readrawdata.java

Read data from a file

//**************************************
// Name: Read data from a file
// Description:There are a number of ways to read data from a file. If you're reading a file as raw binary data, you open a file using a FileInputStream(String) constructor and use one of the various read() methods to read the data into an array of bytes. For example the following program reads raw data from a file specified on the command line. It then writes the same data to the standard output.
(Java FAQ:found on the web at:http://sunsite.unc.edu/javafaq/javafaq.html)
// By: 
//
//
// Inputs:None
//
// Returns:None
//
//Assumes:None
//
//Side Effects:None
//**************************************

import java.io.*;


    class ReadRawData {


        public static void main (String args[]) {
        boolean done = false;
        byte b[] = new byte[1024];
        int num_bytes = 0;
        FileInputStream fin = null;


            try {
            fin = new FileInputStream(args[0]);
        }


            catch(ArrayIndexOutOfBoundsException e) {
            System.out.println("You have to give me the name of a file to open.");
            System.exit(0);
        }


            catch (FileNotFoundException e) {
            System.out.println("Could not open input file " + args[0]);
            System.exit(0);
        }


            catch(IOException e) {
            System.out.println("Error while opening input file" + args[0]);
            System.exit(0);
        }


            catch (Exception e) {
            System.out.println("Unexpected exception: " + e);
            System.exit(0); 
        }


            try {
            num_bytes = fin.read(b);
        } 


            catch(IOException e) {
            System.out.println("Finished Reading: " + e);
            done = true;
        }


            catch (Exception e) {
            System.out.println("Unexpected exception: " + e);
            System.exit(0); 
        }
        


            while(!done) {
            System.out.write(b, 0, num_bytes);


                try {
                num_bytes = fin.read(b);
            }


                catch(IOException e) {
                System.out.println("Finished Reading: " + e);
                done = true;
            }


                catch (Exception e) {
                System.out.println("Unexpected exception: " + e);
                System.exit(0); 
            }
            if (num_bytes == -1) done = true;
        } // end while
    } // end main
    
} // end ReadRawData
On the other hand if you're reading a text file in Java 1.0 you'll probably want to use a DataInputStream which gives you a readLine() method that returns successive lines of the file as Java Strings. You can then process each String as you see fit. 
// Implement the Unix cat utility in java
import java.io.*;


    class cat {


        public static void main (String args[]) {
        
        String thisLine;
        
        //Loop across the arguments


            for (int i=0; i < args.length; i++) {
            
            //Open the file for reading


                try {
                FileInputStream fin = new FileInputStream(args[i]);


                    try {
                    DataInputStream myInput = new DataInputStream(fin);
                    


                        try {
                        while ((thisLine = myInput.readLine()) != null) { // while loop begins here
                        System.out.println(thisLine);
                    } // while loop ends here
                }


                    catch (Exception e) {
                    System.out.println("Error: " + e);
                }
            } // end try


                catch (Exception e) {
                System.out.println("Error: " + e);
            }
            
        } // end try


            catch (Exception e) {
            System.out.println("failed to open file " + args[i]);
            System.out.println("Error: " + e);
        }
    } // for ends here
    
} // main ends here
}
This code emulates the Unix "cat" command. Given a series of filenames on the command line it concatenates the files onto the standard output. 
In Java 1.1 DataInputStream.readLine() is deprecated. You should use a BufferedReader instead as in this class: 
// Implement the Unix cat utility in java
import java.io.*;


class cat {


    public static void main (String args[]) {
    
    String thisLine;
    
    //Loop across the arguments


        for (int i=0; i < args.length; i++) {
        
        //Open the file for reading


            try {
            FileReader fr = new FileReader(args[i]);
            BufferedReader myInput = new BufferedReader(fr);
            while ((thisLine = myInput.readLine()) != null) { // while loop begins here
            System.out.println(thisLine);
        } // while loop ends here
        
    } // end try


        catch (IOException e) {
        System.out.println("Error: " + e);
    }
    
} // for ends here

} // main ends here
}