package examples;
import java.io.*;

public class Replace {

	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();
	}

	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>");
	}
}