At start of method, parameter 'b' is compared with null and NPE is thrown in case of null.

    public int read(byte[] b, int off, int len) throws IOException {
        if (b == null) {
            throw new NullPointerException();

But later variable is compared is null again

                if (b != null) {
                    b[off + i] = (byte)c;
                }

Redundant check could be removed.