import javax.annotation.Nonnull;
import javax.annotation.meta.When;

public class MessageJava {
    @Nonnull(when = When.ALWAYS)
    public String getEchoMessage() {
        return "Hello";
    }

    @Nonnull(when = When.UNKNOWN)
    public String getDummyMessage() {
        return "foo bar";
    }

    @Nonnull(when = When.MAYBE)
    public String getThirdPartyMessage() {
        return fetchFromExternalService();
    }
}