File I/O

Reading assignments

• Java tutorial : How to Use File Streams or textbook 320-322

FileInputStream/FileOutputStream vs FileReader/FileWriter

FileInputStream/FileOutputStream API

Exception

• IOException
• FileNotFoundException : a subclass of IOException.
• You can catch 2 or more exceptions in a try-catch block but you have to put the subclass first.
  Example : (correct version)
  

  import java.io.*;
  
  public class ExceptionTest {
      public static void main(String[] args) {
          FileOutputStream out = null;
          try {
              out = new FileOutputStream("test");
              out.write('A');
          } catch (FileNotFoundException ex) {    
              System.out.println("FileNotFoundException");
          } catch (IOException ex) {
              System.out.println("IOException");  
          } finally {
              if(out != null) {
                  try {
                      out.close();
                  } catch (IOException ex) {
                  }
              }
          }
      }
  }
  

  Incorrect version (compiling error)
  

  import java.io.*;
  
  public class ExceptionTest {
      public static void main(String[] args) {
          FileOutputStream out = null;
          try {
              out = new FileOutputStream("test");
              out.write('A');
          } catch (IOException ex) {
              System.out.println("IOException");  
          } catch (FileNotFoundException ex) {    
              System.out.println("FileNotFoundException");        
          } finally {
              if(out != null) {
                  try {
                      out.close();
                  } catch (IOException ex) {
                  }
              }
          }
      }
  }
  

Examples from Java tutorial

import java.io.*;

public class CopyBytes {
    public static void main(String[] args) throws IOException {
        File inputFile = new File("farrago.txt");
        File outputFile = new File("outagain.txt");

        FileInputStream in = new FileInputStream(inputFile);
        FileOutputStream out = new FileOutputStream(outputFile);
        int c;

        while ((c = in.read()) != -1)
           out.write(c);

        in.close();
        out.close();
    }
}

Here is the code that the CopyByte program uses to create a file:

File inputFile = new File("farrago.txt"); 
FileReader in = new FileReader(inputFile);

After you've run the program, you should find an exact copy of farrago.txt in a file named outagain.txt in the same directory. Here is the content of the file:

So she went into the garden to cut a cabbage-leaf, to
make an apple-pie; and at the same time a great
she-bear, coming up the street, pops its head into the
shop. 'What! no soap?' So he died, and she very
imprudently married the barber; and there were
present the Picninnies, and the Joblillies, and the
Garyalies, and the grand Panjandrum himself, with the
little round button at top, and they all fell to playing
the game of catch as catch can, till the gun powder ran
out at the heels of their boots.

Samuel Foote 1720-1777

Exercise

import java.io.*;

public class CopyFile {
    public static void main(String[] args) throws IOException {
        String copyFromFileName = null;
        String copyToFileName = null;
        FileInputStream fileIn = null;
        FileOutputStream fileOut = null;
        if(args.length < 2) {
            usage();
            return;
        } else {
            copyFromFileName = args[0];
            copyToFileName = args[1];
            try {
                fileIn = new FileInputStream(copyFromFileName);
            } catch (FileNotFoundException ex) {
                System.out.println("File " + copyFromFileName + "not found");
                return;
            }
            
            if(new File(copyToFileName).exists()) {
                System.out.println("File " + copyToFileName + " exists.");
                fileIn.close();
                return;
            }
            
            try {
                fileOut = new FileOutputStream(copyToFileName);
            } catch (FileNotFoundException ex) {
                System.out.println("File " + copyToFileName + "cannot be created");
                fileIn.close();
                return;
            }
        }
        
        int c;
        
        while((c = fileIn.read()) != -1) {
            fileOut.write(c);
        }
        
        fileIn.close();
        fileOut.close();

    }
    
    private static void usage() {
        System.out.println("Usage : CopyFrom filenameIn filenameOut");
    }
}

Increase performance

import java.io.*;

public class FilePerformance {
    public static void copy1(File from, File to) throws IOException {
        FileInputStream in = new FileInputStream(from);
        FileOutputStream out = new FileOutputStream(to);
        int ch;
        while((ch = in.read())!= -1) {
            out.write(ch);
        }
        in.close();
        out.close();
    }
    
    public static void copy2(File from, File to) throws IOException {
        FileInputStream in = new FileInputStream(from);
        FileOutputStream out = new FileOutputStream(to);
        int length = 32; // 32 is a power of 2. 
                         // use a power of 2 can increase the performance
        byte[] ch = new byte[length];
        while((length = in.read(ch))!= -1) {
            out.write(ch,0,length);
        }
        in.close();
        out.close();
    }
    
    public static void main(String[] args) throws IOException {
        File from = new File("pic.jpg");
        File to = new File("pic2.jpg");
        long start = System.currentTimeMillis();
        for(int i = 0; i < 10; i++) {
            System.out.println(i);
            copy1(from, to);
        }
        long end = System.currentTimeMillis();
        System.out.println("Time : " + (end - start) + "ms." );
        start = System.currentTimeMillis();
        for(int i = 0; i < 10; i++) {
            System.out.println(i);
            copy2(from, to);
        }
        end = System.currentTimeMillis();
        System.out.println("Time : " + (end - start) + "ms." );
    }
}