The solution and a test-case (for single digit number) have been added.

src/main/java/org/jugbd/marathon/day2/BiggestNumber.java

@@ -5,6 +5,29 @@ public class BiggestNumber {
     public static long findTheNextBiggerNumber(long n) {
         //write your awesome code here
 
-        return n;
+        StringBuilder originalNumStr = new StringBuilder(String.valueOf(n));
+        int numOfDigits = originalNumStr.length();
+        return finderHelper(n, n, 1, numOfDigits);
+    }
+    private static long finderHelper(long originalNumber, long currentNumber, int index, int numOfDigits) {
+        if (currentNumber / 10 != 0) {
+            long swappedNumber;
+            swappedNumber = swap(originalNumber, numOfDigits - index, numOfDigits - index - 1);
+            if (swappedNumber > originalNumber) {
+                return swappedNumber;
+            }
+            return finderHelper(originalNumber, currentNumber / 10, index + 1, numOfDigits);
+        } else {
+            return -1;
+        }
+    }
+
+    private static long swap(long number, int firstIndex, int secondIndex) {
+        StringBuilder numberStr = new StringBuilder(String.valueOf(number));
+        char temp;
+        temp = numberStr.charAt(firstIndex);
+        numberStr.setCharAt(firstIndex, numberStr.charAt(secondIndex));
+        numberStr.setCharAt(secondIndex, temp);
+        return Long.valueOf(numberStr.toString());
     }
 }

src/test/java/org/jugbd/marathon/day2/BiggestNumberTest.java

@@ -14,6 +14,7 @@ public void basicTests() {
         assertEquals(2071, BiggestNumber.findTheNextBiggerNumber(2017));
         assertEquals(441, BiggestNumber.findTheNextBiggerNumber(414));
         assertEquals(414, BiggestNumber.findTheNextBiggerNumber(144));
+        assertEquals(-1, BiggestNumber.findTheNextBiggerNumber(7));
         assertEquals(-1, BiggestNumber.findTheNextBiggerNumber(111));
         assertEquals(-1, BiggestNumber.findTheNextBiggerNumber(531));
     }