stubbing-spock.groovy

class Publisher {
    private Subscriber subscriber

    Publisher(Subscriber subscriber) {
        this.subscriber = subscriber
    }

    String sendAndGetStatus(String message) {
        return subscriber.receive(message)
    }
}

interface Subscriber {
    String receive(String message)
}

class PublisherSpec extends Specification {
    Publisher publisher
    Subscriber subscriber = Mock()

    def setup() {
        subscriber.receive(_) >> "ok"  // Stubbing
        publisher = new Publisher(subscriber)
    }

    def 'should send messages and get the subscriber status'() {
        when:
        def result = publisher.sendAndGetStatus("hello")

        then:
        result == "ok"
    }
}