Develop Reference

Why I'm not getting any response in Console in Eclipse with the following program?

what's wrong in this code? I'm trying to make compiler accept my keyboard input; but compilation is not ending while it's also not throwing any error or warning. I'm using Eclipse 4.14 for Mac OS X. Can anyone suggest.
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
public class TestInputStream {
public static void main(String args[]) throws IOException {
InputStreamReader Obj = new InputStreamReader(System.in);
BufferedReader br = new BufferedReader(Obj);
int a = Integer.parseInt(br.readLine());
System.out.println("Please enter your choice");
System.out.println("You have entered " + a);
}
}

Java Client-Server Programming: Shared Memory Between Clients

I am developing a simple 2D multiplayer game in Java, following the Client-Server model. The functionality in terms of the socket programming is simple:
A Client sends the Server their character data (x position, y position, health points, etc.)
The Server registers the updated character data in a table which is storing all the different Clients' character data
The Server sends back the table to the Client so that they can render the other characters
I am running into a weird issue that I would like help understanding so I can move forward with the project. Here is the code I'm running right now.
import java.io.IOException;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Set;
import java.util.Vector;
import java.util.HashSet;
import java.util.Scanner;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class Server
{
private static Vector<Integer> IDs = new Vector<>();
public static void main(String[] args) throws Exception
{
ExecutorService pool = Executors.newFixedThreadPool(32);
try (ServerSocket listener = new ServerSocket(40000))
{
System.out.println("The server is up.");
while (true)
{
pool.execute(new Player(listener.accept()));
}
}
}
private static class Player implements Runnable
{
private Socket socket;
private Scanner input;
private PrintWriter output;
private int ID;
public Player(Socket socket)
{
this.socket = socket;
this.ID = IDs.size() + 1;
IDs.add(ID);
}
public void run()
{
try
{
input = new Scanner(socket.getInputStream());
output = new PrintWriter(socket.getOutputStream(), true);
while(input.hasNextLine())
{
String result = "";
for(int i = 0; i < IDs.size(); i++)
{
result += (IDs.get(i) + " ");
}
output.println(result);
}
}
catch(Exception e)
{
System.out.println(e);
}
}
}
}
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.Socket;
import java.util.Scanner;
public class Client {
public static void main(String[] args) throws Exception {
try (Socket socket = new Socket("127.0.0.1", 40000)) {
Scanner scanner = new Scanner(System.in);
Scanner in = new Scanner(socket.getInputStream());
PrintWriter out = new PrintWriter(socket.getOutputStream(), true);
while (scanner.hasNextLine()) {
out.println(scanner.nextLine());
System.out.println(in.nextLine());
}
in.close();
scanner.close();
}
}
}
What I want to happen is: every time a Client sends any message to the Server, the Server responds with a list of IDs (each Client that connects gets a fresh ID).
What is actually happening:
The second Client that connects is able to see that there is a first Client that is already connected. But the first Client is not able to see that there is a second Client connected.
Thank you for your help.

The server loop
while(input.hasNextLine())
{
String result = "";
for(int i = 0; i < IDs.size(); i++)
{
result += (IDs.get(i) + " ");
}
output.println(result);
}
never calls input.nextLine(), which means that input.hasNextLine() is always true, so the server floods the socket with output until the buffer is full.
This can be seen by starting a client, killing the server, then keep pressing enter. The client keeps receiving data, even though the server is gone, until the buffer is emptied.
Add the following line inside the loop:
System.out.println(this.ID + ": " + input.nextLine());
This will make the server consume the line from the client, as was intended, and allow you to see / verify the data flowing.

Connect multiple clients to one server

I'm trying to connect all my clients to one server. I've done some research and found out that the easiest way to do it is create a new thread for every client that connects to the server. But I am already stuck on the part where a client disconnects and reconnect.
Client
import java.io.IOException;
import java.io.PrintStream;
import java.net.Socket;
import java.net.UnknownHostException;
import java.util.Scanner;
public class Test {
private static int port = 40021;
private static String ip = "localhost";
public static void main(String[] args) throws UnknownHostException,
IOException {
String command, temp;
Scanner scanner = new Scanner(System.in);
Socket s = new Socket(ip, port);
while (true) {
Scanner scanneri = new Scanner(s.getInputStream());
System.out.println("Enter any command");
command = scanner.nextLine();
PrintStream p = new PrintStream(s.getOutputStream());
p.println(command);
temp = scanneri.nextLine();
System.out.println(temp);
}
}
}
Server
import java.io.IOException;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;
public class MainClass {
public static void main(String args[]) throws IOException {
String command, temp;
ServerSocket s1 = new ServerSocket(40021);
while (true) {
Socket ss = s1.accept();
Scanner sc = new Scanner(ss.getInputStream());
while (sc.hasNextLine()) {
command = sc.nextLine();
temp = command + " this is what you said.";
PrintStream p = new PrintStream(ss.getOutputStream());
p.println(temp);
}
}
}
}
When I connect once it works correctly but as soon as I disconnect the client and try to reconnect (or connect a second client) it does not give an error or anything it just does not function. I am trying to keep it as basic as possible.
The output with one client:
When I try and connect a second client:
I hope someone could help me out. Thanks in advance.

