Hello,

I am creating a client/server application that verifies math equations for addition, subtraction, and division. I think I just need someone else’s eyes to possibly see where I am going wrong with this. Everything seems to be working fine except for one variable. This is the String serverAnswer variable in my Server class. When I debug the application, the serverAnswer variable remains null, can anyone tell me what I’m doing wrong? I wrote the code for where it should be populated in the processConnection() method of my server class. I know this is a lot of code, but it is needed to debug the application. Thank you.

import java.io.EOFException;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.ServerSocket;
import java.net.Socket;
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.script.ScriptEngine;
import javax.script.ScriptEngineManager;
import javax.script.ScriptException;
import java.lang.*;
import sun.io.Converters;
import javax.script.*;

public class Server extends JFrame 
{
   private JTextField enterField; // inputs message from user
   private JTextArea displayArea; // display information to user
   private ObjectOutputStream output; // output stream to client
   private ObjectInputStream input; // input stream from client
   private ServerSocket server; // server socket
   private Socket connection; // connection to client
   private int counter = 1; // counter of number of connections
   String serverAnswer;
   //private String serverAnswer;

   // set up GUI
   public Server()
   {
      super( "Server" );

      enterField = new JTextField(); // create enterField
      enterField.setEditable( false );
      enterField.addActionListener(
         new ActionListener() 
         {
            // send message to client
            public void actionPerformed( ActionEvent event )
            {
               sendData( event.getActionCommand() );
               enterField.setText( "" );
            } // end method actionPerformed
         } // end anonymous inner class
      ); // end call to addActionListener

      add( enterField, BorderLayout.NORTH );

      displayArea = new JTextArea(); // create displayArea
      add( new JScrollPane( displayArea ), BorderLayout.CENTER );

      setSize( 300, 150 ); // set size of window
      setVisible( true ); // show window
   } // end Server constructor

   // set up and run server 
   public void runServer()
   {
      try // set up server to receive connections; process connections
      {
         server = new ServerSocket( 12345, 100 ); // create ServerSocket

         while ( true ) 
         {
            try 
            {
               waitForConnection(); // wait for a connection
               getStreams(); // get input & output streams
               processConnection(); // process connection
            } // end try
            catch ( EOFException eofException ) 
            {
               displayMessage( "\nServer terminated connection" );
            } // end catch
            finally 
            {
               closeConnection(); //  close connection
               counter++;
            } // end finally
         } // end while
      } // end try
      catch ( IOException ioException ) 
      {
         ioException.printStackTrace();
      } // end catch
   } // end method runServer

   // wait for connection to arrive, then display connection info
   private void waitForConnection() throws IOException
   {
      displayMessage( "Waiting for connection\n" );
      connection = server.accept(); // allow server to accept connection            
      displayMessage( "Connection " + counter + " received from: " +
         connection.getInetAddress().getHostName() );
   } // end method waitForConnection

   // get streams to send and receive data
   private void getStreams() throws IOException
   {
      // set up output stream for objects
      output = new ObjectOutputStream( connection.getOutputStream() );
      output.flush(); // flush output buffer to send header information

      // set up input stream for objects
      input = new ObjectInputStream( connection.getInputStream() );

      displayMessage( "\nGot I/O streams\n" );
   } // end method getStreams

   // process connection with client
   private void processConnection() throws IOException
   {
      
      String message = "Connection successful";
      sendData( message ); // send connection successful message
      

      // enable enterField so server user can send messages
      setTextFieldEditable( false );
      ScriptEngine engine = new ScriptEngineManager().getEngineByExtension("js");

      do // process messages sent from client

      { 
         try // read message and display it
         {
            message = ( String ) input.readObject(); // read new message
          if (message.contains("+")){
            Object result = engine.eval(message);
            serverAnswer = String.valueOf(result);
            }
          else if(message.contains("/")){
            Object result = engine.eval(message);
            serverAnswer = String.valueOf(result);
          }
          else if(message.contains("-")){
            Object result = engine.eval(message);
            serverAnswer = String.valueOf(result);
          }
          else if (message.equals(serverAnswer)){
              displayMessage("\n" + "You are correct");
          }
          else{
            displayMessage("\n" + "Incorrect!");

          }
            
            //displayMessage("\n" + serverAnswer);

