Java: Accessing an ArrayList inside an ArrayList?

Question

In FleetTUI.java I have a list of Fleets (Each fleet will hold its own list of Trucks).

private static ArrayList<Fleet> fleetCollection;

In Fleet.java, I have a list of Truck objects.

ArrayList<Truck> fleetList;

which gets initialized in it's constructor.

    public Fleet(String businessName){
    this.businessName = businessName;
    this.fleetList = new ArrayList<Truck>();
}

So every time I make a new fleet, I am also making a list of Trucks along with it.

In FleetTUI.java, I have a method that adds a truck to a Fleet, made prior:

    public static void addTruck(){
    printFleets();
    System.out.println("Please enter the fleet number where this truck will be added:");
    inputText = scan.nextLine();
    int inputFleetId = Integer.parseInt(inputText);
    System.out.println("Please enter the truck's horn sound:");
    inputText = scan.nextLine();
    String inputHorn = inputText;
    System.out.println("Please enter the truck's fuel capactity:");
    inputText = scan.nextLine();
    int inputFuelCapacity = Integer.parseInt(inputText);
    System.out.println("Please enter the amount of gas remaining in the tank:");
    double inputGasRemaining = Double.parseDouble(scan.nextLine());
    **Fleet.fleetList**.add(inputFleetId, new Truck(inputHorn, inputFuelCapacity, inputGasRemaining));
    System.out.println("--- A truck that goes " + inputHorn + " with " + inputGasRemaining + " gallon(s) in a " + inputFuelCapacity + " gallon tank has been added to " + **fleetCollection.getBusinessName()** + "'s fleet ---" );
    System.out.println("");
}

I have bolded where I am getting errors. The errors are:

Cannot make a static reference to the non-static field Fleet.fleetList

Cannot make a static reference to the non-static method getBusinessName() from the type Fleet

I don't know any other way to access the list of trucks from inside the fleet I created with the method:

    public static void createFleet(){
    System.out.println("");
    System.out.println("Please enter the name of the fleet.");
    inputText = scan.nextLine();
    
    fleetCollection.add(new Fleet(inputText));
    printFleets();
    System.out.println("");
    System.out.println("--- Fleet: " + inputText + " added ---");
    System.out.println("");
    
}

So I guess really, my question is: How do I correctly add a new Truck into the list of Trucks, which is inside the Fleet I just made using createFleet(), which is housed in it's own collection of fleets?

Update: Thanks :) you guys really helped me! once again!

This looks like an initial poor design to me. static should be used with care and when relevant. My guess is that you should get rid of all your static keywords and instantiate objects properly. If tomorrown one ask you to handle several fleetCollections, your whole code has to refactored. — Guillaume Polet
– Guillaume Polet, Commented Feb 28, 2012 at 7:31
To avoid confusion rename your fleetList to truckList, that's what it represents. — Kuldeep Jain
– Kuldeep Jain, Commented Feb 28, 2012 at 7:44
@user1176922 please don't delete the question details with a "thank you" message. The details are important so that others understand the issue and the answers. — jackJoe
– jackJoe, Commented Feb 29, 2012 at 18:02

npinti · Accepted Answer · 2012-02-28 07:54:23Z

2

Your problem is that you are creating static methods within which you are then trying to access a non-static variable, in this case this being fleetlist. This will cause the error you are showing.

Since you are using a constructor and everything, you do not need to call the method static. Calling it static will allow the method to be called without the object needing to be constructed, which from what I am seeing is not something you are after.

So in short, make your methods non-static by removing the static keyword from the method declaration and access your methods such as addTruck() and createFleet() through the use of the initialized Fleet object.

On the other hand, you can declare your fleetlist to be static, like so:

private static ArrayList<Truck> fleetList;

EDIT: Seeing your comment, this is how you can do it:

In Java, it is considered good practice to encapsulate your object variables so that you can better control the access to such variable, so you can do something like so in your Fleet Class:

private List<Truck> fleetList;

public Fleet(String businessName){
    this.businessName = businessName;
    this.fleetList = new ArrayList<Truck>();
}

public List<Truck> getFleetList()
{
    return this.fleetList;
}

In your FleetUI Class you can do something like so:

private ArrayList<Fleet> fleetCollection;    
...   
for (Fleet fleet : this.fleetCollection)
{
    for (Truck truck : fleet.getFleetList())
    {
         //Do something with your truck here
    }
}

EDIT 2: To add a truck to your array, you can do it in 2 ways. Let us assume that the object fleet is the Fleet to which you want to add your truck. In your FleetUI class you can do like so:

fleet.getFleetList().add(new Truck(...))

The other (and in my opinion, better way) is to have an addTruck method in your fleet object like so:

public void addTruck(Truck truck)
{
    this.fleetList.add(truck);
}

edited Feb 28, 2012 at 7:54

answered Feb 28, 2012 at 7:26

npinti

52.2k5 gold badges74 silver badges98 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

npinti Over a year ago

@user1176922: I have modified my answer. Hope it makes things more clear to understand.

npinti Over a year ago

I have further improved my answer. Just as an FYI, the helper method encapsulates the field of the object. This encapsulation is one of the corner stones of OOP, and I would recommend you would learn to use it since it can be quite useful.

npinti Over a year ago

@user1176922: Yes, if you want you can do something like so: for(Fleet fleet : fleetCollection){if (fleet.getName().equals("myFleet"){fleet.addTruck(new Truck(...));break;}}. In this case I assumed that the Fleet object had a name and you wanted to find a fleet with a specific name. You can obviously swap in any other mechanism you want.

aviad · Accepted Answer · 2012-02-28 07:23:59Z

1

You are calling

Fleet.fleetList.add()

method from the static method addTruck.
It is bad practice to reference a member like this:

Fleet.fleetList.add

you should introduce some accessor method to add a truck to a flletList in your Fleet class

answered Feb 28, 2012 at 7:23

aviad

8,2789 gold badges55 silver badges100 bronze badges

8 Comments

aviad Over a year ago

as I said the problem is that the list is not static and you call its add method from static method addTruck.

user1176922 Over a year ago

I changed all my methods to non-static.

aviad Over a year ago

did it help the situation? :)

aviad Over a year ago

that is new one but the idea is the same you are calling non-static run from some static method in FleetUI

aviad Over a year ago

first create an instance of the class whose method you going to call in static code.

|

Collectives™ on Stack Overflow

Java: Accessing an ArrayList inside an ArrayList?

2 Answers 2

3 Comments

8 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

8 Comments

Your Answer

Sign up or log in

Post as a guest

Related