The 30 second answer
Concat itself is fine with null elements in either list, but sorted uses natural ordering, so comparing a null throws NullPointerException at the terminal operation because streams are lazy. Filter nulls out first, or sort with Comparator nullsFirst or nullsLast so nulls have a defined place in the result.
concat is fine with nulls; sorted() uses natural ordering, so comparing null throws **NullPointerException** (at the terminal operation, since streams are lazy). Fix: filter(Objects::nonNull) or sorted(Comparator.nullsFirst(Comparator.naturalOrder())) / nullsLast.
.filter(Objects::nonNull)
.sorted()
// or keep nulls in a defined position:
.sorted(Comparator.nullsFirst(Comparator.naturalOrder()))Sources:
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