Error in taking input from file using scanner class in java

Question

I want to take input from a file using the scanner class in Java. I want to read two coordinates of different cities from the file and then store them in an ArrayList of type City objects.

The input file format is as follows:

NAME : att48
COMMENT : 48 capitals of the US (Padberg/Rinaldi)
TYPE : TSP
DIMENSION : 5
EDGE_WEIGHT_TYPE : ATT
NODE_COORD_SECTION
1 6734 1453
2 2233 10
3 5530 1424
4 401 841
5 3082 1644

My sample code fragment is as follows: TourManager is a class containing an ArrayList of City objects. I haven't shown it here. City class contains every details (x,y coordinates) of a city.

try 
    {
        Scanner in = new Scanner(new File("att48.tsp"));
        String line = "";
        int n;
        //three comment lines
        in.nextLine();
        in.nextLine();
        in.nextLine();
        //get n
        line = in.nextLine();
        line = line.substring(11).trim();
        n = Integer.parseInt(line);
                    City[] city= new City[n];
       for (int i = 0; i < n; i++) {
                    city[i].x=0;
                    city[i].y=0;
        }

        //two comment lines
        in.nextLine();
        in.nextLine();

        for (int i = 0; i < n; i++)
        {
            in.nextInt();
            city[i].x =  in.nextInt();
            city[i].y =  in.nextInt();
            TourManager.addCity(city[i]);
        }
    } 
    catch (Exception e) 
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

But I am getting a NullPointerException in the line.

  city[i].x =  in.nextInt();

Although I have initialized it by 0 previously, the code throws the NullPointerException .

For clarity, City class is as follows:

public class City {
    int x;
    int y;

  // Constructs a city at chosen x, y location
  public City(int x, int y){
    this.x = x;
    this.y = y;
  }
}

Is there a problem in the code?

Answer 1

You are getting error because after doing :

City[] city= new City[n];

You have not allocated memory to individual city[i] elements.

You have to do something like this, but for this you need to add a default constructor to your city class as you are assigning x and y afterwards:

for (i=0;i <n ;i++){
   city[i]= new City();     
}

Add the following constructor in your City class:

public City(){

}

or I would suggest you to revise your City class:

public class City {
    private int x;
    private int y;

    // Constructs a city at chosen x, y location
    public City(int x, int y) {
        this.setX(x);
        this.setY(y);
    }

    public City() {

    }

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

}