private void goToPreviousVersion(Document doc)
{
    Document oldVersion = hashmap.get(doc.getVersionId());
    doc.setContent(oldVersion);
    //copy all properties from old version the the document

    //Update the view (GUI)
}

private void saveVersion(Document doc)
{
    hashmap.put(doc.getVersionId(), doc.clone());
    doc.setVersionId(doc.getVersionId +1);
}