Digit factorials: https://projecteuler.net/problem=34

DigitsFactorial.java

public class Solution {
  private Map<Integer, Integer> factorialMemo = new ConcurrentHashMap<Integer, Integer>();
  
  private int factorial(final int n) {
    final Integer f = factorialMemo.get(n);
    if (f != null) {
      return f;
    }
    
    if (n == 0) {
      return 1;
    } else {
      final int fm = n * factorial(n - 1);
      
      factorialMemo.put(n, fm);
      
      return fm;
    }
  }
  
  private List<Integer> digits(final int n) {
    if (n < 10) {
      return ImmutableList.of(n);
    } else {
      final List<Integer> result = new ArrayList<Integer>();
      
      result.add(n % 10);
      result.addAll(digits(n / 10));
                    
      return result;
    }
  }

  private int sumDigitFactorial(final int n) {
    final List<Integer> d = digits(n);
    
    int sum = 0;
    for (int i = 0; i < d.size(); ++i) {
      sum += factorial(d.get(i));
    }
    
    return sum;
  }

  private int sumOfValid() {
    int sum = 0;
    
    // Valid solutions can't be larger than 9!
    for (int i = 10; i < factorial(9); ++i) {
      if (sumDigitFactorial(i) == i) {
        sum += i;
      }
    }
    
    return sum;
  }
                    
  @Test
  public void eyeballTest() {
    System.out.println(sumOfValid());
  }
    
  public static void main(String[] args) {
    JUnitCore.main("Solution");
  }
}