How would I test for a void message?

public Contact findContact (String name){
     if(name == null){
          throw new IllegalArgumentException("name must not be null");
     }
     Contact contact = null;
     for (int length=this.contacts.size(); length > 0; length--) {
            Contact currContact = this.contacts.get((new Random()).nextInt(length));
            if(currContact.getName().equals(name)) {
                contact = currContact;
                break;
            }
            else {
                this.contacts.remove(currContact);
                this.contacts.add(currContact);
            }
        }
        return contact;
}

I can think of a way for the for loop to be covered in a test case. Could someone tell me or would I need to post more of my code?