用Java制作一款租车APP

制作租车APP需要考虑以下几个方面：

1. 用户登录和注册功能
2. 车辆库存管理
3. 车辆租赁预订功能
4. 车辆归还和结算功能
5. 费用计算和支付功能

下面是一个基本的Java代码框架，可供参考：

public class User {
    private String username;
    private String password;
    private String email;
    private String phone;
    // getter and setter methods
}

public class Car {
    private String make;
    private String model;
    private int year;
    private double pricePerDay;
    private boolean available;
    // getter and setter methods
}

public class Rental {
    private User user;
    private Car car;
    private Date startDate;
    private Date endDate;
    private double totalCost;
    private boolean paid;
    // getter and setter methods
}

public class CarRentalApp {
    private List<User> users;
    private List<Car> cars;
    private List<Rental> rentals;
    // methods for user registration and login
    public void addCar(Car car) {
        cars.add(car);
    }
    public List<Car> displayAvailableCars() {
        List<Car> availableCars = new ArrayList<>();
        for (Car car : cars) {
            if (car.isAvailable()) {
                availableCars.add(car);
            }
        }
        return availableCars;
    }
    public Rental rentCar(User user, Car car, Date startDate, Date endDate) {
        Rental rental = new Rental(user, car, startDate, endDate);
        rentals.add(rental);
        car.setAvailable(false);
        return rental;
    }
    public void returnCar(Rental rental) {
        rental.setPaid(true);
        rental.getCar().setAvailable(true);
        double cost = calculateCost(rental);
        rental.setTotalCost(cost);
    }
    private double calculateCost(Rental rental) {
        // calculate the cost based on the rental period and car price per day
        return rental.getCar().getPricePerDay() * (rental.getEndDate().getTime() - rental.getStartDate().getTime()) / (24 * 60 * 60 * 1000);
    }
}

以上代码只是一个简单的示例，实际开发需要根据具体需求进行完善和优化。