Is there a downside to doing this:
import java.util.*;
rather than importing a specific part:
import java.util.Stack;
Does it take more time to build for example?
You wouldn't feel the difference, but for the sake of clear and readable code you should import only the classes you actially need.
No. Some people consider .* as a bad practice since it is less readable and does not allow you to understand the class' relationships with other types just by looking at imports.