Java (Read and write to file)

Question

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Scanner;

public class backup {

    public static void main(String[] args) {
        System.out.println("\nThe names in the file are:\n");
        readFromFile();
    }

    public static void readFromFile() {
        try {
            Scanner file = new Scanner(new File("Names.txt"));
            PrintWriter pass = new PrintWriter("pass.txt");
            PrintWriter fail = new PrintWriter("fail.txt");

            Scanner input = new Scanner(System.in);
            while (file.hasNext()) {
                String name = file.nextLine();
                System.out.printf("Please enter " + name + "'s mark:");
                int mark = input.nextInt();
                Scanner kybd = new Scanner(System.in);
                {
                    if (mark >= 40) {
                        // System.out.println(name + " has passed");
                        pass.println(name);
                    } else {
                        // System.out.println(name + " has failed");
                        fail.println(name);
                    }
                }
            } // end while
        } // end try
        catch (IOException ioe) {
            System.out.println("Error handling file");
        } // end catch
    }// readFromFile
}

Unable to write to the file pass and fail the Names file is just a list of names if the user scores over 40 they go into the pass however i cant get the txt file to populate the output.

When i insert my printwriter in the while it is only the last entry which gets added to either pass or fail txt file.

Answer 1

The PrintWriter only really writes when they are closed ( close ) or flushed ( flush or maybe when it thinks it is time ;-)).

Try to use try-with-resources whenever possible, that way you do not need to think about closing/flushing a stream or writer. try-with-resources takes care that AutoClosable -implementations are closed automatically at the end of the try -block.

Your sample rewritten with try-with-resources (only showing the initial part):

try(
  Scanner file = new Scanner(new File("Names.txt"));
  PrintWriter pass = new PrintWriter("pass.txt");
  PrintWriter fail = new PrintWriter("fail.txt")
) {

The rest can stay as is... or if you like: you may also want to lookup the Files-API . Maybe using Files.lines is something for you?

Answer 2

You should call the function close on your file for it to be written on the dsisk.

Otherwise you modifications are only in memory

Something like: file.close()

Answer 3

If you use PrintWriter , you have to close the stream at the end. Otherwise, it will be only in memory. Try this:

    try {
        Scanner file = new Scanner(new File("Names.txt"));
        PrintWriter pass = new PrintWriter("pass.txt");
        PrintWriter fail = new PrintWriter("fail.txt");

        Scanner input = new Scanner(System.in);
        while (file.hasNext()) {
            String name = file.nextLine();
            System.out.printf("Please enter " + name + "'s mark:");
            int mark = input.nextInt();
            //Scanner kybd = new Scanner(System.in);
            //{
                if (mark >= 40) {
                    // System.out.println(name + " has passed");
                    pass.println(name);
                } else {
                    // System.out.println(name + " has failed");
                    fail.println(name);
                }
            //}
        } // end while
        pass.close();
        fail.close();
    } // end try
    catch (IOException ioe) {
        System.out.println("Error handling file");
    }

Answer 4

You need to make sure that you add a finally block at the end and call the close method on anything that is closeable so in this case on the printwriter. In this case pass.close() and fail.close().