Htykuut

Why am I getting null pointer while running this program , at line 13 : dragons[i].strength = Integer.parseInt(input[0]);
??

public final class practice {

public static void main(String args) throws IOException {

    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

    String input = br.readLine().split(" ");

    int n = Integer.parseInt(input[1]);

    int s = Integer.parseInt(input[0]);

    class dragon{

        int strength; int points;

    };

    dragon dragons = new dragon[n];

    for(int i = 0 ; i < n ; i = i + 1) {

        input = br.readLine().split(" ");

        dragons[i].strength = Integer.parseInt(input[0]);

        dragons[i].points = Integer.parseInt(input[1]);

    }



    Arrays.sort(dragons, new Comparator<dragon>() {

        @Override

        public int compare(dragon o1, dragon o2) {

            if(o1.strength < o2.strength)

                return -1;

            else if(o1.strength == o2.strength) {

                if(o1.points < o2.points)

                    return -1;

                return 1;

            }

            return 1;

        }

    });



    for(int i = 0 ; i < n ; i = i + 1) {

        System.out.print(dragons[i].strength + " " + dragons[i].points);

    }

}

}

Sample input :
2 2
99 1
100 1

dragon dragons = new dragon[n];

Keep in mind that this line does not initalize any objects of the type dragon. It only initalizes an array of the size n and type dragon.

To fix this you should create an object of type dragon in your for loop.

for(int i = 0 ; i < n ; i = i + 1) {

    input = br.readLine().split(" ");



    dragons[i] = new dragon(); //<-- new line

    dragons[i].strength = Integer.parseInt(input[0]);

    dragons[i].points = Integer.parseInt(input[1]);

}

Also keep in mind that by convention your class names should start with an capital letter (i.e. Dragon).

The array you have created here dragon dragons = new dragon[n]; has null values.

You haven't added any dragon Objects to that one. So when you try to access dragons[i].strength you are trying to access a attribute of a null value. So it will give you a NullPointerException

Maybe you have to write

dragon dragons = new dragon[n];

instead of

dragon dragons = new dragon[n];