findFirst抛出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"); 

Answer 2

What do you think happens when you do:

System.out.println(req.substring(numberOfRetry - i));

where i for the first time is zero ? An out of bounds exception will be thrown - which you catch and return null ; so you have a Stream.of(null) which you call findFirst on - which is documented to throw that NullPointerException is case the element is null.

Answer 3

From the javadoc for findFirst :

Throws: NullPointerException - if the element selected is null

Optional cannot differentiate between "present but null" and "not present"

Answer 4

It happens because you return null in your map function. Try to add .filter(s -> s != null) after map operation.