Enter the following code.  Use "Buffered" and press ctrl-space to get
"BufferedReader", "Input" ctrl-space to get InputStreamReader.  You will see a
list of choices in code completion for both.

Now delete the code and do the same thing again.  This time code completion
automatically completes Buffered as BufferedReader and shows no list of options;
 same thing for InputStreamReader.  It should still show the list of classes.

    public static void main(String[] args) throws IOException {
        System.out.println("How many rabbits");
        BufferedReader b = new BufferedReader (new InputStreamReader (System.in));
        String s = b.readLine();

        int ct = Integer.parseInt(s);
        int val = fib (ct);
        System.out.println("After " + ct + " generations there are " + val + "
rabbits");
    }
    
    private static final int fib (int n) {
        return n == 0 ? 0 : n == 1 ? 1 : fib (n - 2) + fib (n - 1);
    }