In the program, a class Car and its subclass Truck are defined.

Write a method
public static int totalLoad(ArrayList<Car> cars)

that receives a list of Truck objects as a parameter. 
The method calculates and returns the total load of the trucks.
Note that the list type is Car, not Truck!



import java.util.Random;
import java.util.ArrayList;

public class Test{
    public static void main(String[] args){
        final Random r = new Random();
        
        String[] brands = "Sisu Scania Volvo Ford Fiat Citroen".split(" ");
        
        for(int test=1; test<=3; test++) {
            
            System.out.println("Test " + test);
        
            ArrayList<Car> cars = new ArrayList<>();
            
            final int size = r.nextInt(4) + 3;
            for (int i=0; i<size; i++) {
                cars.add(new Truck(brands[r.nextInt(brands.length)], r.nextInt(20) + 5));
            }
            System.out.println("Cars:");
            cars.stream().forEach(a -> System.out.println("" + a));
            
            System.out.println("Total load: " + totalLoad(cars));
            System.out.println("");
    
        }
    }



    //add
    // round 1 - NO
    // public static int totalLoad(ArrayList<Car> cars) {
    //     int totalLoad = 0;
    //     for (Truck truck : cars) {
    //         totalLoad += truck.getLoad();
    //     }
    //     return totalLoad;
    // }


    // round 2 - YES
    public static int totalLoad(ArrayList<Car> cars) {
        int totalLoad = 0;
        for (Car car : cars) {
            // explicit type casting
            Truck truck = (Truck) car;
            totalLoad += truck.getLoad();
        }
        return totalLoad;
    }




}





class Car {
    protected String brand;
    
    public Car(String brand) {
        this.brand = brand;
    }

    public String getBrand() {
        return brand;
    }
}

class Truck extends Car {
    private int load;

    public Truck(String brand, int load) {
        super(brand);
        this.load = load;
    }
    
    public int getLoad() {
        return load;
    }
    
    public String toString() {
        return brand + ", load: " + load;
    }
}











Test 1
Cars:
Sisu, load: 5
Volvo, load: 24
Sisu, load: 23
Total load: 52

Test 2
Cars:
Fiat, load: 9
Fiat, load: 10
Scania, load: 13
Ford, load: 16
Total load: 48

Test 3
Cars:
Volvo, load: 18
Ford, load: 22
Scania, load: 20
Sisu, load: 20
Volvo, load: 7
Sisu, load: 21
Total load: 108