code: public Validation getValidation(Object key){
          return validations == null ? null : validations.get(key);
      }
docs: Retrieve the validation object corresponding to the given key.
  If no validation with this key has been added, <code>null</code>
  is returned.
  
  @param key key specified when adding the validation
  @return retrieved validation object, <code>null</code> if no validation with this key was added