How to extract only one allowed element from a stream?

Question

I have a list of elements, and want to extract the value of the fields' propery. Problem: all elements should have the same property value.

Can I do better or more elegant than the following?

Set<String> matches = fields.stream().map(f -> f.getField()).collect(Collectors.toSet());
if (matches.size() != 1) throw new IllegalArgumentException("could not match one exact element");
String distrinctVal = matches.iterator().next(); //continue to use the value

Is this possible directly using the stream methods, eg using reduce?

Answer 1

Your current solution is good. You can try this way also to avoid collecting.

Use distinct() then count()

if (fields.stream().map(f -> f.getField()).distinct().count() != 1) 
      throw new IllegalArgumentException("could not match one exact element");

To get the value

String distrinctVal = fields.get(0).getField();