code: public void setOutputPackagePrefix(String packagePrefix) {
          if (packagePrefix == null || packagePrefix.length() == 0) {
              this.packagePrefix = null;
              return;
          }
          Matcher matcher = PACKAGE_NAME_PATTERN.matcher(packagePrefix);
          if (matcher.matches()) {
              this.packagePrefix = packagePrefix;
          } else {
              throw (new IllegalArgumentException("The name " + packagePrefix
                      + " is not a valid java package prefix name"));
          }
  
      }
docs: Check that package prefix name provided is valid.
  <p/>
  Null is acceptable and interpreted as no prefix.
  
  @param packagePrefix the generated java classes package name