for(int i=0;i<n;i++)
{
   sum=sum+arr[i];
}


0 1 2 3 4 5 7 8 9

arr[mid]==mid

every where my element value equal to Mid value;

Who so ever the part you got mismatch in that part only you can expect missing element
==========================================================
class Solution {
    public int missingNumber(int[] arr) {
        Arrays.sort(arr);
        int start=0;
        int end=arr.length-1;

        while(start<=end){
            int mid=start+(end-start)/2;
            if(arr[mid]==mid){
                start=mid+1;
            }else{
                end=mid-1;
            }
        }

        return start;
    }
}
=========================================================
class Main {
    public static void main(String[] args) {
        System.out.println("Try programiz.pro");
        int arr[]={1,2,3,5,6,9,-11,-12};
        int max=Integer.MIN_VALUE;
        for(int i=0;i<arr.length;i++){
            if(arr[i]>max){
                max=arr[i];
            }
        }
        System.out.println(max);
    }
}
==========================================================
class Main {
    public static void main(String[] args) {
        System.out.println("Try programiz.pro");
        int arr[]={1,2,3,5,6,9,9,9,-11,-12};
        int fmax=Integer.MIN_VALUE;
        int smax=Integer.MIN_VALUE;
        for(int i=0;i<arr.length;i++){
            if(arr[i]>fmax){
                smax=fmax;
                fmax=arr[i];
            }else if(arr[i]>smax && arr[i]!=fmax){
                smax=arr[i];
            }
        }
        System.out.println(smax);
    }
}
=========================================================