Question

( IN JAVA): Write a program that reads each line in a file, and writes them out in reverse order into another file. This program should ask the user for the name of an input file, and the name of the output file. If the input file contains:

Mary had a little lamb

Its fleece was white as snow

And everywhere that Mary went

The lamb was sure to go

Then the output file should end up containing:

The lamb was sure to go

And everywhere that Mary went

Its fleece was white as snow

Mary had a little lamb

Hint: You'll need to read in all the lines from the input file, store them somewhere, and then write then out in reverse order. Try to write this using methods for each piece -- the reading, and then the writing out in backwards order.

Be sure that you handle all possible exceptions appropriately -- your main() can write messages to standard output (the console) explaining to the user what any problems were. However, any methods (other than main() should not write messages to the console. This means that your main needs to make sure that all pathnames are good before calling the methods to read and write the files.

Answer 1

import java.io.*;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

class Main {

    //Method to read the lines of data and return a list
    static List<String> readLines(String name) throws IOException {
        //List to store lines
        List<String> lines = new ArrayList<>();
        //Open file for reading
        BufferedReader br = new BufferedReader(new FileReader(new File(name)));
        //Read all the lines
        String data = br.readLine();
        while (data != null) {
            lines.add(data);
            data = br.readLine();
        }
        return lines;
    }

    //Method to write lines to file
    static void writeLines(List<String> lines) throws IOException {
        //Open writer to write lines
        BufferedWriter bw = new BufferedWriter(new FileWriter(new File("reverse.txt")));
        //Loop through list in reverse order
        for(int i=lines.size()-1;i>=0;i--){
            bw.write(lines.get(i)+"\n");
        }
        //Flush the writer
        bw.flush();
        bw.close();
    }

    public static void main(String[] args) {
        //Prompt file name
        System.out.print("Enter the file name : ");
        Scanner obj = new Scanner(System.in);
        String name = obj.nextLine();
        List<String> lines = null;
        try {
            //Read lines
            lines = readLines(name);
        } catch (IOException e) {
            System.out.println("File not found");
            return;
        }

        try {
            //Write lines
            writeLines(lines);
        } catch (IOException e) {
            System.out.println("Exception occured while writing");
        }

    }
}

OUTPUT :

input.txt

OUTPUT :