error: bad operand types for binary operator '+'
<p>Im getting error in the following code which says "error: bad operand types for binary operator '+'. The error appears at the line <code>sum+=arr[i]</code>
    This Code is to calculate diagonal difference of an integer matrix</p>

<pre><code>import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int a[][] = new int[n][n];
        for(int a_i=0; a_i &lt; n; a_i++){
            for(int a_j=0; a_j &lt; n; a_j++){
                a[a_i][a_j] = in.nextInt();
            }
        }
        Solution solution= new Solution();
        solution.diagonalDifference(a,n);
    }
    public void diagonalDifference(int arr[][],int n){
        int sum=0,sum1=0,sum2=0;
        for(int i=0; i &lt; n;i++){
            for(int j=0; j &lt; n;j++){
                if(i==j){
                    sum1+=arr[i];//calculating sum of primary diagonal
                }
            }
        }
        for(int i=0; i &lt; n;i++){
            for(int j=n; j &gt;0;j--){
                 sum2+=arr[i];//calculating sum of secondary diagonal
            }
        }
        sum=Math.abs(sum1-sum2);
        System.out.println(sum);
    }
}
</code></pre>

<p>Error:-</p>

<pre><code>Solution.java:26: error: bad operand types for binary operator '+'
                    sum+=arr[i];//calculating sum of primary diagonal
                       ^
  first type:  int
  second type: int[]
Solution.java:32: error: bad operand types for binary operator '+'
                 sum+=arr[i];//calculating sum of secondary diagonal
                    ^
  first type:  int
  second type: int[]
2 errors
</code></pre>