在 Java 中的类之间传递参数

Question

I'm very stuck trying to solve an excercise consisting of a java pack being tested with some tests I find impossible to pass.

There are two classes in the pack, namely Car and Parking. The relationship between them is an aggregation - Parking is the compound and Car is the component:

Parking -parking (0...1)<>------- -cars(*) Car

-parking and -cars are the attributes which with the classes Parking and Car are respectively related. The attribute -parking can have two values, 0 or 1, and -cars is an array of undefined dimension.

A car can be assigned to one or none parkings. Likewise, a parking is compound of a variable number of parking lots.

The code is as follows:

package package;

public class Parking {

    private String name; // Parking's name

    private String address; // Parking's address

    private int capacity; // Parking's capacity (number of cars can be parked in)

    private Car[] cars; // Array of cars that can be parked

    public Parking() { // Default constructor
        name = "Default parking";
        address = "59th Street";
        capacity = 10;
        cars = new Car[capacity];
    }

    public Car[] getCars() { // Getter of the array 'cars'
        return cars;
    }

    public int getFirstFreeParkingLot() { // Returns first free empty slot of the
                                         // array 'cars'. A car can be parked there.
                                         // Otherwise, returns -1
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == null) {
                b = true;
                break;
            }
        }
        if (!b) return -1;
        else return i;
    }

    public void addCar (Car car) throws Exception { // Adds a car to a parking lot
        if (car == null) throw new Exception("[ERROR] The car cannot be null");
        else if (getParkingLotNumberByCar(car) != -1) throw new Exception("[ERROR] This car is already in this parking");
        else if (isFull()) throw new Exception("[ERROR] This parking is full");
        else if (getFirstFreeParkingLot() != -1) {
            cars[getFirstFreeParkingLot()] = car;
            car.setParking(car.getParking());
            }
    }

    public void removeCar (Car car) throws Exception { // remove a car from a parking 
                                                       // lot
        if(getParkingLotNumberByCar(car) != -1) {
            cars[getParkingLotNumberByCar(car)] = null;
            car.setParking(null);
            }
        else throw new Exception("[ERROR] This car does not exist in this parking");
    }

    public boolean isFull() { // Checks if the parking is full. 
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == null) {
                b = true;
                break;
            }
        }
        return !b;
    }

    public boolean isFree() { // Checks if there's at least one empty parking lot
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == null) {
                b = true;
                break;
            }
        }
        return b;
    }

    public boolean isEmpty() { // Checks if the entire parking lot is empty
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] != null) {
                b = true;
                break;
            }
        }
        return !b;
    }

    public int getParkingLotNumberByCar (Car car) { // Return the index of the array 
                                                    // 'cars' where the car given as
                                                    // argument is present in the 
                                                    // parking lot. Otherwise, returns -1
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == car) {
                b = true;
                break;
            }
        }
        if (!b) return -1;
        else return i;
    }

    public int getNumFreeParkingLots() { // Return the number of free parking lots in a parking
        int i;
        int n = 0;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == null) n++;
        }
        return n;
    }

}

package package;

import javax.management.ObjectName;
import java.time.LocalDate;
import java.util.Objects;
import java.util.UUID;

public class Car {

    private Parking parking;

    public Car() {
        parking = null;
    }

    public Parking getParking() {
        return parking;
    }

    public void setParking(Parking parking) throws Exception {
        if (parking == null)
            this.parking = null;
        else {
            parking.addCar(parking.getCars()[parking.getParkingLotNumberByCar(new Car())]);
            if (this.parking != parking) this.parking.removeCar(parking.getCars()[parking.getParkingLotNumberByCar(new Car())]);
        }
    }
}

And here's one test which I don't pass as an example:

package package;

import org.junit.jupiter.api.*;
import org.junit.jupiter.api.MethodOrderer.OrderAnnotation;
import org.junit.jupiter.api.TestInstance.Lifecycle;

import static org.junit.jupiter.api.Assertions.*;

@TestInstance(Lifecycle.PER_CLASS)
@TestMethodOrder(OrderAnnotation.class)
class IntegrationTest {
    Parking parking1;
    Car car1;
    
    @BeforeAll
    void init(){
        try {
            parking1 = new Parking();
            car1 = new Car();
        } catch (Exception e) {
            e.printStackTrace();
            fail("Init failed");
        }
    }

