Updated Addon

import java.util.ArrayList;
import java.util.List;

import org.bukkit.Bukkit;
import org.bukkit.entity.Player;

import me.mrCookieSlime.QuickSell.boosters.Booster;

public class PermBoosters extends Booster {

    public PermBoosters(double multiplier, boolean silent, boolean infinite) {
        super(multiplier, silent, infinite);
    }

    public void onEnable() {
        new PermBoosters(1.5, true, true);
    }

    @Override
    public List<String> getAppliedPlayers() {

        List<String> one5 = new ArrayList<String>();

        for (Player p : Bukkit.getOnlinePlayers()) {
            if (p.hasPermission("quicksell.boosters.1.5")) {
                one5.add(p.getName());
            }
        }
        return one5;
    }
}