how to compress file using Arithmetic coding, and then design simple file format in which results of compression and CRC/hash will be stored

I have to take plain .txt file and calculate its CRC, or hash code. I did that. Then I must compress that file using Arithmetic coding. After those two steps, I need to design a simple file format, in which results of compression and CRC/hash will be stored.

I got a hint about this that says I can make something like XML

<hash> 4234nm2n42nmnwmr </hash>
<content> filecontents </content>

and suggestion to use some library, like XStream.

Even with this hint, I have no clue how to make this work, and I tried everything. Stackoverflow is my only hope to make this right.