code: public void setElementType (Class type, String fieldName, Class elementType) {
  		ObjectMap<String, FieldMetadata> fields = getFields(type);
  		FieldMetadata metadata = fields.get(fieldName);
  		if (metadata == null) throw new JsonException("Field not found: " + fieldName + " (" + type.getName() + ")");
  		metadata.elementType = elementType;
  	}
docs: Sets the type of elements in a collection. When the element type is known, the class for each element in the collection
  does not need to be written unless different from the element type.