位移大于32位长

Question

I am trying to extract the first 49 bits from a 7 byte array. I approached this byte using masks and bit shifting as follows:

long byteVal = ((decryptedVCW[6] & 0xff)&((decryptedVCW[6] & 0xff)<<7)) | ((decryptedVCW[5] & 0xff) << 8) | ((decryptedVCW[4] & 0xff) << 16) | ((decryptedVCW[3] & 0xff) << 24) | ((decryptedVCW[2] & 0xff) << 32) | ((decryptedVCW[1] & 0xff) << 40) | ((decryptedVCW[0] & 0xff) << 48);

Where decryptedVCW is a 56 bit byte array.

The masking and bit shifting work as expected until the 32 bit shift '<<32'.

As an example, the hex for decryptedVCW is E865037A9C6424 of which in binary is:

11101000011001010000001101111010100111000110010000100100

When I perform the above shifting I get 7AFC6503 in binary:

1111010111111000110010100000011

Does anyone have any idea why the bit shifting falls apart at 32 upwards and how to go about solving this issue?

Many thanks Shiv

Answer 1

The type of decryptedVCW[2] & 0xff is int , since the first operand is byte and the second is an int literal.

When the first operand of the << operator is int , you are shifting an int , so if the second operand is 32, you'll get int overflow.

You can cast the first operand of the << operator to long :

(((long)(decryptedVCW[2] & 0xff)) << 32)

or you can force the first operand to be a long by using a long literal in the & operation, as suggested by @shmosel :

(decryptedVCW[2] & 0xFFL) << 32