Example.groovy

class Example extends Specification {

  def "test"() {
    given:
    Cls cls = Spy(new Cls())

    when:
    cls.m()

    then:
    1 * cls.onChanges({ List<String> lst ->
      check(lst)
    })
  }

  def check(List<String> lst) {
    assert lst.size() == 4
    assert lst.every { it instanceof String } // не работает
  }

  class Cls {
    def m() {
      onChanges(List.of("a", "a", "a", "b"))
    }

    def onChanges(List<String> list) {
      list.forEach { println it }
    }
  }
}