variable access from a constructor

[akila sekaran]

i got command line argument for a user name. I then initialized it in constructor of the class.

i created a thread from this constructor and tried using this name variable in run method.

It returns null . How do i use this variable inside a run method.

i tried using getName method which return name. But it returns null too.

 public static void main(String args[]) 
 {
 
  int port=Integer.parseInt(args[0]);
  String name=args[1];
  AppServer server = new AppServer(port,name);
 }
} 

public AppClient(int port,String ip,String name)
 { 
  port=port;
  name=name;
  try{  }
  catch(Exception e) { }
  th=new Thread(this);
  th.start();
 }

public void run()
 {
  while(true)
  {
   try{
    ta.append(getName()+":"+br.readLine()+"\n");//Append to TextArea
   }catch(Exception e) {}
  }
 }

 public String getName()
 { 
   return name;
 }

[Michael Krimgen]

Hi,

It would be useful if you posted the whole code.

You are saying that you initialize the name variable in the constructor of the AppServer class and start a thread from it.
However, in the code you posted you start a thread from the constructor of the AppClient class....

Cheers,
Michael

[akila sekaran]

Michael Krimgen wrote:Hi,

It would be useful if you posted the whole code.

You are saying that you initialize the name variable in the constructor of the AppServer class and start a thread from it.
However, in the code you posted you start a thread from the constructor of the AppClient class....

Cheers,
Michael

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;

public class AppServer extends Frame implements ActionListener,Runnable
{
 //Declarations
 Button b1;
 TextField tf;
 TextArea ta;
 ServerSocket ss;
 Socket s;
 PrintWriter pw;
 BufferedReader br;
 Thread th;
 String name;
 
 public AppServer(int port,String name)
 {
  Frame f=new Frame("Server Side Chatting");//Frame for Server
  f.setLayout(new FlowLayout());//set layout
  
  b1=new Button("Send");//Send Button
  
  b1.addActionListener(this);//Add action listener to send button.
  tf=new TextField(15);
  ta=new TextArea(12,20);
  port=port;
  //name=name;
  //System.out.println(name);
  f.addWindowListener(new W1());//add Window Listener to the Frame
  f.add(tf);//Add TextField to the frame
  f.add(b1);//Add send Button to the frame
  f.add(ta);//Add TextArea to the frame
  try{
   ss=new ServerSocket(port);//Socket for server
   s=ss.accept();//accepts request from client
   System.out.println(s);
   //below line reads input from InputStreamReader
   br=new BufferedReader(new InputStreamReader(s.getInputStream()));
   //below line writes output to OutPutStream
   pw=new PrintWriter(s.getOutputStream(),true);
  }catch(Exception e)
  {
  }
  th=new Thread(this);//start a new thread
  //th.setDaemon(true);//set the thread as demon
  th.start();
  //setFont(new Font("Arial",Font.BOLD,20));
  f.setSize(200,200);//set the size
  f.setLocation(300,300);//set the location
  f.setVisible(true);
  //f.validate();
 }
 //method required to close the Frame on clicking "X" icon.
 private class W1 extends WindowAdapter
 {
  public void windowClosing(WindowEvent we) 
  {
   System.exit(0);
  }
 }
 public String getName()
 {
  return name;
 }
 //This method will called after clicking on Send button.
 public void actionPerformed(ActionEvent ae)
 {
  ta.append("myself:"+tf.getText()+"\n");
  pw.println(tf.getText());//write the value of textfield into PrintWriter
  tf.setText("");//clean the textfield
 }
 //Thread running as a process in background
 public void run()
 {
  while(true)
  {
   try{
    String s=br.readLine();//reads the input from textfield
    ta.append(name +": "+s+"\n");//Append to TextArea
   }catch(Exception e)
   {
   }
  } 
 }
 //Main method
 public static void main(String args[]) 
 {
  //Instantiate AppServer class
  int port=Integer.parseInt(args[0]);
  String name=args[1];
  AppServer server = new AppServer(port,name);//name
 }
} 

[akila sekaran]

akila sekaran wrote:

Michael Krimgen wrote:Hi,

It would be useful if you posted the whole code.

You are saying that you initialize the name variable in the constructor of the AppServer class and start a thread from it.
However, in the code you posted you start a thread from the constructor of the AppClient class....

