Develop Reference

Unsure why I'm getting a null value

I just started learning Java and I am creating a simple TCP application where an input will be entered by a user and this input will be sent from the client side to the server side. This will then be processed and the result will be sent back to the client.
Here are my codes:
Client.java
public class Client {
public static void main(String[] args) {
String hostName = "127.0.0.1";
int port = 23456;
try {
Socket socket = new Socket(hostName, port);
Scanner sc = new Scanner(System.in);
System.out.print("Input command:");
String input = sc.nextLine();
OutputStream os = socket.getOutputStream();
PrintWriter pw = new PrintWriter(os);
pw.println(request);
pw.flush();
InputStream is = socket.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String header = br.readLine();
System.out.println(header);
} catch (IOException ex) {
ex.printStackTrace();
}
}
}
Server.java
public class Server
{
public static void main(String[] args)
{
int port = 23456;
try
{
ServerSocket ss = new ServerSocket(port);
System.out.println("Server is ready to receive command!");
while(true)
{
Socket socket = ss.accept();
InputStream is = socket.getInputStream();
InputStreamReader isr = new
InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String command = br.readLine();
String response = "";
int val = 0;
try
{
String[] arr = command.trim().split("\\s+");
String operator = arr[0];
int firstNumber = Integer.parseInt(arr[1]);
int secondNumber = Integer.parseInt(arr[2]);
if (operator.equals("add"))
{
val = firstNumber + secondNumber;
response = "The add result is: " + String.valueOf(result);
}
else if (operator.equals("subtract"))
{
val = firstNumber - secondNumber;
response = "The substract result is: " + String.valueOf(result);
}
else if (operator.equals("multiply"))
{
val = firstNumber * secondNumber;
response = "The multiply result is: " + String.valueOf(result);
}
else if (operator.equals("divide"))
{
if (secondNumber != 0)
{
val = firstNumber * secondNumber;
response = "The multiply result is: " + String.valueOf(result);
}
else
{
response = "Error: Divided by zero exception";
}
} else {
response = "Error: Invalid command " + "\"" + operator + "\"";
}
}
catch (NumberFormatException e)
{
System.out.println(command + " is not a number");
}
OutputStream os = socket.getOutputStream();
PrintWriter pw = new PrintWriter(os);
pw.println(response);
pw.flush();
pw.close();
socket.close();
}
}
catch (IOException ex)
{
ex.printStackTrace();
}
}
}
When I run, I get a null as the output.

Why is my program running twice on my Server?

