java - Number クラス doubleValue の例外

Question

メイン メソッドを実行すると、次の例外が発生します: Exception in thread "main" java.lang.StackOverflowError, 続いて this: at Rational.doubleValue(Rational.java:62) like 100+ times. これは、メイン クラスが提供されている課題のためのものなので、そこでは何も変更できません。これが私のコードです：

---

public class Rational extends Number implements Comparable<Rational> 
{
   private int num;
   private int denom; 
   public Rational(int num, int denom)
   {
      this.num = num;
      this.denom = denom;
   }
   public int compareTo(Rational r)
   {
      if ((num/denom) - (r.num/r.denom) > 0)
      {
         return +1;
      }
      else if ((num/denom) - (r.num/r.denom) < 0)
      {
         return -1;
      }
      return 0;
   }

   public Rational add(Rational r)
   {
      return new Rational(num + r.num, denom + r.denom);
   }

   public Rational sub(Rational r)
   {
      return new Rational(num - r.num, denom - r.denom);
   }

   public Rational mul(Rational r)
   {
      return new Rational(num * r.num, denom * r.denom);
   }

   public Rational div(Rational r)
   {
      return new Rational(num * r.denom, denom * r.num);
   }

   public int gcd(Rational r)
   {
      int i = 1;
      while(i != 0)
      {
         i = denom % r.denom;
         denom = r.denom;
         r.denom = i;
      }
      return denom;
   }

   public String toString()
   {
      String a = num + "/" + denom;
      return a;
   }
   public double doubleValue()
   {
      return (double)doubleValue();
   }
   public float floatValue()
   {
      return (float)floatValue();
   }
   public int intValue()
   {
      return (int)intValue();
   }
   public long longValue()
   {
      return (long)longValue();
   }
}

---

そして、これがメインクラスです（これは変更できません）

---

public class Main {

    public static void main(String[] args) {

        Rational a = new Rational(2, 4);
        Rational b = new Rational(2, 6);

        System.out.println(a + " + " + b + " = " + a.add(b));
        System.out.println(a + " - " + b + " = " + a.sub(b));
        System.out.println(a + " * " + b + " = " + a.mul(b));
        System.out.println(a + " / " + b + " = " + a.div(b));

        Rational[] arr = {new Rational(7, 1), new Rational(6, 1),
                          new Rational(5, 1), new Rational(4, 1),
                          new Rational(3, 1), new Rational(2, 1),
                          new Rational(1, 1), new Rational(1, 2),
                          new Rational(1, 3), new Rational(1, 4),
                          new Rational(1, 5), new Rational(1, 6),
                          new Rational(1, 7), new Rational(1, 8),
                          new Rational(1, 9), new Rational(0, 1)};

        selectSort(arr);

        for (int i = 0; i < arr.length-1; ++i) {
            if (arr[i].compareTo(arr[i+1]) > 0) {
                System.exit(1);
            }
        }

//        for (Rational r : arr) {
//            System.out.println(r);
//        }

        Number n = new Rational(3, 2);

        System.out.println(n.doubleValue());
        System.out.println(n.floatValue());
        System.out.println(n.intValue());
        System.out.println(n.longValue());
    }

    public static <T extends Comparable<? super T>> void selectSort(T[] array) {

        T temp;
        int mini;

        for (int i = 0; i < array.length - 1; ++i) {

            mini = i;

            for (int j = i + 1; j < array.length; ++j) {
                if (array[j].compareTo(array[mini]) < 0) {
                    mini = j;
                }
            }

            if (i != mini) {
                temp = array[i];
                array[i] = array[mini];
                array[mini] = temp;
            }
        }
    }
}