Exercise04SPOC.java created by reu - https://repl.it/NKMw/20

Exercise04SPOC.java.java

import java.util.Scanner;

 // YOU NEED TO CHANGE THIS LINE BACK TO WHAT IT WAS, BUT IT WOULD ONLY WORK ON THIS WEBSITE IF I CHANGED IT :) 
 // public class Exercise04SPOC {
public class Main {
 public static void main(String[] args) {
  Scanner input = new Scanner(System.in);
  Scanner input2 = new Scanner(System.in);
  
  int num1, den1, num2, den2, numRes = 0, denRes = 0, numSimplified = 0, denSimplified = 0, i;
  
  System.out.println("Please enter the numerator of your fraction:");
  num1 = input.nextInt();
  
  System.out.println("Please enter the denominator:");
  den1 = input.nextInt();
  
  // For the operations we have to declare a String:
  String op;
  System.out.println("Choose the operation you want to perform: \n Addition (+) \n Subtraction (-) \n Multiplication (*) \n Division (:) \n Calculate the Opposite (O) \n Inverse (I) \n Quit (S)");
  op = input2.nextLine();
  
  
  // Exit Program
  if (op.equalsIgnoreCase("S")) {
   System.exit(-1);
  }
  
  // Opposite ??? Wtf is the opposite of a fraction
  if (op.equalsIgnoreCase("O")) {
  // This is a guess
   numRes = -num1;
   denRes = den1;
  }
  
  // Inverse
  if (op.equalsIgnoreCase("I")) {
   numRes = den1;
   denRes = num1;
  }
  
  // Multiplication
  if (op.equalsIgnoreCase("*")) {}
  
  // Division
  if (op.equalsIgnoreCase(":")) {}
  
  
  // Addition
  if (op.equals("+")) {
   System.out.println("Please enter the second numerator:");
   num2 = input.nextInt();
   
   System.out.println("Please enter the second denominator:");
   den2 = input.nextInt();
   
   // We have to operate the fractions now, looking for a common denominator.
   if (den1 == den2) {
    numRes = num1 + num2;
    denRes = den1;
   } else {
    numRes = (den2 * num1) + (den1 * num2);
    denRes = den1 * den2;
   }
  }
  
  
  // Subtraction
  if (op.equals("-")) {
   System.out.println("Please introduce the second numerator: ");
   num2 = input.nextInt();
   
   System.out.println("Please introduce the second denominator: ");
   den2 = input.nextInt();
   
   // We have to operate the fractions now
   if (den1 == den2) {
    numRes = num1 - num2;
    denRes = den1;
   } else {
    numRes = (den2 * num1) - (den1 * num2);
    denRes = den1 * den2;
   }
  }
  
  // If numerator == denominator, then we should output 1
  if(numRes == denRes) {
    System.out.println("The result of your operation is: 1");
    
    // This just skips the rest of the code
    return;
  }
  
   // Simplification
   int highestCommonFactor = 1;
   
   // Math.min returns the smallest of the 2 numbers - as it's pointless trying when i is bigger than one of the number, bcs you'll get something < 1
   // I think your problem was that if you hadn't found a common factor, you set the num/denSimplified to num/denRes
   // So say 44/22, you find 11 is a common factor, then you try 12 and it doesn't work, so your code would have set num/denSimplified back to 44 and 11...which is wrong
   // So in mine, if a number isn't a common factor, you do nothing, bcs it doesn't matter. However you need to continue looking, bcs there may be a higher one like 22 in the example ^^, but when you try 23, and it doesn't work, the code does nothing....which is correct
   
   for (i = 1; i <= Math.min(numRes, denRes); i++) {
     if((numRes % i) == 0 && (denRes % i) == 0) {
       highestCommonFactor = i;
     }
   }
   
   // Then here obviously you divide by the highest common factor to get the numbers
   numSimplified = numRes / highestCommonFactor;
   denSimplified = denRes / highestCommonFactor;
   
   // If the denominator is < 0, then swap the signs (this also handles the case where both numbers are < 0)
   if(denSimplified < 0) {
     numSimplified = -numSimplified;
     denSimplified = -denSimplified;
   }
   
   // Now we check for fractions where the denominator is 1, and to see if the numerator is 0
   // If either are true, we should just print the numerator
   if(denSimplified == 1 || numSimplified == 0) {
     System.out.println("The result of your operation is: " + numSimplified);
   } else {
     System.out.println("The result of your operation is: " + numSimplified + "/" + denSimplified);
   }
 }
}