Sliding_window_demo.java

public static void main(String[] args) 
    { 
        FastReader s=new FastReader(); 
        int n = s.nextInt();
        int k = s.nextInt();
        int a[] = new int[n];
        for(int i=0; i<n; i++)a[i] = s.nextInt();
        
        int maximum_sum = Integer.MIN_VALUE;
        
        //Making two pointer (left end to right end of the window)
        int left=0, right=0, curr_sum=0;
        while(right < n){
            //Adding the values of array till the window size is reached
            curr_sum += a[right];

            //if window size is small increase the right pointer/end the window by 1
            if(right-left+1 < k)
                right++; 

            //When window size is reached calculate the max value and update 
            else if(right-left+1 == k){
                maximum_sum = Math.max(curr_sum, maximum_sum);

                //Moving the left end of window forward
                curr_sum -= a[left];
                left++;
                right++;
            }
        }
        System.out.println(maximum_sum);
            
    }