File and FileDescriptor

[Siva kandasamy]

Hi some basic questions.

1. - How File and FileDescriptors class are different in java
- What it represents.
- What we can not achive through File class,
we have to depend on "FileDescriptors"

2. What is Filter Streams ? Can you give me an example.

thanks in advance
siva

[Joe Ess]

The documentation answers all of your questions:
java.io.File

An abstract representation of file and directory pathnames.

java.io.FileDescriptor

The main practical use for a file descriptor is to create a FileInputStream or FileOutputStream to contain it.
Applications should not create their own file descriptors.

Filter streams are specalized streams which transform the data moving through them. There are examples in Java Tutorial

[Siva kandasamy]

Joe,
Thank You. Code appended here is example for Filter streams. Please help me to identify the transformation of data.
"which transform the data moving through them", what we mean here.
Thanks you
siva

 import java.io.*;
 
public class DataIODemo {
    public static void main(String[] args) throws IOException {
 
        // write the data out
        DataOutputStream out = new DataOutputStream(new
   FileOutputStream("invoice1.txt"));
 
        double[] prices = { 19.99, 9.99, 15.99, 3.99, 4.99 };
        int[] units = { 12, 8, 13, 29, 50 };
        String[] descs = { "Java T-shirt",
   "Java Mug",
   "Duke Juggling Dolls",
   "Java Pin",
   "Java Key Chain" };
        
        for (int i = 0; i < prices.length; i ++) {
            out.writeDouble(prices[i]);
            out.writeChar('\t');
            out.writeInt(units[i]);
            out.writeChar('\t');
            out.writeChars(descs[i]);
            out.writeChar('\n');
        }
        out.close();
 
        // read it in again
        DataInputStream in = new DataInputStream(new
 FileInputStream("invoice1.txt"));
 
        double price;
        int unit;
        StringBuffer desc;
        double total = 0.0;
 
        try {
            while (true) {
                price = in.readDouble();
                in.readChar();       // throws out the tab
                unit = in.readInt();
                in.readChar();       // throws out the tab
char chr;
desc = new StringBuffer(20);
char lineSep = System.getProperty("line.separator").charAt(0);
while ((chr = in.readChar()) != lineSep)
    desc.append(chr);
                System.out.println("You've ordered " +
    unit + " units of " +
    desc + " at $" + price);
                total = total + unit * price;
            }
        } catch (EOFException e) {
        }
        System.out.println("For a TOTAL of: $" + total);
        in.close();
    }
}
 

[Joe Ess]

DataInputStream and DataOutputStream are transforming raw bytes into Java primitive types or vice-versa.

[Siva kandasamy]

Thank You Joe. I think, it makes sense. I will try more.
thanks
siva