PsAndQs Encoder

PsAndQs.java

import java.io.*;
import java.nio.charset.StandardCharsets;

/**
 * Encode streams into Ps and Qs. 
 * Why? Because it's mannerly.
 * P = 0
 * Q = 1
 */
public class PsAndQs {

    public static void main(String[] args) throws IOException {
        String inputText = "hello world";
        System.out.printf("Encoding: %s%n", inputText);
        ByteArrayOutputStream  baos = new ByteArrayOutputStream();
        PsAndQs.EncoderStream out = new PsAndQs.EncoderStream(baos);

        out.write(inputText.getBytes(StandardCharsets.UTF_8));
        String encoded = baos.toString();
        System.out.printf("Encoded Text: %s%n", encoded);

        PsAndQs.DecoderStream decoderStream = new PsAndQs.DecoderStream(new ByteArrayInputStream(encoded.getBytes(StandardCharsets.UTF_8)));
        System.out.printf("Decoded: %s%n", new String(decoderStream.readAllBytes()));
    }

    private static final byte[] BIT_MASKS =
            new byte[]{0b1000000, 0b1000000, 0b100000, 0b10000, 0b1000, 0b100, 0b10, 0b1};

    public static class EncoderStream extends FilterOutputStream {

        protected EncoderStream(OutputStream out) {
            super(out);
        }

        @Override
        public void write(int b) throws IOException {
            for (byte mask : BIT_MASKS) {
                if ((b & mask) == mask) {
                    out.write('q');
                } else {
                    out.write('p');
                }
            }
        }
    }

    public static class DecoderStream extends FilterInputStream {

        private boolean eos;
        private boolean closed;

        public DecoderStream(InputStream in) {
            super(in);
        }

        @Override
        public void close() throws IOException {
            if (!closed) {
                super.close();
                closed = true;
                eos = true;
            }
        }

        @Override
        public int read() throws IOException {
            ensureOpen();
            byte read = readByte();
            if (eos) {
                return -1;
            } else {
                return read;
            }
        }

        @Override
        public int read(byte[] b, int off, int len) throws IOException {
            if (eos) {
                return -1;
            }

            int i = off;
            int v;
            while (i < off + len && (v = read()) != -1) {
                b[i] = (byte) v;
                i++;
            }

            return i - off;
        }

        private void ensureOpen() throws IOException {
            if (closed) {
                throw new IOException("Stream closed");
            }
        }

        private byte readByte() throws IOException {
            byte value = 0;
            for (int i = 0; i < 8; i++) {
                int read = in.read();
                if (read == 'q') {
                    value |= BIT_MASKS[i];
                } else if (read == 'p') {
                    // empty
                } else if (read == -1) {
                    eos = true;
                } else {
                    throw new IllegalArgumentException("Invalid character " + read);
                }
            }
            return value;
        }
    }
}