    @Test
    @Order(1)
    void testIntegration1() {
        try {
            parking1.addCar(car1);
            assertEquals(0, parking1.getParkingLotNumberByCar(car1));
            assertEquals(9, parking1.getNumFreeParkingLots());
            assertEquals(1, parking1.getFirstFreeParkingLot());
            assertEquals(car1, parking1.getCars()[parking1.getParkingLotNumberByCar(car1)]);
            assertEquals(parking1, car1.getParking());
        } catch (Exception e) {
            e.printStackTrace();
            fail("Integration1 failed");
        }
    }

The critical part comes when utilizing the setParking (Parking parking) method in the Car class and the addCar (Car car) and removeCar (Car car) methods in the Parking class. The setParking method in the Car class establishes the attribute parking and also calls the methods addCar and remove Car of the Parking class, which in turn add to and remove a car from a parking, and finally call the mentioned setParking method.

I get to add a Car to a parking, but I fail in adding the parking's information to that car.

To refer to a Car object in the Car class, I use 'new Car', and when it comes to the Parking class, I use 'new Parking'. Am I proceeding correctly?

All the tests I'm failing at are related with the unsuccessful intertwining of these above-mentioned methods - parking.addCar, parking.removeCar, car.setParking.

Could somebody help me?

Thank you a lot in advance

Answer 1

I finally found a solution.

public class Parking {

    private String name; // Parking's name

    private String address; // Parking's address

    private int capacity; // Parking's capacity (number of cars can be parked in)

    private Car[] cars; // Array of cars that can be parked

    public Parking() { // Default constructor
        name = "Default parking";
        address = "59th Street";
        capacity = 10;
        cars = new Car[capacity];
    }

    public Car[] getCars() { // Getter of the array 'cars'
        return cars;
    }

    public int getFirstFreeParkingLot() { // Returns first free empty slot of the
                                         // array 'cars'. A car can be parked there.
                                         // Otherwise, returns -1
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == null) {
                b = true;
                break;
            }
        }
        if (!b) return -1;
        else return i;
    }

    public void addCar (Car car) throws Exception {
        if (car == null) throw new Exception("[ERROR] The car cannot be null");
        else if ((getParkingLotNumberByCar(car) != -1)) throw new Exception("[ERROR] This car is already in this parking");
        else if (isFull()) throw new Exception("[ERROR] This parking is full");
        else if (isFree()) {
            cars[getFirstFreeParkingLot()] = car;
            car.setParking(this);
            }
    }

    public void removeCar (Car car) throws Exception {
        if(getParkingLotNumberByCar(car) != -1) {
            cars[getParkingLotNumberByCar(car)] = null;
            car.setParking(null);
            }
        else throw new Exception("[ERROR] This car does not exist in this parking");
    }

    public boolean isFull() { // Checks if the parking is full. 
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == null) {
                b = true;
                break;
            }
        }
        return !b;
    }

    public boolean isFree() { // Checks if there's at least one empty parking lot
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == null) {
                b = true;
                break;
            }
        }
        return b;
    }

    public boolean isEmpty() { // Checks if the entire parking lot is empty
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] != null) {
                b = true;
                break;
            }
        }
        return !b;
    }

    public int getParkingLotNumberByCar (Car car) { // Return the index of the array 
                                                    // 'cars' where the car given as
                                                    // argument is present in the 
                                                    // parking lot. Otherwise, returns -1
        int i;
        boolean b = false;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == car) {
                b = true;
                break;
            }
        }
        if (!b) return -1;
        else return i;
    }

    public int getNumFreeParkingLots() { // Return the number of free parking lots in a parking
        int i;
        int n = 0;
        for (i = 0; i < cars.length; i++) {
            if (cars[i] == null) n++;
        }
        return n;
    }

}

public class Car {

    private Parking parking;

    public Car() {
        parking = null;
    }

    public Parking getParking() {
        return parking;
    }

    public void setParking(Parking parking) throws Exception {
        if (parking != null) {
            if (parking.getParkingLotNumberByCar(this) == -1) {
                    parking.addCar(this);
            }
            if (this.parking != null && this.parking != parking) {
                        this.parking.removeCar(this);
                    }
            this.parking = parking;
                }
        else {
            if (this.parking != null) {
                if (this.parking.getParkingLotNumberByCar(this) != -1)
                    this.parking.removeCar(this);
            }
            this.parking = null;
        }
    }
}