Could a sensible unit test be written for this code which extracts a rar archive by delegating it to a capable tool on the host system if one exists? I can write a test case based on the fact that my machine runs linux and the unrar tool is installed, but if another developer who runs windows would check out the code the test would fail, although there would be nothing wrong with the extractor code. I need to find a way to write a meaningful test which is not binded to the system and unrar tool installed. How would you tackle this?

public class Extractor {

private EventBus eventBus;
private ExtractCommand[] linuxExtractCommands = new ExtractCommand[]{new LinuxUnrarCommand()};
private ExtractCommand[] windowsExtractCommands = new ExtractCommand[]{};
private ExtractCommand[] macExtractCommands = new ExtractCommand[]{};

@Inject
public Extractor(EventBus eventBus) {
    this.eventBus = eventBus;
}

public boolean extract(DownloadCandidate downloadCandidate) {
    for (ExtractCommand command : getSystemSpecificExtractCommands()) {
        if (command.extract(downloadCandidate)) {
            eventBus.fireEvent(this, new ExtractCompletedEvent());
            return true;
        }
    }

    eventBus.fireEvent(this, new ExtractFailedEvent());
    return false;
}

private ExtractCommand[] getSystemSpecificExtractCommands() {
    String os = System.getProperty("os.name");
    if (Pattern.compile("linux", Pattern.CASE_INSENSITIVE).matcher(os).find()) {
        return linuxExtractCommands;
    } else if (Pattern.compile("windows", Pattern.CASE_INSENSITIVE).matcher(os).find()) {
        return windowsExtractCommands;
    } else if (Pattern.compile("mac os x", Pattern.CASE_INSENSITIVE).matcher(os).find()) {
        return macExtractCommands;
    }

    return null;
}

}