I'm developing some reports in JaspesoftStudio 6.0.1 and can't find any description to built-in functions that are represented in Expression Editor. So I don't understand which parameters this functions can accept. It would be great if anyone could give me reference to the description of all built-in functions.
I'm not aware of any "official" documentation of this kind.
My advice is to give a look directly to the code.
Here it is the location to check: https://sourceforge.net/p/jasperreports/code/ci/master/tree/jasperreports/demo/samples/functions/src/net/sf/jasperreports/functions/standard/
Look at the following classes that refer to the different functions presented in the expression editor:
As you might see, some functions are working with more "generic" parameter types like Object or Number in order to be more "flexible".
P.S: you could open an enhancement request in the Jaspersoft Studio tracker, asking for showing this info somehow in the expression editor (i.e. detailed tooltip).