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?

Answer 1

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...

Answer 2

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.

Answer 3

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));