How return null by using Stream API?

Question

So, I have an ArrayList of autogenerated strings. I want to find the first element that contains some character or else if there is no one element that matches this filter, to apply another filter. In another way, I want to return null object.

So I write this lambda expression:

str.stream()
   .filter(s -> s.contains("q"))
   .findFirst()
   .orElseGet(() -> str.stream()
                       .filter(s -> s.contains("w"))
                       .findFirst()
                       .orElseGet(null))

But if there is no one element that matches this two filters I will have NullPointerException. How, can I get something like: return null?

Don't. Instead, return an `Optional` and let the client decide what to do with it. — Luiggi Mendoza, Jul 27 '18 at 13:37
but think about it, you really want to return that `null` so that you move the problem further? May be a default value? or even better throw the exception earlier? — Eugene, Jul 27 '18 at 13:49
@Eugene using FP and null... it's not the best option, but if you have to, I'd prefer to throw the exception prior rather than handling null. — Luiggi Mendoza, Jul 27 '18 at 13:55

score 6 · Answer 1 · answered Jul 27 '18 at 13:42

6

Unless I'm missing something obvious, simply .orElse(null) instead of the last orElseGet. orElseGet accepts a Supplier and you pass a null, calling get() on a null reference, well...

answered Jul 27 '18 at 13:42

Eugene

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Nikolas Charalambidis · Answer 2 · 2018-07-27T13:47:23.003

The problem is Optional::orElseGet(null) which accepts a Supplier<T> and throws the NullPointerException.

Use the Optional::orElse(null) which accepts T which is String in your case.

The following code works:

List<String> str = Arrays.asList("a", "b", "c");
String output = str.stream()
                   .filter(s -> s.contains("q"))
                   .findFirst()
                   .orElseGet(() -> str.stream()
                                       .filter(s -> s.contains("w"))
                                       .findFirst()
                                       .orElse(null));                 // <-- HERE

System.out.println(output);    // Prints null

Alternatively, use the Optional::orElseGet(Supplier<? extends T> other) to return null. The Supplier itself must be not null:

.orElseGet(() -> null));

score 4 · Accepted Answer · answered Jul 27 '18 at 13:49

4

An additional, simpler approach: you can filter on strings containing q or w, then sort to move those containing q first, find first, return null if the optional is empty:

str.stream()
    .filter(s -> s.contains("q") || s.contains("w"))
    .sorted((s1, s2) -> s1.contains("q") ? -1 : 1 )
    .findFirst()
    .orElse(null);

.sorted((s1, s2) -> s1.contains("q") ? -1 : 1 ) will move the strings containing "q" first. Since the stream has been filtered only on values containing either q or w, then returning null will only happen no element is retained.

answered Jul 27 '18 at 13:49

ernest_k

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I thought about this, too complicated :( – Andrew Tobilko Jul 27 '18 at 13:50
@AndrewTobilko I'll second this, I wanted to edit my answer with sort of like this, but since you posted... – Eugene Jul 27 '18 at 13:51
@AndrewTobilko I would prefer it as I'm sure to traverse the stream only once. It's a better alternative for large streams. – ernest_k Jul 27 '18 at 13:54
@ernest_k *unless* measured, this is called guessing, no offense – Eugene Jul 27 '18 at 13:56
@Eugene Of course, that's why I said it's just an alternative. But wouldn't you choose to have just one traversal even if it's a small stream? – ernest_k Jul 27 '18 at 13:58
1

@ernest_k, I support the idea of iterating once (added it to my answer), but, IMHO, filtering complicates things – Andrew Tobilko Jul 27 '18 at 14:02
1

Upvoted, definitely a better approach as the traversal is done one time no matter what the content is. – marsouf Jul 27 '18 at 14:21

Andrew Tobilko · Answer 4 · 2018-07-27T13:57:04.943

3

orElseGet expects a Supplier, you need to pass a value, which is the case for orElse.

I wouldn't use Stream API here, one iteration is enough to solve the problem:

String wString = null;
for (String s : str) {
    if (s.contains("q")) return s;
    if (s.contains("w") && wString == null) wString = s;
}
return wString;

edited Jul 27 '18 at 13:57

answered Jul 27 '18 at 13:44

Andrew Tobilko

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How return null by using Stream API?

4 Answers4