将多个Java对象数组传递给另一个方法

Question

I just started learning java, with some minor prior experience in python and a bit of javascript, but not using classes. I have an issue with this code (just for reference, below that I point out exact issue):

public class Race {
    Boolean isThereABrokenTruck = false;
    private Car[] cars;
    private Motorcycle[] motorcycles;
    private Truck[] trucks;
    private void createVehicles() {
        cars = new Car[10];
        motorcycles = new Motorcycle[10];
        trucks = new Truck[10];

    } // creates 10 cars, 10 trucks and 10 motorcycles.

    private void simulateRace() {
        Weather.setRaining();
        for (Car car : cars) {
            for (int i = 0; i < 50; i++) {
                car.moveForAnHour();
            }
        }
        for (Motorcycle motorcycle : motorcycles) {
            for (int i = 0; i < 50; i++) {
                motorcycle.moveForAnHour();
            }
        }
        for (Truck truck : trucks) {
            for (int i = 0; i < 50; i++) {
                truck.moveForAnHour();
            }
        }

    } 
    private void printRaceResults() {
        for (Car car : cars) {
            System.out.println("Name: " + car.name);
            System.out.println("\n Distance Travelled: " + car.distanceTraveled);
            System.out.println("\n Type:" + car.getClass().getName());
        }
        for (Motorcycle motorcycle : motorcycles) {
            System.out.println("Name: " + motorcycle.name);
            System.out.println("\n Distance Travelled: " + motorcycle.distanceTraveled);
            System.out.println("\n Type:" + motorcycle.getClass().getName());
        }
        for (Truck truck : trucks) {
            System.out.println("Name: " + truck.name);
            System.out.println("\n Distance Travelled: " + truck.distanceTraveled);
            System.out.println("\n Type:" + truck.getClass().getName());
        }
    } // prints each vehicle's name, distance traveled ant type.

    protected Boolean isThereABrokenTruck() {
        return isThereABrokenTruck;
    }

    public static void main(String[] args) {
        Race race = new Race();
        race.createVehicles();
        race.simulateRace();
        race.printRaceResults();
    }
}

This code compiles (classes Car, Motorcycle and Truck are defined in my code as well, but are not relevant to the question), however I get runtime null pointer exception on

   for (Car car : cars) { // null pointer exception here
        for (int i = 0; i < 50; i++) {
            car.moveForAnHour();
        }
    }

so I guess I am not assigning value to cars properly. I am forced to have separate methods to create those vehicles, operate on them and print the results to console. In python I'd probably just return multiple arrays (or lists) and assign their values to different variables, but how do I do it here in Java?

Answer 1

initial arrays in constructor

public Race() {
  createVehicles();
}

Answer 2

You initialize your vehicle array:

private void createVehicles() {
    cars = new Car[10];
    motorcycles = new Motorcycle[10];
    trucks = new Truck[10];

}

But your array now contains only null-car, null-trucks...

You need to initialize them as well:

private void createVehicles() {
    cars = new Car[10];
    for (int i = 0; i < cars.length; i++) {
        cars[i] = new Car();
    }
    motorcycles = new Motorcycle[10];
    trucks = new Truck[10];
    // Init other vehicles as well
}

Answer 3

You are creating cars variable to hold 10 instances of Car but not storing any value. Add some values to the array and try.

Answer 4

when you initial an array you fill the array with nulls. so Car[] cars = new Car[10]; means you have an array with 10 nulls in it. and you are iterating on it. and you will get NullPointerException here car.moveForAnHour(); in createVehicles() you must fill it with objects. for example :

private void createVehicles() {
    cars = new Car[10];
    for(int i = 0; i < cars.length; i++) {
      cars[i] = new Car();
    }

}