https://qiita.com/Puroguramingu_Bennkyoutyuu/questions/44f4e871bbdd02db6093#answer-89a65b43ee42f279e603

ParkingSimulator.java

public class ParkingSimulator {

    public static void main(String[] args) {
        Parking parking = new Parking("駅前駐車場", 2);
        Car car1 = new Car("品川33 あ 1234");
        Car car2 = new Car("横浜555 い 5678");
        Car car3 = new Car("多摩345 う 9012");
        car1.park(parking);
        car2.park(parking);
        car3.park(parking);
        car1.exit();
        car3.park(parking);
        car1.exit();
        car2.park(parking);
    }
}

interface Vehicle {
}

class Parking {
    public final String name;
    private ParkingSpace[] spaces;

    Parking(String name, int capacity) {
        this.name = name;
        spaces = new ParkingSpace[capacity];
        for (int i = 0; i < capacity; i++) {
            spaces[i] = new ParkingSpace(i + 1);
        }
    }

    public ParkingSpace searchAvailableSpace() {
        for (var space: spaces) {
            if (space.isAvailable()) {
                return space;
            }
        }
        return null;
    }
}

class ParkingSpace {
    public final int number;
    private Vehicle vehicle;

    ParkingSpace(int number) {
        this.number = number;
    }

    boolean isAvailable() {
        return vehicle == null;
    }

    void park(Vehicle vehicle) throws IllegalStateException {
        if (!isAvailable()) {
            throw new IllegalStateException("not available");
        }
        this.vehicle = vehicle;
    }

    void exit(Vehicle vehicle) throws IllegalStateException {
        if (this.vehicle != vehicle) {
            throw new IllegalStateException("not park");
        }
        this.vehicle = null;
    }
}

class Car implements Vehicle
{
    public final String number;
    private ParkingSpace parkingSpace;

    public Car(final String number)
    {
        this.number = number;
    }

    void park(Parking parking) {
        if (parkingSpace != null) {
            System.out.println("「" + number + "」は既に駐車しています");
            return;
        }
        parkingSpace = parking.searchAvailableSpace();
        if (parkingSpace == null) {
            System.out.println("「" + number + "」は満車で停められませんでした");
            return;
        }
        parkingSpace.park(this);
        System.out.println("「" + number + "」は駐車番号" + parkingSpace.number + "に停めました");
    }

    void exit() {
        if (parkingSpace == null) {
            System.out.println("「" + number + "」は駐車していません");
            return;
        }
        parkingSpace.exit(this);
        parkingSpace = null;
        System.out.println("「" + number + "」は出庫しました");
    }
}