I am writing tcp client-server program. When the client types "Hello" the server returns a list of files and directories of his current directory. When the client types "FileDownload " it downloads the selected file from the server.
When I type "Hello" it works fine, but when I type "FileDownload ", on the server side it runs twice the else if(received.contains("FileDownload")) block. Because of this the server is sending twice the data which is causing other issues on the client side.
Here is the server code:
public static void main(String[] args) throws IOException {
ServerSocket servSock = new ServerSocket(1333);
String received="";
String[] s = null;
File[] f1;
int i=0;
File f=new File(System.getProperty("user.dir"));
f1=f.listFiles();
for(File f2:f1) {
if(f2.isDirectory())
System.out.println(f2.getName() + "\t<DIR>\t" + i);
if(f2.isFile())
System.out.println(f2.getName() + "\t<FILE>\t" + i);
i++;
}
while (true) {
Socket client = servSock.accept();
InputStream in = client.getInputStream();
OutputStream out = client.getOutputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(in));
PrintWriter pw = new PrintWriter(out, true);
s=f.list();
while(true) {
received = reader.readLine();
if(received.equals("END")) break;
if(received.equals("Hello")) {
System.out.println("Hello-Start");
int length=f1.length;
pw.println(length);
i=0;
for(File f2:f1) {
if(f2.isDirectory())
pw.println(f2.getName() + "\t<DIR>\t" + i);
if(f2.isFile())
pw.println(f2.getName() + "\t<FILE>\t" + i);
i++;
}
pw.println("Options: " + "\tFileDownload <FID>" + "\tFileUpload <name>" + "\tChangeFolder <name>");
System.out.println("Hello-End");
}
else if(received.contains("FileDownload")) {
System.out.println("FileDownload-Start");
int j=-1;
try {
j=Integer.parseInt(received.substring(13).trim());
}catch(NumberFormatException e) {
System.err.println("error: " + e);
}
if(j>0 && j<s.length) {
FileInputStream fi=new FileInputStream(s[j]);
byte[] b=new byte[1024];
System.out.println("file: "+s[j]);
pw.println(s[j]);
fi.read(b,0,b.length);
out.write(b,0,b.length);
System.out.println("FileDownload-End");
}
}
Here is the client code:
public static void main(String[] args) throws IOException {
if ((args.length != 2))
throw new IllegalArgumentException("Parameter(s): <Server> <Port>");
Socket socket = new Socket(args[0], Integer.parseInt(args[1]));
Scanner sc = new Scanner(System.in);
InputStream in = socket.getInputStream();
OutputStream out = socket.getOutputStream();
BufferedReader reader = new BufferedReader(new InputStreamReader(in));
PrintWriter pw = new PrintWriter(out, true);
while(true) {
String msg="", received="";
String length;
msg = sc.nextLine();
pw.println(msg);
if(msg.equals("END")) break;
if(poraka.equals("Hello")) {
System.out.println();
length = reader.readLine();
for(int i=0;i<Integer.parseInt(length);i++) {
received = reader.readLine();
System.out.println(received);
}
System.out.println("\n"+reader.readLine());
}
else if(msg.contains("FileDownload")) {
System.out.println("FileDownload-Start");
pw.println(msg);
byte[] b=new byte[1024];
File file=new File(reader.readLine().trim());
System.out.println(file.getName().trim());
FileOutputStream fo=new FileOutputStream("D:\\Eclipse WorkSpace\\proekt\\src\\client\\"+file.getName().trim());
System.out.println("file: "+file.getName().trim());
in.read(b,0,b.length);
fo.write(b,0,b.length);
System.out.println("FileDownload-End");
}
I could not find what's causing this issue, so any help possible would be very highly appreciated!

It is because your client requests the data twice:
msg = sc.nextLine();
pw.println(msg); // <-- this is the first time
and then later
else if(msg.contains("FileDownload")) {
System.out.println("FileDownload-Start");
pw.println(msg); // <-- this is the second time

Java Sockets - Sending an object from the server and receiving/displaying it to the client

I am trying to send an object from my server and then receiving it/displaying it on the client side. The object in question has a few parameters tied to it, such as int values and string values. Do I also need to have a version of my server class on the client side in which to store the values from the input stream?
I have tried the following:
Server
public void run() {
int height = 6;
int width = 9;
int moves = height * width;
System.out.println("Connected: " + socket);
try {
Server server = new Server(val1, val2, str1, str2);
Scanner scanner = new Scanner(socket.getInputStream());
PrintWriter printWriter = new PrintWriter(socket.getOutputStream(), true);
ObjectOutputStream serverOutputStream = new ObjectOutputStream(socket.getOutputStream());
ObjectInputStream serverInputStream = new ObjectInputStream(socket.getInputStream());
// while (scanner.hasNextInt()) {
// printWriter.println(scanner.nextInt());
// }
System.out.println(server );
// printWriter.println(server );
serverOutputStream.writeObject(server );
// while (scanner.hasNextInt()) {
for (int player = 0; moves-- > 0; player = 1 - player) {
char symbol = PLAYERS[player];
server.doSomething(symbol, scanner);
// printWriter.println(scanner.nextInt());
System.out.println(server);
// printWriter.println(server);
serverOutputStream.writeObject(server);
if (server.hasWon()) {
System.out.println("\nPlayer " + symbol + " wins!");
return;
}
// }
}
} catch (Exception exception) {
System.out.println("Error: " + socket);
} finally {
try {
socket.close();
} catch (IOException e) {
}
System.out.println("Closed: " + socket);
}
}
Client
public static void main(String[] args) throws Exception {
// if (args.length != 1) {
// System.err.println("Pass the server IP as the sole command line argument");
//
// return;
// }
try (Socket socket = new Socket("127.0.0.1", 59898)) {
System.out.println("Enter a move: ");
Scanner scanner = new Scanner(System.in);
Scanner in = new Scanner(socket.getInputStream());
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
ObjectInputStream serverInputStream = new ObjectInputStream(socket.getInputStream());
ObjectOutputStream serverOutputStream = new ObjectOutputStream(socket.getOutputStream());
Object object = serverInputStream.readObject();
while (scanner.hasNextLine()) {
out.println(scanner.nextLine());
serverOutputStream.writeObject(object);
}
}
}

