FitNesse. UserGuide.
DataTypesInFixtures [add child]
You can use all the standard Java data types in RowFixture and ColumnFixture. These types are automatically translated in and out of cell contents for you. They include:

Managing your own types with Fit.

Any user defined type that implements toString and parse will be managed correctly by RowFixture and ColumnFixture. The signatures of these methods are:
String toString();
static Object parse(String s);