It's OK to do it if you can guarantee it will work. If there's any chance that intf is pointing to an instance of another class, you should check first (using instanceof).
However, while casting is sometimes necessary, it can be an indication that the design is flawed. In the particular example you've given, if you want to treat the object as an Example, then assign it to an Example reference variable in the first place. In a more realistic example, there may well be a better way but it's impossible to say what that would be without knowing more about it.