对象ArrayList循环错误

Question

I have an Object ArrayList and I need to use the toString() method of the Motor object, which is a parameter of the Vehicle object. My vehicle objects are in an ArrayList which is iterated through with a for-loop (I know a foreach loop would be easier, but this is part of the assignment)

Here is the code for the loop:

for (int i = 0; i < VehicleList.size(); i++) {
    System.out.println();
    String info = VehicleList.get(i).toString();
    Motor m = VehicleList.get(i).motor;
    String motorInfo = m.toString();
    System.out.println(info);
    System.out.println(m);
    }

There is an error that says " motor cannot be resolved or is not a field ".

All of the classes should allow this to work, unless of course there is a simple mistake I am missing.

Here is the Motor class:

 public class Motor {
    protected String name;
    protected int cylinders;
    protected int bhp;
    protected double displacement;

    public Motor(String name, int cylinders, int bhp, double displacement) {
        this.name = name;
        this.cylinders = cylinders;
        this.bhp = bhp;
        this.displacement = displacement;
    }

    public String toString() {
        return "Motor name= " + name + ", cylinders= " + cylinders + ", bhp= 
     " + bhp + ", displacement= " + displacement;
    }
}

Motors and Vehicles are intitialized here (In the TestVehicle class):

        //Motors
        Motor EcoBoost = new Motor("EcoBoost", 6, 310, 2.3);
        Motor Hemi = new Motor("Hemi", 8, 707, 5.7);
        Motor P90D = new Motor("P90D", 0, 762, 0.0);

        //Vehicles
        Vehicle v0 = new PassCar("Ford", "Mustang", 2016, 44500.0, 5, true, EcoBoost);
        Vehicle v1 = new PassCar("Tesla", "Model S", 2016, 121000.0, 2, true, P90D);
        Vehicle v2= new Truck("Dodge", "Ram", 2016, 46000.0, "pickup", 1500, Hemi);

PassCar and Truck are inherited classes of Vehicle with a few more attributes. I can post the PassCar or Truck class if needed but I do not think that is where the problem is arising from. I believe it is coming from the For-Loop, specifically the line Motor m = VehicleList.get(i).motor; but I am not sure of how to fix it.

Vehicle Class:

public class Vehicle {
protected String make;
protected String model;
protected int year;
protected double price;

public Vehicle(String make, String model, int year, double price) {
    this.make = make;
    this.model = model;
    this.year = year;
    this.price = price;
}


public void description() {
    System.out.println("Description");
}

public String toString() {
    return "make= " + make + ", model= " + model + ", year= " + year + 
  ", price= " + price;
 }

}

EDIT: There cannot be any Getters or Setters as per the assignment requirements, and it must be an ArrayList, not a regular List. When I switch to I get the error "Type mismatch: cannot convert from ArrayList to ArrayList

Here is an image of the classes:

Answer 1

 ArrayList<Object> VehicleList = new ArrayList<>(Arrays.asList(vehicles)); 

VehicleList is declared to contain instances of Object , so the compiler will only let you access methods and fields it knows exist on all instances of Object .

Change it to ArrayList<Vehicle> .

Answer 2

Create an interface IMotor which is used by Vehicle class and Implemented in PassCar and other implementation of vehicle.

IMotor.java

public interface IMotor {
    public Motor getMotor();
}

Motor.java

public class Motor {
    protected String name;
    protected int cylinders;
    protected int bhp;
    protected double displacement;

    public Motor(String name, int cylinders, int bhp, double displacement) {
        this.name = name;
        this.cylinders = cylinders;
        this.bhp = bhp;
        this.displacement = displacement;
    }

    public String toString() {
        return "Motor name= " + name + ", cylinders= " + cylinders + ", bhp=" + bhp + ", displacement= " + displacement;
    }
}

Vehicle.java

public abstract class Vehicle implements IMotor{
    protected String make;
    protected String model;
    protected int year;
    protected double price;

    public Vehicle(String make, String model, int year, double price) {
        this.make = make;
        this.model = model;
        this.year = year;
        this.price = price;
    }

    public String toString() {
        return "make= " + make + ", model= " + model + ", year= " + year + 
      ", price= " + price;
     }
}

PassCar

public class PassCar extends Vehicle{

    protected Motor motor;

    public PassCar(String make, String model, int year, double price, Motor motor) {
        super(make, model, year, price);
        this.motor = motor;
    }

    public Motor getMotor() {
        return motor;
    }
}

Test.java

import java.util.Arrays;
import java.util.List;

public class Test {
    public static void main(String[] args) {
        Motor EcoBoost = new Motor("EcoBoost", 6, 310, 2.3);
        Vehicle v0 = new PassCar("Ford", "Mustang", 2016, 44500.0, EcoBoost);

        List<Vehicle> vehicles = Arrays.asList(v0);
        System.out.println(vehicles.get(0).getMotor());
    }
}

Answer 3

First, mind the naming convention. Variables should be named in camcelCase eg vehicleList instead of VehicleList`

I have an Object ArrayList

I believe you mean declaration of vehicleList looks like ArrayList<Object> vehicleList

Then behavior is expected because compiler only knows that VehicleList.get(i) is going to return you an Object reference. It can be a Vehicle , but it can also be anything else. So it won't allow you to access the motor field, as there is simply no such field in Object .

Change your declaration to something like List<Vehicle> vehicleList

---

However, as mentioned in other answer, it is not a good idea to access the field directly because of various reason. A slightly less evil way is to have getter of motor . (A better way is to provide meaningful behaviors instead of providing access to internal data)

Answer 4

Your problem is that motor is not a member of the Vehicle class, but you are trying to access it through an expression of type Vehicle - namely vehicleList.get(i) . This is forbidden, because the compiler has no way of knowing that every possible kind of Vehicle has a motor . After all, what would happen if you added a Bicycle class?

To make this work, you should remove motor from the Truck and PassCar classes, and add it to the Vehicle class. That way, vehicleList.get(i).motor would actually make sense, since the Vehicle expression would be guaranteed to refer to a Vehicle with a Motor .

It would also be recommended to use a getter for the motor field - that is, have motor as a private field of the Vehicle class, and write a method getMotor() to return it. You could then write vehicleList.get(i).getMotor() to get the Motor object associated with one Vehicle in the list.

Answer 5

Thanks to the help of all of your comments and my Java textbook, I managed to piece it together. Here is how I got it to work:

for (int i = 0; i < vehicleList.size(); i++) {
    String motorInfo = "";
    String info = "";
    System.out.println();
    if (vehicleList.get(i) instanceof PassCar) {
        info = ((PassCar)vehicleList.get(i)).toString();
        **motorInfo = ((PassCar)vehicleList.get(i)).motor.toString();**

    }
    else if(vehicleList.get(i) instanceof Truck) {
        info = ((Truck)vehicleList.get(i)).toString();
        **motorInfo = ((Truck)vehicleList.get(i)).motor.toString();**
    }

Basically I had to use a polymorphic call and check if it was an instance of a PassCar or Truck. And as for the Array and ArrayList used during the Class, I edited them like this:

Vehicle [] vehicles = new Vehicle [3]; 
vehicles[0] = v0;
vehicles[1] = v1;
vehicles[2] = v2;

showVehicle(vehicles);

ArrayList<Vehicle> vehicleList = new ArrayList<Vehicle>(Arrays.asList(vehicles));
System.out.println();
System.out.println("Output from ArrayList in main: ");

Thank you for the help everyone!