer.parseInt(message[1]));
+            String response = "Invalid request";
+            if (message[0].equals("open")){ //if message is an open session message, add the key value pair of the socket details (a unique identifier of each session) and a string array containing the username and the current chat
+                sessions.put(sd,new String[]{message[2],"notInAChat"});
+                response = "Session started"; //set response to confirm the session has started
+            }
+            else if (message[0].equals("close")){ //if message is an open session message, add the key value pair of the socket details (a unique identifier of each session) and a string array containing the username and the current chat
+                if (chats.containsKey(sessions.get(sd)[1])){ //if the user is in a chat
+                    if (chats.get(sessions.get(sd)[1]).size() == 1) { //if the chat contains only 1 member, delete it
+                        chats.remove(sessions.get(sd)[1]);
+
+                    }
+                    else{ //else remove the user from the chat and inform all members on the chat
+                        chats.get(sessions.get(sd)[1]).remove(sd);
+                        sendMessage(sessions.get(sd)[0] + " has left the chat", sessions.get(sd)[1]);
+                    }
+                }
+                sessions.remove(sd);
+                response = "Session ended"; //set response to confirm the session has ended
+            }
+            else if (message[0].equals("join")){ //if the message is a join chat message
+                sessions.get(sd)[1] = message[2]; //set the chat in the session
+                if (chats.containsKey(message[2])){ //if the chat already exists, add the socket details to the chat members list and send a message to all chat members that the new user has joined
+                    sendMessage(sessions.get(sd)[0] + " has joined the chat", message[2]);
+                    chats.get(message[2]).add(sd);
+                    response = "Joined chat " + message[2]; //set response to confirm the user has joined the chat
+                }
+                else { //else we need to create a new chat, so add a new key value pair of the chat name and list of users and add the socket details of the current user
+                    chats.put(message[2], new ArrayList<SocketDetails>());
+                    chats.get(message[2]).add(sd);
+                    response = "Created chat " + message[2]; //set response to confirm the chat has been created
+                }
+            }
+            else if (message[0].equals("leave")){ //if the message is a join chat message
+                if (chats.containsKey(sessions.get(sd)[1])){ //if the user is in a chat
+                    if (chats.get(sessions.get(sd)[1]).size() == 1) { //if the chat contains only 1 member, delete it
+                        chats.remove(sessions.get(sd)[1]);
+                        response = "Deleted chat " + sessions.get(sd)[1]; //set response to confirm the chat has been deleted
+                        sessions.get(sd)[1] = "NotInAChat";
+                    }
+                    else{ //else remove the user from the chat and inform all members on the chat
+                        chats.get(sessions.get(sd)[1]).remove(sd);
+                        sendMessage(sessions.get(sd)[0] + " has left the chat", sessions.get(sd)[1]);
+                        response = "Left chat " + sessions.get(sd)[1]; //set response to confirm the user has left the chat
+                        sessions.get(sd)[1] = "NotInAChat";
+                    }
+                }
+                else { //else the user is not in a chat and hence cannot leave
+                    response = "Invalid request as not in chat"; //set response to say that you need to join a chat first
+                }
+            }
+            else if (message[0].equals("message")) { //if the message is a chat message
+                if (chats.containsKey(sessions.get(sd)[1])) { //if the user is in a chat, send the message
+                    sendMessage(sessions.get(sd)[0] + " - " + message[2], sessions.get(sd)[1]);
+                    response = "Sent message"; //set response to confirm the user has joined the chat
+                } else { //else set response to say that you need to join a chat first
+                    response = "Invalid request as not in chat";
+                }
+            }
+            PrintWriter sender = new PrintWriter(socket.getOutputStream()); //once the message is processed send back a response and close the socket
+            sender.println(response);
+            sender.flush();
+            socket.close();
+        }
+    }
+
+    public static void sendMessage(String message, String chat) throws Exception{ //method to send a specified message to every member in a chat
+        for (SocketDetails sd: chats.get(chat)) { //loops through socket details
+            Socket socket = sd.getSocket(); //opens socket
+            PrintWriter sender = new PrintWriter(socket.getOutputStream()); //sends message
+            sender.println(message);
+            sender.flush();
+            socket.close(); //closes socket
+        }
+    }
+}