findFirst throws java.lang.NullPointerException

Question

I have this code below. The findfirst call is throwing NullPointerException even though I have an orElseGet call chained

int numberOfRetry = 5;
String req = "abc";
String res =
    Stream.iterate(0, n -> n + 1).map(i -> {
        try {
            return req.substring(numberOfRetry - i);
        } catch (Exception e) {
            // log exception
        }
        return null;
    })
    .limit(1)
    .findFirst()
    .orElseGet(() -> "Exception");

However, it works fine if I put a filter call as shown below:

int numberOfRetry = 5;
String req = "abc";
String res =
    Stream.iterate(0, n -> n + 1).map(i -> {
        try {
            return req.substring(numberOfRetry - i);
        } catch (Exception e) {
            // log exception
        }
        return null;
    })
    .limit(1)
    .filter(Objects::nonNull)
    .findFirst()
    .orElseGet(() -> "Exception");

I guess we cannot explicitly return null in some situations, and it is not quite clear what those situations are at first glance. In the first case it returns a stream with the null element in it which throws NullPointerException, in the second case it returns an empty stream which works fine.

Answer 1

Your code is explicitly returning null

return null

which thereafter throws NPE, according to the spec of Optional.findFirst which reads:

@throws NullPointerException if the element selected is null
Optional<T> findFirst();

Also, to clarify the code control couldn't even reach the orElseGet part which anyway assumingly works over an Optional (either empty or with some value).

---

Few suggestions :

• Don't ignore exceptions, especially when you've caught the most generic out of them all.
• Avoid returning null explicitly from within an iteration, it seems contradictory to why you'd iterate then.

• Safer side in your code currently, you can filter-in only nonNull objects using filter as

  Stream.iterate(0, n -> n + 1).map(i -> {
         try {
             return req.substring(numberOfRetry - i);
         } catch (Exception e) {
             err.add(e);
         }
         return null;
  })
  .filter(Objects::nonNull)
  .limit(1)
  .findFirst()
  .orElse("Exception");