Comparing to array list through there given index but its returning null exception at file initialization - arraylist

Question

package adc;
import java.io.*;
import java.math.*;
import java.security.*;
import java.text.*;
import java.util.*;
import java.util.concurrent.*;
import java.util.regex.*;
class Result {
/*
* Complete the 'sockMerchant' function below.
*
* The function is expected to return an INTEGER.
* The function accepts following parameters:
* 1. INTEGER n
* 2. INTEGER_ARRAY ar
*/
public static int sockMerchant(int n, List<Integer> ar) {
// Write your code here
int count=0;
for(int i=0;i<n;i++)
{
for(int j=i+1;j<=n;j++){
if(ar.get(i)==ar.get(j)){
count++;
}
}
}
return(count);
}
}
public class Solution {
public static void main(String[] args) throws IOException {
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
BufferedWriter bufferedWriter = new BufferedWriter(new FileWriter(System.getenv("OUTPUT_PATH"))); ' BufferedWriter(newFileWriter(System.getenv("F:\\")));'
int n = Integer.parseInt(bufferedReader.readLine().trim());
String[] arTemp = bufferedReader.readLine().replaceAll("\\s+$", "").split(" ");
List<Integer> ar = new ArrayList<>();
for (int i = 0; i < n; i++) {
int arItem = Integer.parseInt(arTemp[i]);
ar.add(arItem);
}
int result = Result.sockMerchant(n, ar);
bufferedWriter.write(String.valueOf(result));
bufferedWriter.newLine();
bufferedReader.close();
bufferedWriter.close();
}
}
Working of code
here inputs are n("number of integer arraylist have")=9
and ar input of arraylist [10, 20, 20, 10, 10, 30, 50, 10, 20]
** In this ar arraylist we have to find the even pair of number and return the number of pair .**
In this example the output is 3.