Input and Output
Input and OutputThe following piece of code reads a file and dumps the contents to standard output because we use System.out.println(). We could have written the contents to another file by opening another stream.
import java.io.*;
public class DumpFile
{
// character(non-GUI) application
public static void main(String args[])
{
FileInputStream in = null;
String line;
// We need wrap this sensitive area of code with
// try/catch block to catch any exceptions that
// have occurred
try
{
in = new FileInputStream(args[0]);
}
catch (Throwable e)
{
System.out.println("Usage: java DumpFile file");
System.exit(1);
}
DataInputStream dis = new DataInputStream(in);
// End-of-File is -1, but in this case as long as the
// string returned is not null, continue
try
{
while ((line = dis.readLine()) != null)
System.out.println(line);
in.close();
}
catch(IOException e)
{
System.out.println("I/O Error");
System.exit(1);
}
}
}
Outputting data to a file. Prints Hello World to a file specified by the second parameter. Very foolish
and only useful as an example.
import java.io.*;
public class OutputFile
{
public static void main(String args[])
{
FileOutputStream out = null;
// Attempt to open the file, if we can't display error
// and quit
try
{
out = new FileOutputStream(args[1]);
}
catch (Throwable e)
{
System.out.println("Error in opening file");
System.exit(1);
}
PrintStream ps = new PrintStream(out);
try
{
ps.println("Hello World");
out.close();
}
catch(IOException e)
{
System.out.println("I/O Error");
System.exit(1);
}
}
}
A basic CGI. The server must be either Solaris, Win95 or WinNT.
Another common method is to read a HTML file and dump the contents
to the standard output like DumpFile does. But this demostrates the
concept of how CGIs work. Please read NCSA documentation on CGIs to
learn more.
import java.io.*;
public class BasicCGI
{
public static void main(String args[])
{
// HTML files need this header
System.out.println("Content-type: text/html");
System.out.println();
// Now you could read in a file and dump the contents
System.out.println("<HTML><HEAD>");
System.out.println("<TITLE>Hello World</TITLE>");
System.out.println("</HEAD>");
// Body goes here
System.out.println("</HTML>");
}
}
Q:Can applets use I/O?
A:Yes and No. Applets are restricted to I/O to and from the server. The only way to allow the applet to write to the client's side disk is for the user(client) to install special .class himself and modify the access rights.Q:Best way to implement I/O in Applications?
A:Use FileDialog to save/write to files in addtion to using File/Data/Buffered Input/Output Streams
First thing to do is to open a connection. Lets go over what you can do(applets can't just jump to any site in the world and read files off the server. Everything you can do by Netscape should be possible by an applet )
This simple example displays a button and with a click you goto to the tutorial
// The AWT package, applet package and networking package
import java.awt.*;
import java.applet.*;
import java.net.*;
public class URLTest extends Applet
{
URL u = null;
// This applet may not work if used locally via file://
public void init()
{
setLayout(new BorderLayout());
}
// Add a button that is centered when clicked goes to the
// tutorial
public void start()
{
add("Center", new Button("Go to Tutorial"));
}
// action is called when the button has been clicked
public boolean action(Event evt, Object arg)
{
if("Go to Tutorial".equals(arg))
{
// If there is an error in creating the URL, we will
// set it to the default
try
{
u = getDocumentBase();
u = new URL("http://ugweb.cs.ualberta.ca/~nelson/java/AWT.Tutorial.html");
}
catch(MalformedURLException u)
{
showStatus("Error");
}
// Uses the browser to display the
// new document
getAppletContext().showDocument(u);
return true;
}
return false;
}
}
Note: The JDK Beta-2 and later added showDocument(url, target), where target is string specifying how you want to display the URL. i.e such in a new browser window.
URL u = null;
URLConnection uc = null;
try
{
u = new URL("http://ugweb.cs.ualberta.ca/~nelson/java/AWT.Tutorial.html");
}
catch(MalformedURLException e)
{
System.out.println("Error in given URL");
return;
}
// Now that you have the URL, open the connection
try
{
uc = u.openConnection();
}
catch(IOException e)
{
System.out.println("Error in opening URLConnection");
return;
}
After opening the connection, what do you do? You now can read and write using InputStream/OutputStream. Both similar to FileInputStream and FileOutputStream...
InputStream is = uc.getInputStream(); OutputStream os = uc.getOutputStream();
// In general you do the following, where host is a machine // name, i.e java.sun.com and port say is 80 if http Socket socket = new Socket(host, port_number); in = new DataInputStream(new BufferedInputStream(socket.getInputStream())); out = new DataOutputStream(new BufferedOutputStream(socket.getOutputStream()));Under applets you must do the following.
Socket socket = new Socket(getDocumentBase().getHost(), getDocumentBase().getPort()); // OrBecause you still can not jump to an arbitrary site with applets and this includes sockets. Therefore examples such as "Stock ticker" are difficult without the use of a Server-side CGI and/or inter-applet communication.
Miscellaneous resources
Q: What else is there to know?
A: Quite a bit. You should read up on http://(the http protocol, URLs/URIs and TCP/IP(sockets).Q:How do I do inter-applet communication, Is it useful?
A: Inter-applet communication can get around the one-site for each applet problem. Have one connect to the stock ticker server itself and pass the data to the other applet. Check out the FAQ by Elliotte Harold Russell.Q: Do I really need to do the above?
A: No. But if want more than simple, connect and read/write a file from a server type applications, you will need to. (Note: HTTP may not allow you to write to a file on the server, whereas another protocol will)