code: @Override
      public List<Type> typeParams() {
          if (null == typeParams) {
              if (type instanceof ParameterizedType) {
                  ParameterizedType ptype = $.cast(type);
                  Type[] ta = ptype.getActualTypeArguments();
                  typeParams = C.listOf(ta);
              } else {
                  typeParams = C.list();
              }
          }
          return typeParams;
      }
docs: Returns a list of type parameters if the
  {@link #type()} of the bean is instance of
  {@link ParameterizedType}.
  
  For example the bean spec of a field declared
  as
  
  ```java
  private Map<String, Integer> scores;
  ```
  
  The `typeParams()` method will return a list
  of `String` and `Integer`
  
  @return type parameter list