not to worry!
i fixed it! ;D

i changed the initialization page to
[code]for(int i=0;i y1[i]=-lengthz/2+lengthz/n*i; // AD
y3[i]=lengthz/2-lengthz/n*i; // BC
x2[i]=a-lengthx/n*i; //AB
x4[i]=a-lengthx/n*i; // DC

which then when the slider is moved
setparam ();[/code]

no problem now!!! cool!

really learnt a lot of (problem solving skills & critical thinking) from analyzing, making sense, remixing the virtual lab to suit my design ideas of how to finer customized the virtual lab.
fantastic pedagogy for learning problem solving skills & critical thinking ;D

my physics knowledge comes alive in the java applet. cool way to assess my learning by portfolio like what prof wolfgang is doing i think.