DCO10103 Object-Oriented Programming and Design

File I/O

identify the difference

Console IO	File IO
BufferedReader stdIn = new BufferedReader(new InputStreamReader(System.in));	BufferedReader fileIn = new BufferedReader(new FileReader(filename));
PrintWriter stdOut = new PrintWriter(System.out, true);	PrintWriter fileOut = new PrintWriter(new FileWriter(filename));
// reading from console int x = Integer.parseInt(stdIn.readLine());	// reading from a file int x = Integer.parseInt(fileIn.readLine());
// writing to console stdOut.println(5);	// writing to a file fileOut.println(5);

use different constructor for different PrintWriter e.g. append instead of overwrite
```
PrintWriter  fileApnd = new PrintWriter(new  FileWriter(filename, true);
```
new Exception need to be handled e.g. FileNotFoundException thrown by FileReader(String)
after opening a file for use, you must close the file under all circumstance
finally block
- program that obtain certain type of resources such as memory and file must return those resources to the system
- typical use of finally block is to close any files opened in the try block
- the code in the finally block will be executed whether or not the corresponding try-block throws an exception, and regardless of whether or not any thrown exception there is caught by any of the corresponding catch-blocks

Example: Copy class

import  java.io.*;

/**
 * Sample program:  Copy
 *
 * Makes a copy of a file.
 *
 * @author  Sean Bufano
 * @version  1.1
 */
public class Copy  {

	/* input object for console input */
	private static BufferedReader  stdIn = new
		BufferedReader(new  InputStreamReader(System.in));

	/* output object for normal console output */
	private static PrintWriter  stdOut = new
		PrintWriter(System.out, true);

 	/* output object for alternative console output */
	private static PrintWriter  stdErr = new
		PrintWriter(System.err, true);

	/** 
	 * Main program
	 *
	 * @param args  not  used
	 * @throws IOException
	 */
	public static void  main(String[]  args) throws	IOException  {

            BufferedReader  inFile = getFileForReading("Source filename:  ");
            PrintWriter  outFile = getFileForWriting("Destination filename:  ");

            try {
	
		String  line = inFile.readLine();
		
		while (line != null)  {
			outFile.println(line);
			line = inFile.readLine();
		}
            } catch (Exception e) {
              } finally  {
                       inFile.close();
                       outFile.close();
                }
	}

	/**
	 * opens a file for reading
	 *
	 * @param message  message used as prompt
	 * @return  a reference to the file
	 * @throws IOException
 	 */
	private static BufferedReader
		getFileForReading(String  message) throws IOException  {

		do  {
			try  {
				stdErr.print(message);
				stdErr.flush();
				return  new
					BufferedReader(new
						FileReader(stdIn.readLine()));
			} catch (FileNotFoundException  fnfe)  {
				stdErr.println(fnfe.toString());
			}
		}  while (true);
	}

	/**
	 * opens a file for writing.
	 *
	 * If file exists, its contents are erased.
	 *
	 * @param message  message used as prompt
	 * @return  a reference to the file
	 * @throws IOException
	 */
	private static PrintWriter
		getFileForWriting(String  message) throws IOException  {

		stdErr.print(message);
		stdErr.flush();

		return  new  PrintWriter(new
			FileWriter(stdIn.readLine()));
	}

	/**
	 * opens a file for appending.
	 *
	 * @param message  message used as prompt
	 * @return  a reference to the file
	 * @throws IOException
	 */
	private static PrintWriter
		getFileForAppending(String  message) throws IOException  {

 		stdErr.print(message);
		stdErr.flush();

		return  new  PrintWriter(new
			FileWriter(stdIn.readLine(), true));
	}
}

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