Question

import java.net.*;
import java.io.*;

public class DjPageDownloader {


    public URL url;
    public InputStream is = null;
    public DataInputStream dis;
    public String line;

    public static void main(String []args){
        try {
            url = new URL("http://stackoverflow.com/");
            is = url.openStream();  // throws an IOException
            dis = new DataInputStream(new BufferedInputStream(is));

            while ((line = dis.readLine()) != null) {
                System.out.println(line);
            }
        } catch (MalformedURLException mue) {
             mue.printStackTrace();
        } catch (IOException ioe) {
             ioe.printStackTrace();
        } finally {
            try {
                is.close();
            } catch (IOException ioe) {
                // nothing to see here
            }
        }

    }

this on compilation shows error as:-

Exception in thread "main" java.lang.Error: Unresolved compilation problems: 
    Cannot make a static reference to the non-static field url
    Cannot make a static reference to the non-static field is
    Cannot make a static reference to the non-static field url
    Cannot make a static reference to the non-static field dis
    Cannot make a static reference to the non-static field is
    Cannot make a static reference to the non-static field line
    Cannot make a static reference to the non-static field dis
    Cannot make a static reference to the non-static field line
    Cannot make a static reference to the non-static field is

    at djPageDownloader.DjPageDownloader.main(DjPageDownloader.java:16)

Answer 1

Make all of your fields static if you are just calling them from the Main class. You can’t call an instance field from a static position, as you aren’t currently in an instance. If you wish to keep them non-static, you have to make an instance of DjPageDownloader by putting it in a constructor, and calling that, like so:

public DjPageDownloader() {
    try {
        url = new URL("http://stackoverflow.com/");
        is = url.openStream();  // throws an IOException
        dis = new DataInputStream(new BufferedInputStream(is));

        while ((line = dis.readLine()) != null) {
            System.out.println(line);
        }
    } catch (MalformedURLException mue) {
         mue.printStackTrace();
    } catch (IOException ioe) {
         ioe.printStackTrace();
    } finally {
        try {
            is.close();
        } catch (IOException ioe) {
            // nothing to see here
        }
    }
}

Then call that in your main method:

public static void main(String []args){
    new DjPageDownloader();//If you use it more later, store to a variable, but atm you aren't
}