This says: "jrbtnSub is a variable that can refer to
arrays of JRadioButton references."  It does not actually
create an array, it just says that jrbtnSub can refer to
one if you ever decide to create one.  For example, you
might continue with

    jrbtnSub = new JRadioButton[3];  // or perhaps more

to create a three-element array and set jrbtnSub to point
at it.  Or you could declare and initialize in one line with

    JRadioButton[] jrbtnSub = new JRadioButton[3];

    (Stylistic note: Most Java practitioners prefer
`SomeType[] arrayRef' over `SomeType arrayRef[]', although
both forms are legal.  It is felt that the first form collects
all the type information in one place, while the second spreads
it out in separate chunks.  Using the second form also calls
into question one's dedication to Java by showing that one has
not yet fully renounced C!)

Yup. The more conventional form of the loop might have given you a clue:

for(int i = 0; i < jrbtnSub.length; i++) {

You need to intialize jrbtnSub. You probably mean something like:

JRadioButton jrbtnSub[] = new JRadioButton[temp.length];

-- Adam Maass

In addition to what other replies have said, you might consider using a
(java.util.)List instead of an array for the buttons. With a List you
don't need to know the number of elements when you create it (and there
are many other advantages). So your code could become:

        String[] subjectNames = {
            "Compilers", "Operating Systems",
            "Computer Networks"
        };
        List<JRadioButton> subjectButtons =
            new java.util.ArrayList<JRadioButton>();
        for (String name : subjectNames) {
            JRadioButton button = new JRadioButton(name);
            subjectButtons.add(button);
            subjectPanel.add(button);
        }

(I've changed then identifier names, because I find the likes of "jrbtn"
unreadable and unhelpful.)

Tom Hawtin