Null Pointer Exception with arrays and classes - Java

Question

Ok, so I have a global array as part of the Class "Temp". I want to edit that array from anywhere in the program. That is working fine. However, when I try to use those values to set values in my "Cords" Class, I get a Null Exception Error. I have commented it in my code. Any idea why?

import java.util.Scanner;


public class Solution {

    public static void main(String[] args) {


        Scanner inp = new Scanner(System.in);

        int n = 0;

        for(int k = 0; k<4; k++){

            Temp.tempValues[k] = 0;

        }

        boolean checkNums = false;

        String coords = "";


        while(n<1 || n>2000){

            System.out.println("Enter number of lines");

            n = inp.nextInt();


        }

        Cords[] lines = new Cords[n];

        int proceed = 0;

        inp.nextLine();

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

            proceed = 0;

            checkNums = false;

            while((proceed != 3) || (checkNums == false)){

                System.out.println("Enter line coordinates. "+(i+1));

                coords = inp.nextLine();

                proceed = checkSpaces(coords);

                checkNums = rightNums(coords);

            }

            lines[i] = new Cords();

            lines[i].setValues(Temp.tempValues[0], Temp.tempValues[1], Temp.tempValues[2], Temp.tempValues[3]);


        }

    }

    public static int checkSpaces(String sent){

        int spaces = 0;

        for(int y = 0; y< sent.length(); y++){

            if(sent.charAt(y)==' '){

                spaces++;

            }

        }

        return spaces;

    }

    public static boolean rightNums(String sent){


        int z = 0;

        int l = 0;

        String num = "";

        while(z<sent.length()){

                while((z<sent.length())&&(sent.charAt(z) != ' ')){

                    num += sent.charAt(z);

                    z++;

                }

                if(Integer.parseInt(num) < 0 || Integer.parseInt(num) >=10000){

                    return false;

                }else{

                    Temp.tempValues[l] = Integer.parseInt(num);

                    num = "";
                    z++;

                    l++;
                }

            }

        return true;

    }

    public class Cords{

        int x1 = 0;
        int x2 = 0;
        int y1 = 0;
        int y2 = 0;

        public void setValues(int xx1, int yy1, int xx2, int yy2){

            x1 = xx1;
            y1 = yy1;
            x2 = xx2;
            y2 = yy2;

        }

}

    public static class Temp{

        static int[] tempValues = new int[4];

    }

}

possible duplicate of What is a Null Pointer Exception, and how do I fix it? — ToYonos
– ToYonos, Commented Nov 11, 2014 at 12:06
It is needed, as I will be posting code online, and can only be read from the one file — Greg Peckory
– Greg Peckory, Commented Nov 11, 2014 at 12:22
Try to remove "public" from that cord class, and then try your example — Mohammad Ashfaq
– Mohammad Ashfaq, Commented Nov 11, 2014 at 12:25

stevecross · Accepted Answer · 2014-11-11 12:21:04Z

4

Your lines array does not contain any object. It just contains nulls and you can't invoke methods on null-values.

Since you are iterating over your lines array, initialize the current element before you invoke methods on it:

for (int i = 0; i < lines.length; i++) {
    lines[i] = new Cords();

    lines[i].setValues(...);
}

You also have to make the Cords class static.

edited Nov 11, 2014 at 12:21

answered Nov 11, 2014 at 12:04

stevecross

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7 Comments

Greg Peckory Over a year ago

Sorry, i is in a for loop, my mistake. Forgot to type it out

Greg Peckory Over a year ago

at lines[i] = new Cords() I get error: No enclosing instance of type solution is accessible. Must qualify the allocation with enclosing instance of type solution. (Eg. x.new A()where x is an instance of Solution

stevecross Over a year ago

Please show all of your code. Otherwise we can't help.

Greg Peckory Over a year ago

A lot of it is probably distracting

Greg Peckory Over a year ago

If Cords class is static, can I still make objects?

|

Mohammad Ashfaq · Accepted Answer · 2014-11-11 12:08:42Z

0

public static void main(String[] args){

    Cords[] lines = new Cords[5];

    for(int k = 0; k<4; k++){
        Temp.tempValues[k] = 0;
    }

    for(int i = 0; i < lines.length; i++){
        lines[i] = new Cords();
    lines[i].setValues(Temp.tempValues[0], Temp.tempValues[1], Temp.tempValues[2], Temp.tempValues[3]);
    }
}

answered Nov 11, 2014 at 12:08

Mohammad Ashfaq

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3 Comments

Greg Peckory Over a year ago

No enclosing instance of type solution is accessible. Must qualify the allocation with enclosing instance of type solution. (Eg. x.new A()where x is an instance of Solution

Greg Peckory Over a year ago

at lines[i] = new Cords()

Mohammad Ashfaq Over a year ago

@JohnDoe what do you want to say?

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