很久以前…
class MusicLibrary { private Set<Music> collection ... public Set<Music> getPopMusic() { ... } public Set<Music> getRockMusic() { ... } public Set<Music> getElectronicaMusic() { ... }}然后，您意识到您希望能够按其他流派过滤图书馆的藏书。您可以继续添加新的getter方法。或者您可以使用访客。
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; }}您将数据与算法分开。您将算法卸载到访问者实现中。您可以通过创建 更多 访问者来添加功能，而不是不断修改（和膨胀）保存数据的类。
