Checksumming

• A checksum for an array of data is a value such that if any bit on the data changes the checksum will also change.
• There are two common types: RC32, that's Cycle Redundancy Check, and the Adler32 (its faster).

FileOutputputStream os =  new FileOutputStream("Temp1.tmp");

//CDC32 extends Checksum.
CRC32 crc32 = new CRC32();
CheckedOutputStream cos =  new CheckedOutputStream(os, crc32);
cos.write(1);
cos.write(2);

//Get the checksum for this file
long crc = crc32.getValue();

//Write this checksum to the stream.
cos.write(crc);

• If writing to a stream you can decide that every 512 bytes you will send 4 bytes of checksum. The other side can verify this.
• Note that TCP guarantees that data is not corrupted (it uses checksums).