        }
        catch (ScriptException e) {
            // Something went wrong
            e.printStackTrace();
        }
        catch (ClassNotFoundException classNotFoundException){
            displayMessage ( "\nUnknown object type received" );
        }


            displayMessage( "\n" + message ); // display message
         } // end try
        /* catch ( ClassNotFoundException classNotFoundException )
         {
            displayMessage( "\nUnknown object type received" );
         } // end catch

      } */while ( !message.equals( "CLIENT>>> TERMINATE" ) );


   } // end method processConnection
   

   // close streams and socket
   private void closeConnection() 
   {
      displayMessage( "\nTerminating connection\n" );
      setTextFieldEditable( false ); // disable enterField

      try 
      {
         output.close(); // close output stream
         input.close(); // close input stream
         connection.close(); // close socket
      } // end try
      catch ( IOException ioException ) 
      {
         ioException.printStackTrace();
      } // end catch
   } // end method closeConnection

   // send message to client
   private void sendData( String message )
   {
      try // send object to client
      {
         output.writeObject( "SERVER>>> " + message );
         output.flush(); // flush output to client
         displayMessage( "\nSERVER>>> " + message );
      } // end try
      catch ( IOException ioException ) 
      {
         displayArea.append( "\nError writing object" );
      } // end catch
   } // end method sendData

   // manipulates displayArea in the event-dispatch thread
   private void displayMessage( final String messageToDisplay )
   {
      SwingUtilities.invokeLater(
         new Runnable() 
         {
            public void run() // updates displayArea
            {
               displayArea.append( messageToDisplay ); // append message
            } // end method run
         } // end anonymous inner class
      ); // end call to SwingUtilities.invokeLater
   } // end method displayMessage

   // manipulates enterField in the event-dispatch thread
   private void setTextFieldEditable( final boolean editable )
   {
      SwingUtilities.invokeLater(
         new Runnable()
         {
            public void run() // sets enterField's editability
            {
               enterField.setEditable( editable );
            } // end method run
         }  // end inner class
      ); // end call to SwingUtilities.invokeLater
   } // end method setTextFieldEditable
} // end class Server
import javax.swing.JFrame;

public class ServerTest
{
   public static void main( String args[] )
   {
      Server application = new Server(); // create server
      application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      application.runServer(); // run server application
   } // end main
} // end class ServerTest
import java.io.EOFException;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.net.InetAddress;
import java.net.Socket;
import java.awt.BorderLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JScrollPane;
import javax.swing.JTextArea;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.JLabel;

public class Client extends JFrame 
{
   private JTextField equationField; // enter equation here
   private JTextField answerField;//enter answer here
   private JTextArea displayArea; // display information to user
   private ObjectOutputStream output; // output stream to server
   private ObjectInputStream input; // input stream from server
   private String message = ""; // message from server
   private String chatServer; // host server for this application
   private Socket client; // socket to communicate with server
   private JLabel label;//direction label

   // initialize chatServer and set up GUI
   public Client( String host )
   {
      super( "Client" );

      chatServer = host; // set server to which this client connects
      /*
      label = new JLabel("Enter equation above and answer below.");//create label
      add (label, BorderLayout.NORTH);
      */
      equationField = new JTextField(); // create enterField
      equationField.setEditable( false );
      equationField.addActionListener(
         new ActionListener() 
         {
            // send message to server
            public void actionPerformed( ActionEvent event )
            {
               sendData( event.getActionCommand() );
               equationField.setText( "" );
            } // end method actionPerformed
         } // end anonymous inner class
      ); // end call to addActionListener

      add( equationField, BorderLayout.NORTH);

      answerField = new JTextField(); // create enterField
      answerField.setEditable( false );
      answerField.addActionListener(
         new ActionListener()
         {
            // send message to server
            public void actionPerformed( ActionEvent event )
            {
               sendData( event.getActionCommand() );
               answerField.setText( "" );
            } // end method actionPerformed
         } // end anonymous inner class
      ); // end call to addActionListener

      add( answerField, BorderLayout.SOUTH );

      displayArea = new JTextArea(); // create displayArea
      add( new JScrollPane( displayArea ), BorderLayout.CENTER);

      setSize( 500, 250 ); // set size of window
      setVisible( true ); // show window
   } // end Client constructor

