Question

Variable intVar is primitive int, bb[] is byte array {0x02,0xF7,0x8B,0xF9};

  intVar = bb[0];
  //intVar = 00000002    
  intVar <<= 8;
  //intVar = 00000200
  intVar |= bb[1];
  //intvar = fffffff7

Why????

Answer 1

The reason is sign extension. Use:

  intVar = 0xFF & bb[0];
  //intVar = 00000002    
  intVar <<= 8;
  //intVar = 00000200
  intVar |= 0xFF & bb[1];
  // intVar = 000002F7

Using a negative byte (java bytes are signed) as int results in a negative int, and because java uses two’s complement all leading bits will be 1. Anding that with 0xFF will get you the byte you had in the lowest 8 bits and 24 leading 0s.