CSC 8310 -- Simple Input-Output

The following program uses simple line-oriented input-output to copy the contents of one file to another file.

import java.io.*; public class CopyFile { public static void main (String args []) { BufferedReader in; BufferedWriter out; String oneLine; if (args.length != 2) // Do some minimal error checking { System.err.println ("Usage: java CopyFile <inputfile> <outputfile>"); System.exit (1); } try { in = new BufferedReader (new FileReader (args[0])); out = new BufferedWriter (new FileWriter (args[1])); while ((oneLine = in.readLine ()) != null) { out.write (oneLine); out.newLine (); } out.flush (); in.close (); out.close (); } catch (IOException e) { System.err.println ("*** Error *** " + e); } } }

Notice the following:

• To copy file foo to file bar, do:    java CopyFile foo bar

• The statement import java.io.*; simplifies access to the needed classes.

• Statements that might throw exceptions need to be wrapped in a try...catch statement. This includes most or all of the I/O statements.

• The "files" we want are constructed according to a Decorator pattern: we first create a FileReader, then pass it immediately into a constructor for BufferedReader to add desired functionality (in this case, both buffered reading and the readLine method. Frequently these decorations get quite complex.

• BufferedWriter provides both write and newLine, but not writeln.

• Because writing is buffered, we need to flush our output file to make sure the buffer is emptied before we quit the program.

• It isn't really necessary to close our I/O streams before quitting Java, but it's good style.

• The IOException that we catch, e, can be printed directly, because exceptions have a toString method. We could learn even more by replacing the println statement with a call to e.printStackTrace().