TCP sockets returning strange thing in Java

I have socket application that i use for communication with a device.
When i open socket i can read status outputs from the machine.
Machine sends some data which is separated by comma ',' and i need to parse only numbers.
The problem is when i parse the data i recieve numbers but i also recieve "empty" strings.
Here is my code:
void startListenForTCP(String ipaddress) {
Thread TCPListenerThread;
TCPListenerThread = new Thread(new Runnable() {
#Override
public void run() {
Boolean run = true;
String serverMessage = null;
InetAddress serverAddr = null;
BufferedWriter out = null;
int redni = 0;
try {
Socket clientSocket = new Socket(ipaddress, 7420);
try {
mc.pushNumbers("Connection initiated... waiting for outputs!"
+ "\n");
char[] buffer = new char[2];
int charsRead = 0;
out =
new BufferedWriter(new OutputStreamWriter(
clientSocket.getOutputStream()));
BufferedReader in =
new BufferedReader(new InputStreamReader(
clientSocket.getInputStream()));
while ((charsRead = in.read(buffer)) != -1) {
String message = new String(buffer).substring(0, charsRead);
if (message.equals("I,")) {
mc.pushNumbers("\n");
} else {
String m = message;
m = m.replaceAll("[^\\d.]", "");
String stabilo = m;
int length = stabilo.length();
String result = "";
for (int i = 0; i < length; i++) {
Character character = stabilo.charAt(i);
if (Character.isDigit(character)) {
result += character;
}
}
System.out.println("Result:" + m);
}
}
} catch (UnknownHostException e) {
mc.pushNumbers("Unknown host..." + "\n");
} catch (IOException e) {
mc.pushNumbers("IO Error..." + "\n");
} finally {
clientSocket.close();
}
} catch (IOException e) {
e.printStackTrace();
mc.pushNumbers("Connection refused by machine..." + "\n");
}
}
});
TCPListenerThread.start();
}
And the System.out.println(); returns this:
Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:26Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:Result:13
Result:Result:Result:Result:Result:Result:Result:Result:Result:
I just don't know why I can't parse only numbers, there is probably something that machine sends and it isn't parsed by m = m.replaceAll("[^\\d.]", "");

You're building your String incorrectly. It should be:
String message = new String(buffer, 0, bytesRead);
where 'bytesRead' is the count returned by the read() method. It's a byte count, not a char count.

