And here is my response to my own question. I have been struggling with this for days, and right after I decide to post, the solution strikes me.... It is really, really, really simple. Many apologies for wasting everybody's time, but here is the correct code. I have added comments to indicate the changes that made all the difference.... This might help people out, in case they want to try to do what I have done, since the online SWI-Prolog doc is wrong (!!!!).

<code begins>
import jpl.fli.*;
import jpl.*;

Atom x = new Atom("test.pl" );

Term arg[] = { x } ;

Query q1 = new Query( "consult", arg );
println(q1);
println( "consult " + (q1.query() ));


Query q2 =
   new Query(
       "child_of",
       new Term[] {new Atom("joe"),new Atom("ralf")}
   );
println(
   "child_of(joe,ralf) is " +
   ( q2.query() ? "provable" : "not provable" )
);


//The problem is the next statement. The code in the SWI web site is:
// ....new Variable(X).
//This is wrong: use double quotes instead!

Variable X = new Variable("X");

Query q4 =
   new Query(
       "child_of",
       new Term[] {new Atom("joe"), X}
   );

println(q4);

Hashtable solution;
solution = q4.oneSolution();


//again: use solution.get("X") --include the double quotes!
println( "X = " + solution.get("X"));

java.util.Hashtable[] solutions = q4.allSolutions();
for ( int i=0 ; i<solutions.length ; i++ ) {
//again, use get("X")...
   println( "X = " + solutions[i].get("X"));
}

<code ends>

If anybody wants to know all of the steps needed to make jpl into a library for prolog, I am happy to post them (as far as I can remember what I  have done...)