Sensible unit test possible?

Question

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;
}

}

Answer 1

Could you not pass the class a Map<String,ExtractCommand[]> instances and then make an abstract method, say GetOsName, for getting the string to match. then you could look up the match string in the map to get the extract command in getSystemSpecificExtractCommands method. This would allow you to inject a list containing a mock ExtractCommand and override the GetOsName method to return the key of your mock command, so you could test that when the extract worked, the eventBus is fired etc.

private Map<String,EvenetCommand[]> eventMap;

@Inject
public Extractor(EventBus eventBus, Map<String,EventCommand[]> eventMap) {
    this.eventBus = eventBus;
    this.eventMap = eventMap;
}

private ExtractCommand[] getSystemSpecificExtractCommands() {
    String os = GetOsName();
    return eventMap.Get(os);
}

protected GetOsName();
{
    return System.getProperty("os.name");
}