multi client error with java multithreading

Below is the code for a client and server which handles multi user chat. But when one client writes "quit" my others current connected client also terminates and I can't then connect another client. Can anybody help with this?
Here is my client code:
class TCPClientsc {
public static void main(String argv[]) throws Exception {
String modifiedSentence;
InetAddress inetAddress = InetAddress.getLocalHost();
System.out.println(inetAddress);
Socket clientSocket = new Socket(inetAddress, 6789);
DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());
BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));
CThread write = new CThread(inFromServer, outToServer, 0, clientSocket);
CThread read = new CThread(inFromServer, outToServer, 1, clientSocket);
}
}
class CThread extends Thread {
BufferedReader inFromServer;
DataOutputStream outToServer;
Socket clientSocket = null;
int RW_Flag;
public CThread(BufferedReader in, DataOutputStream out, int rwFlag, Socket clSocket) {
inFromServer = in;
outToServer = out;
RW_Flag = rwFlag;
clientSocket = clSocket;
start();
}
public void run() {
String sentence;
try {
while (true) {
if (RW_Flag == 0) {// write
BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));
sentence = inFromUser.readLine();
// System.out.println("Writing ");
outToServer.writeBytes(sentence + '\n');
if (sentence.equals("quit"))
break;
} else if (RW_Flag == 1) {
sentence = inFromServer.readLine();
if (sentence.endsWith("quit"))
break;
System.out.println("(received)" + sentence);
}
}
} catch (Exception e) {
} finally {
try {
inFromServer.close();
outToServer.close();
clientSocket.close();
} catch (IOException ex) {
Logger.getLogger(CThread.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
}
Server code:
class TCPServersc {
static int i = 0;
static SThread tt[] = new SThread[100];
static SThread anot[] = new SThread[100];
public static void main(String argv[]) throws Exception {
String client;
String capitalizedSentence;
ServerSocket welcomeSocket = new ServerSocket(6789);
while (true) {
Socket connectionSocket = welcomeSocket.accept();
i++;
System.out.println("connection :" + i);
BufferedReader inFromClient = new BufferedReader(newInputStreamReader(connectionSocket.getInputStream()));
DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());
BufferedReader inFromMe = new BufferedReader(new InputStreamReader(System.in));
tt[i] = new SThread(inFromClient, outToClient, tt, 0, connectionSocket, i);
anot[i] = new SThread(inFromMe, outToClient, tt, 1, connectionSocket, i);
}
}
}
// ===========================================================
class SThread extends Thread {
BufferedReader inFromClient;
DataOutputStream outToClient;
String clientSentence;
SThread t[];
String client;
int status;
Socket connectionSocket;
int number;
public SThread(BufferedReader in, DataOutputStream out, SThread[] t, int status, Socket cn, int number) {
inFromClient = in;
outToClient = out;
this.t = t;
this.status = status;
connectionSocket = cn;
this.number = number;
start();
}
public void run() {
try {
if (status == 0) {
clientSentence = inFromClient.readLine();
StringTokenizer sentence = new StringTokenizer(clientSentence, " ");
// ///////////////////////////////////////////////////////////
if (sentence.nextToken().equals("login")) {
String user = sentence.nextToken();
String pass = sentence.nextToken();
FileReader fr = new FileReader("file.txt");
BufferedReader br = new BufferedReader(fr);
int flag = 0;
while ((client = br.readLine()) != null) {
if ((user.equals(client.substring(0, 5))) && (pass.equals(client.substring(6, 10)))) {
flag = 1;
System.out.println(user + " has logged on");
for (int j = 1; j <= 20; j++) {
if (t[j] != null)
t[j].outToClient.writeBytes(user + " has logged on" + '\n');// '\n' is necessary
}
break;
}
}
if (flag == 1) {
while (true) {
clientSentence = inFromClient.readLine();
System.out.println(user + " : " + clientSentence);
for (int j = 1; j <= 20; j++) {
if (t[j] != null)
// '\n' is necessary
t[j].outToClient.writeBytes(user + " : " + clientSentence + '\n');
}
// if(clientSentence.equals("quit"))break;
}
}
}
}
// /////////////////////////////////////////////////////////////////////////////////////////////////////////////////
if (status == 1) {
while (true) {
clientSentence = inFromClient.readLine();
if (clientSentence.equals("quit"))
break;
System.out.println("Server: " + clientSentence);
for (int j = 1; j <= 20; j++) {
if (t[j] != null)
t[j].outToClient.writeBytes("Server :" + clientSentence + '\n');// '\n' is necessary
}
}
}
} catch (Exception e) {
} finally {
try {
// System.out.println(this.t);
inFromClient.close();
outToClient.close();
connectionSocket.close();
} catch (IOException ex) {
Logger.getLogger(SThread.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
}

This code has a number of problems.
First off, in the future, please post smaller, concise code fragments that are well formatted. I just had to basically reformat everything in your post.
I see a couple of places where you are catching but doing nothing with exceptions. This is tremendously bad practice. At the least you should be printing/logging the exceptions you catch. I suspect this is contributing to your problems.
I find the RW_Flag very confusing. You should have two client threads then. One to write from System.in to the server and one to read. Don't have one client thread which does 2 things. Same with status flag in the server. That should be 2 different threads.
Instead of int flag = 0; in the server, that should be boolean loggedIn;. Make use of booleans in Java instead of C-style flags and use better variable names. The code readability will pay for itself. Same for status, RW_flag, etc..
Instead of huge code blocks, you should move contiguous code out to methods: handleSystemIn(), handleClient(), talkToServer(). Once you make more methods in the your code, and shrink down the individual code blocks, it makes it much more readable/debuggable/understandable.
You need to have a synchronized (tt) block around each usage of that array. Once you have multiple threads that are all using tt if the main accept thread adds to it, the updates need to be synchronized.
I don't immediately see the problem although the spagetti code is just too hard to parse. I suspect you are throwing and exception somewhere which is the reason why clients can't connect after the first one quits. Other than that, I would continue to use liberal use of System.out.println debugging to see what messages are being sent where.