Вот мой более компактный вариант, с выводом как в книге:

public class App {
    private static final Integer START_NUM = 1000;
    private static final Integer END_NUM = 9999;
    private static final String ANSWER = "%d = %s * %s\n";

    public static void main(String[] args) {
        for (int num = START_NUM; num < END_NUM; num++) {
            List<Integer> firstComb = new ArrayList<>();
            List<Integer> secondComb = new ArrayList<>();
            char[] number = String.valueOf(num).toCharArray();

            for (int firstChar = 0; firstChar < 4; firstChar++) {
                for (int secondChar = firstChar + 1; secondChar < 4; secondChar++) {
                    String firstHalf = "" + number[firstChar] + number[secondChar];
                    String secondHalf = "" + number[secondChar] + number[firstChar];

                    firstComb.add(Integer.valueOf(firstHalf));
                    secondComb.add(Integer.valueOf(secondHalf));
                }
            }

            for (int i = 0, j = 5; i < firstComb.size(); i++, j--) {
                if ((firstComb.get(i) * secondComb.get(j)) == num) {
                    System.out.printf(ANSWER, num, secondComb.get(j), firstComb.get(i));
                    break;
                } else if ((firstComb.get(i) * firstComb.get(j)) == num) {
                    System.out.printf(ANSWER, num, firstComb.get(j), firstComb.get(i));
                    break;
                } else if ((secondComb.get(i) * secondComb.get(j)) == num) {
                    System.out.printf(ANSWER, num, secondComb.get(j), secondComb.get(i));
                    break;
                }
            }
        }
    }
}

Возможно вам не актуально, но кому-то это точно поможет