You are in the method, the variables are not global.
You take the first value of the array "numbers". But this is only possible, when the array is not null and have values (length > 0). Otherwise the access to the array will force an exception.
So you have to check for null and for a length > 0 before! ;-)

You should take the first two lines (int max =..., int min = ...) with access to numbers after the check for null and length.

And just a hint but no error: Why you go twice through the numbers? You could add the actual number to "sum" in the first for. ;-)