Message.java

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

public class Message {

    private final String greetingMessage = "Hello ";

    @Nonnull
    public String getEchoMessage() {
        return greetingMessage;
    }

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

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