Cheers,
Michael

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.net.*;

public class AppServer extends Frame implements ActionListener,Runnable
{
 //Declarations
 Button b1;
 TextField tf;
 TextArea ta;
 ServerSocket ss;
 Socket s;
 PrintWriter pw;
 BufferedReader br;
 Thread th;
 String name;
 
 public AppServer(int port,String name)
 {
  Frame f=new Frame("Server Side Chatting");//Frame for Server
  f.setLayout(new FlowLayout());//set layout
  
  b1=new Button("Send");//Send Button
  
  b1.addActionListener(this);//Add action listener to send button.
  tf=new TextField(15);
  ta=new TextArea(12,20);
  port=port;
  //name=name;
  //System.out.println(name);
  f.addWindowListener(new W1());//add Window Listener to the Frame
  f.add(tf);//Add TextField to the frame
  f.add(b1);//Add send Button to the frame
  f.add(ta);//Add TextArea to the frame
  try{
   ss=new ServerSocket(port);//Socket for server
   s=ss.accept();//accepts request from client
   System.out.println(s);
   //below line reads input from InputStreamReader
   br=new BufferedReader(new InputStreamReader(s.getInputStream()));
   //below line writes output to OutPutStream
   pw=new PrintWriter(s.getOutputStream(),true);
  }catch(Exception e)
  {
  }
  th=new Thread(this);//start a new thread
  //th.setDaemon(true);//set the thread as demon
  th.start();
  //setFont(new Font("Arial",Font.BOLD,20));
  f.setSize(200,200);//set the size
  f.setLocation(300,300);//set the location
  f.setVisible(true);
  //f.validate();
 }
 //method required to close the Frame on clicking "X" icon.
 private class W1 extends WindowAdapter
 {
  public void windowClosing(WindowEvent we) 
  {
   System.exit(0);
  }
 }
 public String getName()
 {
  return name;
 }
 //This method will called after clicking on Send button.
 public void actionPerformed(ActionEvent ae)
 {
  ta.append("myself:"+tf.getText()+"\n");
  pw.println(tf.getText());//write the value of textfield into PrintWriter
  tf.setText("");//clean the textfield
 }
 //Thread running as a process in background
 public void run()
 {
  while(true)
  {
   try{
    String s=br.readLine();//reads the input from textfield
    ta.append(name +": "+s+"\n");//Append to TextArea
   }catch(Exception e)
   {
   }
  } 
 }
 //Main method
 public static void main(String args[]) 
 {
  //Instantiate AppServer class
  int port=Integer.parseInt(args[0]);
  String name=args[1];
  AppServer server = new AppServer(port,name);//name
 }
} 

this is the code . Sorry that was a typo in the class name of my previous code sample

[Steve Luke]

First - you comment the line which is supposed to do the assignment.

Second - your parameter has the same name as the instance variable, and so the local parameter would hide the instance variable: so doing name = name is a non-action (assigning a value to itself). You want to qualify which name variable you want to assign to using the this keyword: this.name = name.

[akila sekaran]

Steve Luke wrote:First - you comment the line which is supposed to do the assignment.

Second - your parameter has the same name as the instance variable, and so the local parameter would hide the instance variable: so doing name = name is a non-action (assigning a value to itself). You want to qualify which name variable you want to assign to using the this keyword: this.name = name.

ohh yeahhh thanks a lot. "this" worked!! My code works fine now

[Steve Luke]

There are tricks to make this less likely to happen in the future. One such trick is to always make your method parameters final: public AppServer(final int port, final String name). Then if you tried name = name you would get a compile-time error that points to the offending line rather than a run-time behavior problem which occurs in a different method at some later point.

Behaviorally, another thing I like to do is get in the habit of always using the this keyword to qualify instance variable references.

[akila sekaran]

Steve Luke wrote:There are tricks to make this less likely to happen in the future. One such trick is to always make your method parameters final: public AppServer(final int port, final String name). Then if you tried name = name you would get a compile-time error that points to the offending line rather than a run-time behavior problem which occurs in a different method at some later point.

Behaviorally, another thing I like to do is get in the habit of always using the this keyword to qualify instance variable references.[/quote


Oh okay got it! thank you steve!