Strange behaviour of java.nio.charset.StandardCharsets.UTF8.newDecoder()

[Pavel Rappo]

Hello,

As far as I understand one of the major use cases of j.n.c.CharsetDecoder is
incremental decoding. That is, a case where a complete data is available in
chunks.
If the above is true, would the following behaviour be considered an expected
one? Namely, byte-wise decoding fails with an error, while bulk (and complete)
decoding of the same data completes successfully.

Thanks.

    public static void main(String[] args) {
        String hex = "48656c6c6f2dc2b540c39fc3b6c3a4c3bcc3a0c3a12d5554462d382121";
        Matcher m = Pattern.compile("\\p{XDigit}{2}").matcher(hex);
        List<Integer> ints = new ArrayList<>();
        while (m.find()) {
            Integer i = Integer.parseInt(m.group(0), 16);
            ints.add(i);
        }
        if (ints.isEmpty()) {
            throw new AssertionError();
        }

//        decodeByteWise(ints);
        decodeWholesale1(ints);
        decodeWholesale2(ints);
    }

    private static void decodeByteWise(List<Integer> ints) {
        CoderResult r = null;
        CharsetDecoder d = StandardCharsets.UTF_8.newDecoder();
        CharBuffer out = CharBuffer.allocate(ints.size());
        for (Integer i : ints) {
            r = d.decode(ByteBuffer.wrap(new byte[]{i.byteValue()}), out, false);
            if (r != CoderResult.UNDERFLOW) {
                throw new AssertionError();
            }
        }
        if (r != CoderResult.UNDERFLOW) { throw new AssertionError(); }
        r = d.decode(ByteBuffer.allocate(0), out, true);
        if (r != CoderResult.UNDERFLOW) { throw new AssertionError(); }
        r = d.flush(out);
        if (r != CoderResult.UNDERFLOW) { throw new AssertionError(); }
        out.flip();
        System.out.println(out);
    }

    private static void decodeWholesale1(List<Integer> ints) {
        CharsetDecoder d = StandardCharsets.UTF_8.newDecoder();
        CharBuffer out = CharBuffer.allocate(ints.size());
        ByteBuffer in = ByteBuffer.allocate(ints.size());
        for (Integer i : ints) {
            in.put(i.byteValue());
        }
        in.flip();
        CoderResult r = d.decode(in, out, false);
        if (r != CoderResult.UNDERFLOW) { throw new AssertionError(); }
        r = d.decode(in, out, true);
        if (r != CoderResult.UNDERFLOW) { throw new AssertionError(); }
        r = d.flush(out);
        if (r != CoderResult.UNDERFLOW) { throw new AssertionError(); }
        out.flip();
        System.out.println(out);
    }

    private static void decodeWholesale2(List<Integer> ints) {
        CharsetDecoder d = StandardCharsets.UTF_8.newDecoder();
        CharBuffer out = CharBuffer.allocate(ints.size());
        ByteBuffer in = ByteBuffer.allocate(ints.size());
        for (Integer i : ints) {
            in.put(i.byteValue());
        }
        in.flip();
        CoderResult r = d.decode(in, out, true);
        if (r != CoderResult.UNDERFLOW) { throw new AssertionError(); }
        r = d.flush(out);
        if (r != CoderResult.UNDERFLOW) { throw new AssertionError(); }
        out.flip();
        System.out.println(out);
    }