...
  public static int getInt32(byte[] msg, int pos) {
    return (int)(
      ((msg[pos+3] & 0xFF) << 24) +
      ((msg[pos+2] & 0xFF) << 16) +
      ((msg[pos+1] & 0xFF) <<  8) +
       (msg[pos  ] & 0xFF)
    );
  }
...
    case 0x29: //Map checksum
      System.out.println("Receieved: Map checksum");
      String mapName = new String(data, 1, 16);

      long mapCRC = validator.getChecksum(mapName.trim());
      if (mapCRC == -1) break; //File not found
      long mCrc   = getInt32(data, 17) & 0xFFFFFFFFL;
      System.out.println("  map " + mapCRC + "/" + mCrc);
      if (mapCRC != mCrc)
      System.out.println("  " + mapName.trim() + " out of sync");
//Receieved: Map checksum
//  map 1087398166/3002814809
//  smallmap.lvl out of sync
...

import java.io.*;
import java.util.zip.*;

public class validator {
  static final int version = 1;

  public validator() {
    System.out.println("-- validator v" + version + " --");
  }

  public static long getChecksum(byte[] file) {
    CRC32 crc = new CRC32();
    crc.reset();
    crc.update(file);
    return crc.getValue();
  }

  public static long getChecksum(String fileName) {
    byte[] file = {0};
    try {
      RandomAccessFile raf = new RandomAccessFile(fileName, "r");
      raf.readFully(file);
      raf.close();
    } catch(SecurityException e) {
      System.out.println("-- Err: Security settings too high to read local files --");
    } catch(FileNotFoundException e) {
      System.out.println("-- Could not locate " + fileName + " --");
      return -1;
    } catch(EOFException e) {
      System.err.println("-- Err: End of File exception --");
    } catch(IOException e) {
      System.err.println("-- Err: IO exception --");
    }
    //CRC32 crc = new CRC32();
    //crc.reset();
    //crc.update(file);
    //return crc.getValue();
    return getChecksum(file);
  }
}