Cannot invoke "City.getX()" because "city[i]" is null

Question

Hey Im working on a homework and i got stuck, i need to return the name of the two citys with the least amount of distance between them, the citys must be in order that the second one is after the first one (first i, second i+1); Thanks.

public class Maincity {
    public static void main(String[] args) {
        City [] cityArr = new City[10];
        String[] nameArr = new String[] {"Hadera","Beer Sheva","Haifa", "Ashdod", "Eilat", "Jerusalem", "Ashkelon", "Tel Aviv", "Hertzila", "Netanya"};

        for (int i=0;i<cityArr.length;i++) {
            int rRandom = (int)(Math.random() * 100000) + 10000;
            int xRandom = (int)(Math.random() * 10000) + 1000;
            int yRandom = (int)(Math.random() * 10000) + 1000;
            City ir = new City(nameArr[i], rRandom, xRandom, yRandom);
            System.out.println(ir);
        }

        System.out.println(Distance(cityArr));
    }

    public static int Distance(City[] city) {
        int min = 100000000;

        for (int i = 0; i < city.length; i++) {
            int newX = city[i].getX() - city[i + 1].getX();
            int newY = city[i].getY() - city[i + 1].getY();
            newX = (int) Math.pow(newX, 2);
            newY = (int) Math.pow(newY, 2);
            int nDistance = (int) (Math.sqrt(newX) + Math.sqrt(newY));

            if (nDistance < min) {
                min = nDistance;
            }
        }

        return min;
    }
}

You should fill your `cityArr` with the `City` objects you created. — maloomeister, Nov 22 '21 at 07:16
Does this answer your question? [What is a NullPointerException, and how do I fix it?](https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — maloomeister, Nov 22 '21 at 07:17
After doing `City ir = new City(nameArr[i],rRandom, xRandom, yRandom);` put this Object into the array `cityArr[i] = ir;` — Scary Wombat, Nov 22 '21 at 07:17
@Himeshgoswami No, the `-1` there is incorrect -- the OP has it right. For example, if the array had length 3, you would want `i` to take on 3 values: 0, 1, and 2. — yshavit, Nov 22 '21 at 07:23

score 0 · Answer 1 · answered Nov 22 '21 at 07:34

There were 2 problems:

You didn't put the city into array.
The index calculation (i+1) throwed ArrayIndexOutOfBoundsException

    public static void main(String[] args) {
        String[] nameArr=new String[] {"Hadera","Beer Sheva","Haifa", "Ashdod", "Eilat", "Jerusalem", "Ashkelon", "Tel Aviv", "Hertzila", "Netanya"};
        City [] cityArr= new City[nameArr.length];
        for(int i=0;i<cityArr.length;i++) {
            int rRandom = (int)(Math.random()*100000)+10000;
            int xRandom = (int)(Math.random()*10000)+1000;
            int yRandom = (int)(Math.random()*10000)+1000;
            City ir = new City(nameArr[i],rRandom, xRandom,  yRandom);
            cityArr[i] = ir;
            System.out.println(ir);
        }
        System.out.println(Distance(cityArr));
    }
    public static int Distance(City[] city) {
        int min =100000000;
        for(int i=0;i<city.length;i++) {
            int nextCityIndex = (i+1) % city.length;
            int newX =city[i].getX() - city[nextCityIndex].getX();
            int newY =city[i].getY() - city[nextCityIndex].getY();
            newX = (int) Math.pow(newX, 2);
            newY = (int) Math.pow(newY, 2);
            int nDistance = (int) (Math.sqrt(newX)+Math.sqrt(newY));
            if(nDistance<min) {
                min=nDistance;
            }

        }
        return min;
    }

Cannot invoke "City.getX()" because "city[i]" is null

1 Answers1