Sorry - it must be:

int maxtest = (int) Math.sqrt(num)+1;  
return IntStream.range(2, maxtest).noneMatch(divider -> (num % divider) == 0); 

The endvalue of then IntStrem can be the squareroot of num. If no lower divider is found, there is no at all!

int maxtest = (int) Math.sqrt(num);  
return IntStream.range(2, maxtest).noneMatch(divider -> (num % divider) == 0); 

Thats much faster!