I am using a non stream way to get single element from collection.
List<MyCustomClass> list = OtherObject.getMyList();
if (list.size() != 1) {
throw new RuntimeException();
}
MyCustomClass customClass = list.get(0);
Instead of this multi liner approach, is there some way to achieve this via streams?
You can use reduce(accumulator) and orElseThrow(exceptionSupplier) to ensure the stream produces exactly one result.
MyCustomClass customClass = list.stream()
.reduce((a,b) -> { throw new RuntimeException("Too many values present"); })
.orElseThrow(() -> { throw new RuntimeException("No value present"); });
reduce() and orElseThrow() are both built-in. OP didn't ask for cleaner/simpler code, only for a stream solution to do the same in a single statement ("one-liner").I was looking for a version with a single collect statement, although it turned out not as concise or elegant as the solution by Andreas. It uses an implementation of Collector that accumulates to a one-element list, while the combiner raises an exception if we have more than one element; the finisher raises an exception when the list is empty.
list.stream().collect(
Collector.of( ArrayList::new,
(a, t) -> { if (!a.isEmpty())
throw new RuntimeException();
a.add(t); },
(a, b) -> { throw new RuntimeException(); },
a -> { if( a.isEmpty() )
throw new RuntimeException();
return a.get(0);} );
You could try returning an optional from findFirst() or findAny().
List<String> strings = new ArrayList<>();
Optional<String> maybeFirst = strings.stream().findFirst();
// we now have an optional, lets force a value
String value = maybeFirst.orElseThrow(IllegalArgumentException::new);
// if there isn't a value, we'll throw an illegal argument exception.
This can collapsed into the following.
String value = strings.stream()
.findFirst()
.orElseThrow(() -> new IllegalArgumentException("There must be at least one string."));
Hope that helps.
findAny or findFirst would come in handy.list.size() != 1 instead of list.isEmpty().