Consider the following code with the specs quoted

        URLConnection c = new URL("file:/usr").openConnection();
        c.connect();

        System.out.println("c = " + c);
        // "The Map keys are Strings that represent the response-header field names"
        System.out.println(c.getHeaderFields().keySet());

        // Returns the key for the nth header field.
        // It returns null if there are fewer than n+1 fields.
        System.out.println(c.getHeaderFieldKey(0));

        // "It returns null if there are fewer than n+1 fields."
        System.out.println(c.getHeaderField(0));

The output would be:

c = sun.net.www.protocol.file.FileURLConnection:file:/usr
[]
content-type
text/plain

::getHeaderFields indicates that there are zero header fields while getHeaderField/Key(index) say the opposite

FtpURLConnection seem to be affected as well as FileURLConnection