For numerics, the Java format is used. e.g. Integer.parseInt() or Double,parseDouble() calls.
Boolean maps to the strings "true" and "false".
Dates convert in and out of a date format of the form dd-MMM-yyyy. (e.g. 10-Oct-1970)
Lists of strings are mapped to simple strings of the form [a, b, c]. Note the brackets and spaces. This is the standard toString format for Java Lists.
Arrays of Booleans, Integers, Strings, and Doubles map to the form [x, y, z].
The Hash Widget sends HTML as a string to the Slim Fixture. Java_Slim and Ruby_Slim will both convert this HTML string into a Map<String,String> (or a hash in the case of Ruby).