计算类内实例的数量

我被要求在StealthCruiser类中构建方法“ getAnnualMaintenanceCost”，该方法用于计算某物的总成本。总成本的组成部分之一取决于为同一类创建的实例数量，如何计算数量？

import java.util.Arrays;
import java.util.List;
import java.util.Set;

public class StealthCruiser extends AbstractCompbatShip {

    static final int COST_BASIC = 2000;
    static final int COST_PER_UNIT1 = 1000;
    static final int COST_PER_UNIT2 = 100;

    private Weapon defaultWeapon;

    public StealthCruiser(String name, int commissionYear, float maximalSpeed, 
            Set<CrewMember> crewMembers, List<Weapon> weaponArray) {
        super(name, commissionYear, maximalSpeed, crewMembers, weaponArray);
    }

    public StealthCruiser(String name, int commissionYear, float maximalSpeed,
            Set<CrewMember> crewMembers) {
        super(name, commissionYear, maximalSpeed, crewMembers,
                Arrays.asList(new Weapon("Laser Cannons", 10 ,100)));
    }


    @Override
    public int getAnnualMaintenanceCost() {
        return COST_BASIC + 
               (int)(super.getMaximalSpeed() * COST_PER_UNIT1) +                                
               super.getWeaponsCost() +
               (COST_PER_UNIT2 * ...... //should be number of instances of that class;
    }

....

}