   // connect to server and process messages from server
   public void runClient() 
   {
      try // connect to server, get streams, process connection
      {
         connectToServer(); // create a Socket to make connection
         getStreams(); // get the input and output streams
         processConnection(); // process connection
      } // end try
      catch ( EOFException eofException ) 
      {
         displayMessage( "\nClient terminated connection" );
      } // end catch
      catch ( IOException ioException ) 
      {
         ioException.printStackTrace();
      } // end catch
      finally 
      {
         closeConnection(); // close connection
      } // end finally
   } // end method runClient

   // connect to server
   private void connectToServer() throws IOException
   {      
      displayMessage( "Attempting connection\n" );

      // create Socket to make connection to server
      client = new Socket( InetAddress.getByName( chatServer ), 12345 );
      displayMessage("Enter equation in top field and answer in the bottom field\n");
      // display connection information
      displayMessage( "Connected to: " + 
         client.getInetAddress().getHostName() );
      
   } // end method connectToServer

   // get streams to send and receive data
   private void getStreams() throws IOException
   {
      // set up output stream for objects
      output = new ObjectOutputStream( client.getOutputStream() );      
      output.flush(); // flush output buffer to send header information

      // set up input stream for objects
      input = new ObjectInputStream( client.getInputStream() );

      displayMessage( "\nGot I/O streams\n" );
   } // end method getStreams

   // process connection with server
   private void processConnection() throws IOException
   {
      // enable enterField so client user can send messages
      setTextFieldEditable( true );

      do // process messages sent from server
      { 
         try // read message and display it
         {
            message = ( String ) input.readObject(); // read new message
            displayMessage( "\n" + message ); // display message
         } // end try
         catch ( ClassNotFoundException classNotFoundException ) 
         {
            displayMessage( "\nUnknown object type received" );
         } // end catch

      } while ( !message.equals( "SERVER>>> TERMINATE" ) );
   } // end method processConnection

   // close streams and socket
   private void closeConnection() 
   {
      displayMessage( "\nClosing connection" );
      setTextFieldEditable( false ); // disable enterField

      try 
      {
         output.close(); // close output stream
         input.close(); // close input stream
         client.close(); // close socket
      } // end try
      catch ( IOException ioException ) 
      {
         ioException.printStackTrace();
      } // end catch
   } // end method closeConnection

   // send message to server
   private void sendData( String message )
   {
      try // send object to server
      {
         output.writeObject( "CLIENT>>> " + message );
         output.flush(); // flush data to output
         displayMessage( "\nCLIENT>>> " + message );
      } // end try
      catch ( IOException ioException )
      {
         displayArea.append( "\nError writing object" );
      } // end catch
   } // end method sendData

   // manipulates displayArea in the event-dispatch thread
   private void displayMessage( final String messageToDisplay )
   {
      SwingUtilities.invokeLater(
         new Runnable()
         {
            public void run() // updates displayArea
            {
               displayArea.append( messageToDisplay );
            } // end method run
         }  // end anonymous inner class
      ); // end call to SwingUtilities.invokeLater
   } // end method displayMessage

   // manipulates enterField in the event-dispatch thread
   private void setTextFieldEditable( final boolean editable )
   {
      SwingUtilities.invokeLater(
         new Runnable() 
         {
            public void run() // sets enterField's editability
            {
               equationField.setEditable( editable );
               answerField.setEditable(editable);
            } // end method run
         } // end anonymous inner class
      ); // end call to SwingUtilities.invokeLater
   } // end method setTextFieldEditable
} // end class Client
import javax.swing.JFrame;

public class ClientTest 
{
   public static void main( String args[] )
   {
      Client application; // declare client application

      // if no command line args
      if ( args.length == 0 )
         application = new Client( "127.0.0.1" ); // connect to localhost
      else
         application = new Client( args[ 0 ] ); // use args to connect

      application.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
      application.runClient(); // run client application
   } // end main
} // end class ClientTest
private void processConnection() throws IOException {
        String message = "Connection successful";
...
  message = (String) input.readObject(); // read new message
                displayMessage("\n|" + message + "|"); 
                ///"CLIENT>>> " TO REMOVE!
                message = message.substring(10);
...

it's all

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