This code is working fine. Your work would be break this code in multiple function that means creating multiple methods to break it .

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

public class InptOutTemp { public static void main (String args[])throws IOException { Scanner keyboard = new Scanner(System.in); File file = new File("C:\\Users\\erorr\\Desktop\\input.txt"); Scanner inputFile = new Scanner(file); PrintWriter outputFile = new PrintWriter("C:\\Users\\erorr\\Desktop\\OutputFile.txt"); //String str; double numIn, numOut = 0; int option; System.out.println("Enter 1 for F2C"); System.out.println("Enter 2 for C2F"); System.out.println("Enter 3 for C2K"); System.out.println("Enter 4 for K2C"); System.out.println("Enter 5 for F2K"); System.out.println("Enter 6 for K2F"); option = keyboard.nextInt(); while (inputFile.hasNext()) { numIn = inputFile.nextDouble(); switch (option) { case 1: numOut = 5.0/9 *(numIn - 32); break; case 2: numOut = 9.0/5 * numIn + 32; break; case 3: numOut = numIn + 273; break; case 4: numOut = numIn - 273; break; case 5: numOut = 5.0/9 *(numIn - 32) + 273; break; case 6: numOut = 9.0/5 *(numIn - 273) + 32; break; } outputFile.println(numOut); } inputFile.close(); outputFile.close(); System.out.println("done!"); } }