Creating a string attribute in Weka Java API

I’m trying to create a new string Attribute using Weka’s Java API…

Reading through the API javadocs, it appears that the way to do so is to use this constructor:

Attribute

public Attribute(java.lang.String attributeName,
                 FastVector attributeValues)

    Constructor for nominal attributes and string attributes. If a null vector of attribute values is passed to the method, the attribute is assumed to be a string.

    Parameters:
        attributeName - the name for the attribute
        attributeValues - a vector of strings denoting the attribute values. Null if the attribute is a string attribute.

but I’m stuck as to what I should pass into the attributeValues parameter…

when I put in null, Java complains about protected objects
when I put in Null, it’s syntax error
when I put in new FastVector(), it becomes a nominal attribute that is empty rather than a string attribute…
when I create a new object:

FastVector fv = new FastVector();
fv.addElement(null);

and then pass fv into the argument, it returns a null pointer exception…

what exactly should I put into the attributeValues argument so that it becomes a string attribute?

Answer

You have to cast the null to FastVector. Otherwise more methods would apply to the method signature:

    FastVector attributes = new FastVector();
    attributes.addElement(new Attribute("attr", (FastVector) null));

Here is a good resource for creating Instances on the fly: https://waikato.github.io/weka-wiki/formats_and_processing/creating_arff_file/