Request for code review, to make this code simpler, cleaner better. This code will find greatest number smaller than "x" so in an array [10 20 30 40] and x = 25, the output should be 20.

public class GreatestValueLesserThanEqualToX {

    public static Integer findGreatestValueLesserThanOrEqualToX (int[] arr, int x) {
        if (arr == null) {
            throw new NullPointerException(" The input array is null. ");
        }
        if (arr.length >= 1) {
            return findGreatestValueLesserThanOrEqualToX (arr, x, 0, arr.length - 1);
        } else {
            return null;  // note the difference wrt greatest value lesser than X.
        }
    }

    private static Integer findGreatestValueLesserThanOrEqualToX (int[] arr, int x, int lb, int ub) {

        assert arr != null;

        final int mid = (lb + ub) / 2;

        /**
         *  Testing boundaries.
         */
        // testing lower boundary.
        if (mid == 0) {
            if (arr[mid] > x) {
                return null;
            }

            // single element array with value lesser than input value.
            if (arr.length == 1) {
                return arr[0];
            }
        }

        //testing higher boundary.
        if (lb == (arr.length - 1) && arr[mid] < x) {
            return arr[mid]; 
        }

        /**
         *  Testing equalities and duplicates.
         */
        // testing equality and duplicates. eg: consider input like: 1, 2, 2, 5
        if (arr[mid] == x || arr[mid + 1] == x) {
            return x;
        }

        /**
         *  Testing when element is in the range of array elements.
         */
        // input x in range of array elements.
        if (arr[mid] < x && arr[mid + 1] > x) {
             return arr[mid]; // note the difference wrt greatest value lesser than X.
         }

         if (arr[mid] < x) {
             return findGreatestValueLesserThanOrEqualToX (arr, x, mid + 1, ub);
         } else {
             return findGreatestValueLesserThanOrEqualToX (arr, x, lb, mid);
         }
     }
}