Your server currently handles only 1 client at a time, use threads for each client, Modify your server code like this:-
public static void main(String[] args) throws IOException
{
ServerSocket s1 = new ServerSocket(40021);
while (true)
{
ss = s1.accept();
Thread t = new Thread()
{
public void run()
{
try
{
String command, temp;
Scanner sc = new Scanner(ss.getInputStream());
while (sc.hasNextLine())
{
command = sc.nextLine();
temp = command + " this is what you said.";
PrintStream p = new PrintStream(ss.getOutputStream());
p.println(temp);
}
} catch (IOException e)
{
e.printStackTrace();
}
}
};
t.start();
}
}

Why am I getting a NoSuchElementException in this Java client/server setup?

I am making a java client/server socket program in netbeans that is supposed to accept a number and then return the square root/square the number.
It allows me to type in a number and then i get the following statement:
Exception found in Client: java.util.NoSuchElementException.
I am not sure why it is doing this, please help!
Here is my code for both classes:
Client:
package question1.clientserver;
import java.net.*;
import java.io.*;
import java.util.Scanner;
public class Client {
public static void main(String[] args) throws IOException {
try {
int number, temp;
Scanner sc = new Scanner(System.in);
Socket s = new Socket("127.0.0.1", 1452);
Scanner sc1 = new Scanner(s.getInputStream());
System.out.println("Enter any number to be squared");
number = sc.nextInt();
PrintStream p = new PrintStream(s.getOutputStream());
p.println(number);
temp = sc1.nextInt();
System.out.println(temp);
} catch (Exception e) {
System.out.println("Exception found in Client: " + e);
}
}
}
and this is my server code
package question1.clientserver;
import java.io.IOException;
import java.io.PrintStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Scanner;
public class Server {
public static void main(String[] args) {
while (true) {
System.out.println("Waiting for connection, please stand by...");
try {
int number, temp;
ServerSocket s1 = new ServerSocket(1452);
Socket ss = s1.accept();
System.out.println("Connection has been established");
Scanner sc = new Scanner(ss.getInputStream());
number = sc.nextInt();
temp = number * number;
PrintStream p = new PrintStream(ss.getOutputStream());
p.println(temp);
} catch (Exception e) {
System.out.println("Exception in Server while creating connection" + e);
}
}
}
}
}
if you have any ideas or suggestions please let me know!
stacktrace
run: Enter any number to be squared
25 (which is what i type in for example)
Exception found in Client: java.util.NoSuchElementException
Build Successful (total time: 31 seconds)

String data from a Scanner through a PrintWriter

I am trying to pass data from a String into a PrintWriter while simultaneously reading from a BufferedReader between two classes named Server.java and Client.java. My problem is that I am having trouble handling the exceptions that are being thrown from the block of code that reads data from the Scanner object (marked below).
Client.java
package root;
/**
* #author Noah Teshima
*/
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.InetAddress;
import java.net.Socket;
import java.util.Scanner;
public class Client {
private Socket clientSocket;
private BufferedReader bufferedReader;
private InputStreamReader inputStreamReader;
private PrintWriter printWriter;
private Scanner scanner;
public static void main(String[] args) {
new Client("localhost", 1025);
}
public Client(String hostName, int portNumber) {
try {
this.clientSocket = new Socket(hostName, portNumber);
this.bufferedReader = new BufferedReader(this.inputStreamReader = new InputStreamReader(this.clientSocket.getInputStream()));
this.printWriter = new PrintWriter(clientSocket.getOutputStream());
String msg = "",
msg2 = "";
this.printWriter.println(this.getClass());
this.printWriter.flush();
System.out.println(this.getClass().getName() + " is connected to " + this.bufferedReader.readLine());
while(!(msg = this.scanner.nextLine()).equals("exit")) { //Source of problem
this.printWriter.println(this.getClass().getName() + ": " + msg);
this.printWriter.flush();
while((msg2 = this.bufferedReader.readLine()) != null) {
System.out.println(msg2);
}
}
this.clientSocket.close();
}catch(IOException exception) {
exception.printStackTrace();
}
}
}
Stack trace::
Exception in thread "main" java.lang.NullPointerException
at root.Client.<init>(Client.java:47)
at root.Client.main(Client.java:25)
The code used to read from the BufferedReader and write to a PrintWriter is the same for both classes, so I only posted Client.java. If anyone would like to see the other class file, I would be happy to do so.

Before you use
while(!(msg = this.scanner.nextLine()).equals("exit")) { //Source of problem
You should have initialized the scanner.
The scanner object is null when you used it, and hence the NullPointerException.
I do not see a scanner = new Scanner(...); anywhere within your code,
maybe you have forgetten about it?