Files and File I/O

How do you create and access an index-sequential file?

How do I read from a file on a server? Check if a file exists?

See http://www.teleport.com/~idsinga/javacode/FileOnServer.java. Or, http://www.teleport.com/~idsinga contains a java choice widget called 'Java Code' that allows you select which code example you would like to get.

Use the URLConnection class. To read a server file,

try
{
	URL url = new URL("http://www.myserver.com/test.html");
	URLConnection urlConnection = url.openConnection();
	urlConnection.connect();

	// if the file does not exist, the following will throw an exception:
	InputStream is = urlConnection.getInputStream();

	int r = is.read(buf);
}
catch (Exception ex)
{
	System.out.println("Oh no!!!");
	System.exit(1);
}
System.out.println("contents of text.html=" + new String(buf, 0));

Note that this will allow you to actually read the file, regardless of its contents. I.e. the content handler is not called.

How do I write to a file on a server?

See http://www.teleport.com/~idsinga/javacode/FileOnServer.java. Or, http://www.teleport.com/~idsinga contains a java choice widget called 'Java Code' that allows you select which code example you would like to get.

In a Java application running in the interpreter, how do I change directories on the local drive?

// This code will show the current directory of a java application.
// Enter a new directory in the text filed
// The press the ChangeDirectory button to change to the new directory
// Use the ListDirectory button to show a listing of files in the 
// current directory.
//


import java.util.*;
import java.awt.*;
import java.io.*;
import java.applet.*;

public class setProps extends Applet
{
	Label l1 = null;
	TextArea ta1 = null;
	TextArea ta2 = null;
	TextField tf1 = new TextField(30);
	Properties p = null;

	public setProps() {
		setLayout(new BorderLayout());
		Panel p1 = new Panel();
		p1.setLayout(new FlowLayout());
		add("North", p1);
 		p1.add(new Button("ChangeDirectory"));	 
 		p1.add(new Button("ListDirectory"));	 
		p1.add(tf1);
		ta1 = new TextArea(10, 40);			
		ta2 = new TextArea(20, 40);			
		p1.add(ta1);
		Panel p2 = new Panel();
		p2.setLayout(new FlowLayout());
		add("South", p2);
		p2.add(ta2);
		p = System.getProperties();
		ta1.setText("Current directory: " + p.getProperty("user.dir"));
	}
	public boolean action(Event e, Object o) {
	
		if ("ChangeDirectory".equals(o)) {	
			ta1.appendText("\n" + "Current directory before: " + p.getProperty("user.dir"));
//
// This is the code that changes the current directory to a new directory
// The system properties is a hashtable and the values in the table can be
// get/set using the get() and put() methods of the hashtable class. You can also
// get all the system properties using system.getProperties()
//

			p.put("user.dir", tf1.getText());
			System.setProperties(p);
			ta1.appendText("\n" + "Current directory after: " + p.getProperty("user.dir"));
		}
		if ("ListDirectory".equals(o)) {	
			File f1 = new File(p.getProperty("user.dir"));
			String fileList[] = f1.list();
			int i, j = fileList.length;
			ta2.setText("");
			for (i = 0;i < j;i++)
				ta2.appendText("\n" + fileList[i]);
		}
		return true;
	}

	public static void main(String args[]) {
		Frame f1 = new Frame("Standalone Application");
		setProps s1 = new setProps();
		s1.init();
		s1.start();
	        f1.add("North", s1);
	        f1.resize(400, 600);
	        f1.show();	
	}
}