adding Objects to a linked list

Question

I need to implement a combination of 2 methods that add wagon objects to a linked list of wagons. But i dont understand how to build it. Every wagon object has an attribute that references the next wagon( null at start). So i guess i have to change that reference when i add a new waggon. But how can i know where and how to add the waggon and how do i get a waggon in between of 2 others... if the train can be added is checked by the public method but how do i actually add the train with the helpermethod? Or do i actually put it in the locomotive class?

i cant put a dangerousgoods=true waggon together with passengerwagons and passengerwagons are always at the start of the train

package edu.hm.cs.swe2.trains;

public class Wagon {

    private int wagonNumber;
    private boolean isPassengerWagon;
    private boolean carriesDangerousGoods;
    public Wagon nextWagon;

    public Wagon(boolean isPassengerWagon, boolean carriesDangerousGoods) {
        if (isPassengerWagon == true && carriesDangerousGoods == true) {
            this.carriesDangerousGoods = false;
            System.out.println("passengerwagon cant handle dangerous goods");
        } else
            this.isPassengerWagon = isPassengerWagon;
        this.carriesDangerousGoods = carriesDangerousGoods;
        this.setWagonNumber(0);
        this.setNextWagon(null);

    }

    public boolean isPassengerWagon() {
        return isPassengerWagon;
    }

    public boolean isCarriesDangerousGoods() {
        return carriesDangerousGoods;
    }

    public void setWagonNumber(int wagonNumber) {
        this.wagonNumber = wagonNumber;
    }

    public void setNextWagon(Wagon nextWagon) {
        this.nextWagon = nextWagon;
    }

    public void printWagon(int level) {
        if (level == 3) {
            if (nextWagon == null)
                System.out.println("**********");
        } else if (level == 2) {
            if (this.isPassengerWagon == false && this.carriesDangerousGoods == true) {
                System.out.println("* f" + this.wagonNumber + " dg *");
            } else if (this.isPassengerWagon == false && this.carriesDangerousGoods == false) {
                System.out.println("* f" + this.wagonNumber + "    *");
            } else if (this.isPassengerWagon == true) {
                System.out.println("* p" + this.wagonNumber + "    *");
            }
        } else if (level == 1) {
            System.out.println("**********->");
        } else if (level == 0) {
            System.out.println(" ***  *** ");
        }

    }

    public boolean trainCarriesDangerousGoods() {
        if (this.carriesDangerousGoods == true) {
            return true;
        } else if (this.nextWagon == null) {
            return false;
        } else {
            return nextWagon.trainCarriesDangerousGoods();
        }
    }

    public boolean trainHasPassengerWagons() {
        if (this.isPassengerWagon == true) {
            return true;
        } else if (this.nextWagon == null) {
            return false;
        } else {
            return nextWagon.trainHasPassengerWagons();
        }
    }

    public Wagon addWagon(Wagon newWagon) {
        if (this.trainCarriesDangerousGoods() == true && newWagon.isPassengerWagon() == true) {
            System.out.println("cant add Passenger wagon to a train with dangerous goods");
        }
        if (newWagon.carriesDangerousGoods == true && this.trainHasPassengerWagons() == true) {
            System.out.println("cant add dangerous goods to a train with passengerwaggons");
        } else {
            addWagon(this.wagonNumber, newWagon);
        }
        return newWagon;

    }

    private void addWagon(int wagonNumber, Wagon newWagon) {

    }
}

package edu.hm.cs.swe2.trains;

public class Locomotive {

    public int locomotiveCount;
    public int steamLocomotiveCount;
    MotivePower Propulsion;

    public Locomotive(MotivePower Propulsion) {
        this.Propulsion = Propulsion;
        locomotiveCount++;
    }
}

WJS · Accepted Answer · 2020-06-03 17:33:41Z

1

Your Wagon should not point to the next Wagon. Create an inner class Node in the LinkedList<T> class that has a next pointer as well as others. The Node class should be something like the following:

class Node  {
   T object;

   Node next;
   Node previous; // optional depending on linked list type.
}

The LinkedList would be of type Wagon.
E.g. LinkedList<Wagon> list = new LinkedList<>(); and the Wagon instance would be placed in the object field of the node.

edited Jun 3, 2020 at 17:33

answered Jun 3, 2020 at 16:16

WJS

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2 Comments

nikva Over a year ago

and the linked list is implemented in the Locomotive constructor? (i need a new list for every new train) and then i add the append method in the Node class and use it in the private void class of wagon.java?

WJS Over a year ago

For a given linkedList you keep adding nodes with the Wagon object and then linking them via the next reference This means you need to keep a reference to the first node so you can iterate thru the list.

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