Short example demonstrating how to read from a java.nio.ByteBuffer

Example.java

import java.nio.ByteBuffer;
import java.util.Arrays;

class Header {
    private final Short framesize;
    public Header(short framesize) {
        this(new Short(framesize));
    }
    public Header(Short framesize) {
        this.framesize = framesize;
    }
    @Override
    public String toString() {
        return "Header(" + framesize.toString() + ")";
    }
    public Short getFramesize() {
        return framesize;
    }
}

public class Example {

    static byte[] sampleFrame() {
        String header  = "AA 01 00 34 1E 36 44 85 36 00 00 00 41 B1";
        String stat    = "00 00";
        // TODO currently in 16bit integer, change to float!
        String phasors = "39 2B 00 00 E3 6A CE 7C E3 6A 31 83 04 44 00 00";
        String freq    = "09 C4";
        String dfreq   = "00 00";
        String analog  = "42 C8 00 00 44 7A 00 00 46 1C 40 00";
        String digital = "3C 12";
        String crc     = "D4 3F";
        return hexToByte(header + stat + phasors + freq + dfreq + analog + digital + crc);
    }

    public static void main(String[] args) {
        System.out.println(readHeader(ByteBuffer.wrap(sampleFrame())));
    }

    private static Header readHeader(ByteBuffer buffer) {
        // check sync bytes
        byte[] sync = new byte[2];
        buffer.get(sync, 0, 2);
        assertEquals(sync, "AA01");

        // read framesize
        short framesize = buffer.getShort();

        // ignore next 10 bytes
        buffer.position(buffer.position() + 10);
        return new Header(framesize);
    }

    private static void assertEquals(byte[] got, String expected) {
        if (!Arrays.equals(got, hexToByte(expected))) {
            throw new RuntimeException("byte sequences don't match");
        }
    }

    // copied and adopted from answer in
    //   https://stackoverflow.com/questions/140131/convert-a-string-representation-of-a-hex-dump-to-a-byte-array-using-java
    //
    // alternatively use apache commons codec: http://commons.apache.org/proper/commons-codec/
    private static byte[] hexToByte(String hex) {
        String noSpace = hex.replaceAll("\\s+", "");
        int len = noSpace.length();
        byte[] data = new byte[len / 2];
        for (int i = 0; i < len; i += 2) {
            data[i / 2] = (byte) ((Character.digit(noSpace.charAt(i), 16) << 4)
                                 + Character.digit(noSpace.charAt(i + 1), 16));
        }
        return data;
    }
}