Is this a violation of the Law of Demeter? vs. readable code

Question

The code below brakes apparently the Law of Demeter, i.e. methods getServer().methodx(...). From other side it looks pretty compact = better readable?

abstract class BaseManager {
    ResultSet find(String searchText) {
        return getServer().find(searchText);
    }

    ResultSet fetch(String fetchText) {
        return getServer().fetch(fetchText);
    }

    void save(String saveText) {
        getServer().save(saveText);
    }

    abstract BaseManager getServer();
}

class Server1Manager extends BaseManager {
    @Override
    protected BaseManager getServer() {
        return server1;
    }
}

class Server2Manager extends BaseManager {
    @Override
    protected BaseManager getServer() {
        return server2;
    }
}

If the law is violated, them how to refactor this code? Many thanks in advance.

score 1 · Accepted Answer · answered Dec 07 '17 at 14:27

The code below brakes [sic] apparently the Law of Demeter, i.e. methods getServer().methodx(...). From other side it looks pretty compact = better readable?

The point of your design is lost on me. If compactness is your goal, then would not this be even better?

class Manager {
    private Server server;

    public Manager(Server server) {
        this.server = server;
    }

    ResultSet find(String searchText) {
        server.find(searchText);
    }

    ResultSet fetch(String fetchText) {
        server.fetch(fetchText);
    }

    void save(String saveText) {
        server.save(saveText);
    }
}

In addition to being even more compact and clear, that happens to conform to the Law of Demeter. Moreover, it follows the principle of preferring composition over inheritance, which I think you'll see is favored (but not implied) by the Law of Demeter.

If the law is violated, them how to refactor this code?

I'm still liking what I presented above.

Is the following solution acceptable (does it not look like a code duplication)?: [...]

If you presented that to me for code review, I would certainly ask what you think you gain from inheritance there. One might argue a bit over whether your code is technically duplicative, but it is certainly longer and more complex than the non-inheritance version I presented. And the non-inheritance version leaves no question about code duplication.

score 0 · Answer 2 · answered Dec 07 '17 at 14:03

0

For me it's much better after you have implemented 3 methods in manager.

You can have 1 data manager exposing methods for both servers.

There's another rule I like: Don't ask - tell.

answered Dec 07 '17 at 14:03

MicD

197
2
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Is this a violation of the Law of Demeter? vs. readable code

2 Answers2