code: @Override
    public void readHeader(byte[] data) {
      if(ByteArrayUtil.readInt(data, 0) != FILE_VERSION) {
        throw new RuntimeException("PersistentPageFile version does not match!");
      }
  
      this.pageSize = ByteArrayUtil.readInt(data, 4);
    }
docs: Initializes this header from the given Byte array. Looks for the right
  version and reads the integer value of {@link #pageSize} from the file.