[hardwired]

Code:

public class ButtonHandler implements ActionListener
{
    ...
        if(button == add)
        {
            // Problem was here: incrementing num_students too early
            // which skips over the array element at index = 0.
//            num_students++;
            stu[num_students].getData();
            num_students++;
        }

So the first element in the stu array (at index = 0) was skipped and you were
initializing the second element (index = 1). This caused trouble in the loop
of the getCopy method:

Code:

for(int j = 0; j < stu.length; j++)
{
    if(stu[j].getName() == null)
        break;
    count++;
}

which broke out at the first element (j = 0) because it had been skipped.
So getCopy returned a zero-length array to the sortByGPA method.

In conjunction with this you could alter the test method to be able
to add more elements to the array:

Code:

private void test()
{
    // remove the add.setEnabled line
    ...
    stu[2].addCourse(new Course("ES", 220, 5, "A"));
    num_students = 3;
}