Question

I’m building a railway simulator, with the classes simulator, station, train, route and passenger. I’m having a problem with passing a route object as a parameter to a train object – it keeps coming up as null. I’ve included my code as the best way to explain my problem.

import java.util.ArrayList;

public class Simulator {
int numStations;    
Route route1;
ArrayList<Station> stations;
ArrayList<Train> trains;
Route[] routes;

public static void main(String[] args) {

    //set up network
    Simulator sim = new Simulator();                
    ArrayList<Station> stations = new ArrayList<Station>();
    stations = sim.generateStations();
    Route[] routes;
    routes = sim.generateRoutes(stations);
    System.out.println("Route array " + routes);
    ArrayList<Train> trains = new ArrayList<Train>();
    trains = sim.generateTrains(routes);

    //start simulator
    sim.generatePassengers(stations);
    Train currentTrain = trains.get(0);
    System.out.println("current train " + currentTrain);
    //route null here
    System.out.println("Route of current train " + currentTrain.route);


}


public Route[] generateRoutes(ArrayList<Station> stations){
    //initialise routes between stations

    Station[] stationList1 = {stations.get(0), stations.get(1), stations.get(2), stations.get(3), stations.get(4), stations.get(5)};
    int[] stationDist1 = {200,100,200,300,200,300};
    Route route1 = new Route(true, stationList1, stationDist1);
    System.out.println("route1 " + route1);

    //make list of all routes
    Route[] routeList;  
    routeList = new Route[1];
    routeList[0] = route1;

    return routeList;
}

public ArrayList<Train> generateTrains(Route[] routes){
    //initialise trains
    ArrayList<Train> trainList = new ArrayList<Train>();
    trainList.add(new Train(routes[0], 100, 0, true, 5));
    trainList.add(new Train(routes[0], 100, 4, false, 10));

    System.out.println("first train in list " + trainList.get(0));
    System.out.println("first route in array " + routes[0]);
    //route is null here
    System.out.println("route of first train " + (trainList.get(0)).route);

    return trainList;
}
}

Here is my output:

route1 Route@addbf1

Route array [LRoute;@42e816

Route1 in array Route@addbf1

first train in list Train@190d11

first route in array Route@addbf1

route of first train null

current train Train@190d11

Route of current train null

Can anyone explain where I’m going wrong?

Edit:
Train class declaration:

public class Train {
Route route;
int capacity;
int recentLoc;
boolean forwards;
boolean atStation;
int speed;
Train currentTrain;
int timeAtStation;

//train constructor
public Train(Route r, int c, int i, boolean f, int s){
    r = route;
    c = capacity;
    i = recentLoc;
    f = forwards;
    s = speed;
    atStation = true;
    timeAtStation = 0;
}

Answer 1

You’re not assigning values to your instance variables. Default value for object is null. Change “r = route” into “route = r” inside your Train constructor (the same for all the other fields).

public class Train {
Route route;
int capacity;
int recentLoc;
boolean forwards;
boolean atStation;
int speed;
Train currentTrain;
int timeAtStation;

//train constructor
public Train(Route r, int c, int i, boolean f, int s){
    route = r;
    capacity = c;
    recentLoc = i;
    forwards = f;
    speed = s;
    atStation = true;
    timeAtStation = 0;
}