Rules the owner enforces
So far every add put its object on the list, no questions asked. But the object that owns the list can also have rules about what goes on it. A parking garage has a fixed number of spots, and the same car can't be parked in it twice:
public class Main {
public static void main(String[] args) {
ParkingGarage garage = new ParkingGarage(2);
System.out.println(garage.park(new Car("ABC-123")));
System.out.println(garage.park(new Car("ABC-123")));
System.out.println(garage.park(new Car("XYZ-789")));
System.out.println(garage.park(new Car("JKL-456")));
garage.leave("ABC-123");
System.out.println(garage.park(new Car("JKL-456")));
System.out.println(garage);
}
}import java.util.ArrayList;
public class ParkingGarage {
private int spots;
private ArrayList<Car> cars;
public ParkingGarage(int spots) {
this.spots = spots;
this.cars = new ArrayList<>();
}
public boolean park(Car car) {
if (this.cars.size() >= this.spots) {
return false;
}
if (this.isParked(car.getPlate())) {
return false;
}
this.cars.add(car);
return true;
}
public boolean isParked(String plate) {
for (Car car : this.cars) {
if (car.getPlate().equals(plate)) {
return true;
}
}
return false;
}
public void leave(String plate) {
for (int i = this.cars.size() - 1; i >= 0; i--) {
if (this.cars.get(i).getPlate().equals(plate)) {
this.cars.remove(i);
}
}
}
public String toString() {
return this.cars.size() + "/" + this.spots + " spots taken: " + this.cars;
}
}public class Car {
private String plate;
public Car(String plate) {
this.plate = plate;
}
public String getPlate() {
return this.plate;
}
public String toString() {
return this.plate;
}
}true
false
true
false
true
2/2 spots taken: [XYZ-789, JKL-456]parkchecks its rules first, and adds the car only when all of them hold: there's a free spot, and no car with the same plate is parked. Each rule that fails returnsfalseright away.- The
booleanit returns tells the caller whether the car got in.mainprints it here; a real program could printGarage fullinstead. parkuses another method of the same object,this.isParked(...). Small methods like this one can be useful to the caller too:garage.isParked("ABC-123").- After
leave, there's a free spot again, and the car that was turned away gets in.
The rules always hold because the list is private. The only way onto the list is through park, and park checks every car. No other part of the program can put a car in past the rules, however it's written.
Don't hand out the list
Suppose ParkingGarage had a getter for its list:
public ArrayList<Car> getCars() {
return this.cars;
}It returns a reference to the garage's own list, not a copy (module 22). With it, garage.getCars().add(new Car("NEW-001")); puts a car straight on the list, past every rule, even into a full garage. So keep the list to the class, and give the class methods for what the rest of the program needs: park, leave, isParked, a toString.
The module's tools
| Tool | Example |
|---|---|
| A list as an instance variable | private ArrayList<Book> books; |
| Made in the constructor | this.books = new ArrayList<>(); |
| A loop over the object's list | for (Book book : this.books) |
| A toString built in a loop | text = text + "\n" + book; |
| Removing while going through | an index loop that goes backwards |
| A total from the objects | total = total + book.getPages(); |
| A search that may find nothing | returns the object, or null |
| A rule checked before adding | if (this.cars.size() >= this.spots) |