Java NullPointerException - Short Program

Question

I'm new in programming in Java and I do not understand what's going on in my code.

It tells me:

Exception in thread "main" java.lang.NullPointerException
    at Main.Country.addMine(Country.java:37)
    at Main.Main.main(Main.java:21)
Java Result: 1

My main.java is simple:

    Continent Europe = new Continent("Europe");
    Country asd = new Country("asd", Europe);
    Mine mine = new Mine(100,100,100,100);
    System.out.println(mine == null);
    asd.addMine(mine); //dies here

this is addMine method:

public void addMine(Mine mine) {
     System.out.println(mine == null);
     this.mines.add(mine); //dies here
     this.iron += mine.iron;
     this.gold += mine.gold;
     this.stone += mine.stone;
     this.wood += mine.wood;
     System.out.println("Mine has been successfully added to the country with the given values."
);

and Mine.java is:

public class Mine implements Building { //Building is an empty interface :)
    protected int iron;
    protected int gold;
    protected int stone;
    protected int wood;
    public Mine(int iron, int gold, int stone, int wood) {
        this.iron += iron;
        this.gold += gold;
        this.stone += stone;
        this.wood += wood;
    }
}

As You can see I wrote 2 println-s and both of them were false, so the object exists! I don't understand why it shows NullPointerException :(

Answer 1

If this is failing:

this.mines.add(mine); //dies here

... then I suspect mines is a null reference. You haven't shown any declaration for it or initialization - but that should be your first port of call. Chances are it's just a case of changing:

private List<Mine> mines;

to

private List<Mine> mines = new ArrayList<Mine>();

or something similar.

Answer 2

Yes, mine could be not null but what about mines? Which I guess it's a ArrayList<Mine> or something like that, did you inizialize it as mines = new ArrayList<Mine>()? (or whichever collection it is)