APIQuestPackage.java

package api.quest;

public class QuestModule implements Listener {
    private final QuestManager manager;
    private final YourAPI api;
    
    public QuestModule(YourAPI api) {
        this.api = api;
        this.manager = new QuestManager(api);
    }
    
    @Override
    public void onPlayerLogin(PlayerLoginEvent event) {
        Player player = event.getPlayer();
        manager.addLocal(player);
        PlayerInfo info = api.getPlayerInfo(player);
        
        if(info.hasQuests()) {
            localQuests.put(player, info.quests());
        }
    }
    
    @Override
    public void onPlayerLogout(PlayerLogoutEvent event) {
        Player player = event.getPlayer();
        manager.removeLocal(player);
    }

    public static QuestModule load(YourAPI api) {
        // prepare base quests if wanted
        return new QuestModule(api);
    }
}

public class QuestManager {
    private final Map<Player, QuestGroup> localQuests = new HashMap<>();
    private final YourAPI api;
    
    public QuestManager(YourAPI api) {
        this.api = api;
    }
    
    public void removeLocal(Player player, Quest quest) {
        if(localQuests.contains(player) {
            QuestGroup quests = localQuests.get(player);
            api.deregisterListener(quests);
            localQuests.remove(player);
        }
    }
    
    public void addLocal(Player player, Quest quest) throws QuestAlreadyExistsException {
        if(!localQuests.contains(player))
            localQuests.put(player, new QuestGroup(quest)
        
        QuestGroup quests = localQuests.get(player);
        if(quests.contains(quest))
            throw new QuestAlreadyExistsException();
        
        quests.add(quest);
    }
    
    public static final class QuestAlreadyExistsException extends Exception { }
}

public class QuestGroup extends Group<Quest> {

}

public class Quest {
    private final String name;
    private int progress;
    
    public Quest(String name) {
        this.name = name;
    }
    
    public void increaseProgressBy(int amount) {
        progress += amount;
        if(progress > 100)
            progress = 100;
    }
    
    public void decreaseProgressBy(int amount) {
        progress -= amount;
        if(progress < 0)
            progress = 0;
    }
    
    @Override
    public String toString() {
        return name;
    }
}