Question

I’m really confused about the visitor pattern and its uses. I can’t really seem to visualize the benefits of using this pattern or its purpose. If someone could explain with examples if possible that would be great.

Answer 1

Once upon a time…

class MusicLibrary {
    private Set<Music> collection ...
    public Set<Music> getPopMusic() { ... }
    public Set<Music> getRockMusic() { ... }
    public Set<Music> getElectronicaMusic() { ... }
}

Then you realize you’d like to be able to filter the library’s collection by other genres. You could keep adding new getter methods. Or you could use Visitors.

interface Visitor<T> {
    visit(Set<T> items);
}

interface MusicVisitor extends Visitor<Music>;

class MusicLibrary {
    private Set<Music> collection ...
    public void accept(MusicVisitor visitor) {
       visitor.visit( this.collection );
    }
}

class RockMusicVisitor implements MusicVisitor {
    private final Set<Music> picks = ...
    public visit(Set<Music> items) { ... }
    public Set<Music> getRockMusic() { return this.picks; }
}
class AmbientMusicVisitor implements MusicVisitor {
    private final Set<Music> picks = ...
    public visit(Set<Music> items) { ... }
    public Set<Music> getAmbientMusic() { return this.picks; }
}

You separate the data from the algorithm. You offload the algorithm to visitor implementations. You add functionality by creating more visitors, instead of constantly modifying (and bloating) the class that holds the data.