Another problem with porting Delphi to Java

Question

I am porting an old application written in Delphi in Java. I am having some problems with this function, which calculates the CRC of the transmitted message.

Here is the source code:

if(ReceivedMessage[slot].Message[1] = $FD) and (ReceivedMessage[slot].MessageLength in [1..16]) and (ReceivedMessage[slot].Message[counter] = $FF) then begin index := 2; ReceivedMessage[slot].DataReady := TRUE; for counter := 1 to ReceivedMessage[slot].MessageLength do begin Inc(index); if ReceivedMessage[slot].Message[index] < $F8 then ReceivedMessage[slot].Data[counter] := ReceivedMessage[slot].Message[index] else if ReceivedMessage[slot].Message[index] = $F8 then begin Inc(index); ReceivedMessage[slot].Data[counter] := ReceivedMessage[slot].Message[index] or $F0; end else ReceivedMessage[slot].DataReady := FALSE; // Invalid data end; if ReceivedMessage[slot].DataReady = TRUE then begin Inc(index); if ReceivedMessage[slot].Message[index] < $F8 then ReceivedMessage[slot].CRC := ReceivedMessage[slot].Message[index] shl 8 else if ReceivedMessage[slot].Message[index] = $F8 then begin Inc(index); ReceivedMessage[slot].CRC := (ReceivedMessage[slot].Message[index] or $F0) shl 8; end; Inc(index); if ReceivedMessage[slot].Message[index] < $F8 then ReceivedMessage[slot].CRC := ReceivedMessage[slot].CRC or ReceivedMessage[slot].Message[index] else if ReceivedMessage[slot].Message[index] = $F8 then begin Inc(index); ReceivedMessage[slot].CRC := ReceivedMessage[slot].CRC or ReceivedMessage[slot].Message[index] or $F0; end; 

And here is my Java code:

  if(array[1]==0xFD && (array[2]>0 && array[2]<17) && array[pos]==(byte)0xFF) { index=2; for(int counter=1;counter<splMsg.nbytes+1;counter++) { index++; if(array[index]<0xF8) data[counter]=array[index]; else if(array[index]==0xF8) { index++; data[counter]=(byte)(array[index] | 0xF0); } else return 0xFC; } index++; short crc=0x0000; if(array[index]<0xF8) crc=(short) (array[index]<<8); else if(array[index]==0xF8) { index++; crc=(short)((array[index] | 0xF0) << 8); } index++; if(array[index]<0xF8) crc=(short) (crc | array[index]); else if(array[index]==0xF8) { index++; crc=(short)(crc | array[index] | 0xF0); } msgcrc=new byte[] {(byte)(crc >> 8 & 0xff),(byte)(crc & 0xff)}; 

My function returns the passed CRC code most often, but sometimes it crashes and returns the last two bytes of the message. The last 3 bytes of the message are the CRC code (2 bytes) and the 0xff byte of the end of the message.

Any help?

Thanks Pedro

Answer 1

Remember:

Java byte signed. So:

 byte test = (byte)255; System.out.println(test); 

will output: -1

and

  byte test = (byte)255; System.out.println(test == 255); 

will print false but

  byte test = (byte)255; System.out.println((test & 255) == 255); 

will do what I think you want to achieve (in this case prints true ).

To get unsigned (byte) values, use (array[index] & 255) .

You will need to do the masking with 0xff everywhere, otherwise you will get the sign of extended integers for byte values ​​greater than 127.

For comparison, you can also cast to byte (for example, if(test == (byte)255) ), but I think that you need to stick to a single conversion, so I recommend switching to masking & 0xff .