Description of problem:

I just wanted to call an add operation for calculator which requires 2 parameters, x and y. http://soaptest.parasoft.com/calculator.wsdl
First I tried to set an array of parameters for web service operation through the process variable and it didn't work https://bugzilla.redhat.com/show_bug.cgi?id=1024946 . So I've changed it to set the variable in script task like this:
kcontext.setVariable("parameters", new Float[]{9.0f, 12.0f});
When I run it, it will throw java.lang.ClassCastException: [Ljava.lang.Float; cannot be cast to java.lang.Float
on the line https://github.com/droolsjbpm/jbpm/blob/master/jbpm-workitems/src/main/java/org/jbpm/process/workitem/webservice/WebServiceWorkItemHandler.java#L92
When I change the set variable command to use just one float, it will fail with java.lang.IndexOutOfBoundsException: Index: 1, Size: 1 which is obvious because it requires 2 parameters.

The problem is that you consider parameter to be something of type Object https://github.com/droolsjbpm/jbpm/blob/master/jbpm-workitems/src/main/java/org/jbpm/process/workitem/webservice/WebServiceWorkItemHandler.java#L81 but an array cannot be used like an object - if you run the example below it will print [Ljava.lang.Float;@39c931fb
instead of 
9.0
12.0

public static void main(String[] args) {
    Object o = new Float[]{9.0f, 12.0f};
    test(o);
}
    
public static void test(Object... array) {
    for (Object o : array) {
        System.out.println(o);
    }
}

so I recommend to change the call to something like:
if (o instanceof Object[]) {
    test((Object[])o);
} else {
    test(o);
}