Question

Write a complete Java program that does the following:

• Open an input file named data.txt that consists of a series of unknown number of integers. If data.txt does not exist, give an appropriate error message and terminate the program.
• Define a constant MAX of value 100 and create an array of size MAX to hold items from the input file. Make sure your program will not generate ArrayIndexOutOfBounds exception.
• Open an output file named result.txt and write the array elements in reverse order to result.txt. Make sure you write only the array elements that contain valid data items to the output file.

Answer 1

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

public class ReadWriteNumbers {
        public static void main(String[] args) {
                final int MAX = 100;
                //creating array with size 100
                int arr[] = new int[MAX];
                //opening the file to read data
                Scanner sc=null;
                try{
                        sc = new Scanner(new File("data.txt"));
                }
                catch (Exception e) {
                        System.out.println("data.txt does not exist");
                        return;
                }
                int index = 0;
                // looping through file and reading upto 100 integers
                while (sc.hasNextInt() && index < 100) {
                        arr[index] = sc.nextInt();
                        index++;
                }
                // opening file to write the data
                PrintWriter pw=null;
                try {
                        pw = new PrintWriter(new File("result.txt"));
                } catch (FileNotFoundException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                }
                // iterating array from backside to write results in reverse order
                for (int i = index - 1; i >= 0; i--)
                        pw.println(arr[i]);
                pw.close();
                System.out.println("Succes...");
        }
}

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