package examples;
import java.io.*;

/**
 * Replace is a utility for replacing all occurrences of one string by another in a file.
 * @author kjg
 *
 */
public class Replace {

	/**
	 * REQUIRES: The input file 'in' must exist.  The 'in' and 'out' files must not be the same file.
	 * MODIFIES: The output file.
	 * EFFECTS: The input file is copied to the output file, with each occurence of the oldstring
	 * replaced by newstring.
	 * @param oldstring the String to be replaced
	 * @param newstring the replacement String
	 * @param in the input File
	 * @param out the output File
	 * @throws IOException if the input file does not exist or the output file cannot be created.
	 */
	public static void replace(String oldstring, String newstring, File in, File out) throws IOException {
		BufferedReader reader = new BufferedReader(new FileReader(in));
		PrintWriter writer = new PrintWriter(new FileWriter(out));
		String line = null;
		while ((line = reader.readLine()) != null) {
			writer.println(line.replaceAll(oldstring,newstring));
		}
		reader.close();
		writer.close();
	}

	/**
	 * Allows use on the command line, as<BR>
	 * <TT>java Replace oldstring newstring inputfilename outputfilename</TT>
	 * @param args array of the four String arguments provided by the user
	 */
	public static void main(String[] args) {
		try {
			String oldString = args[0];
			String newString = args[1];			
			File in = new File(args[2]);
			if (!in.exists()) {
				System.out.println("The input file " + in + " does not exist.");
				return;
			}
			File out = new File(args[3]);
			if (out.exists()) {
			      System.out.println("The output file " + out + " already exists.");
			      return;
			}
			replace(oldString, newString, in, out);
		} catch (Exception e) {
			usage();
		}
	}
	
	static void usage() {
		System.out.println("Usage: Replace <oldstring> <newstring> <input filename> <output filename>");
	}
}