在java中将两个字节（一个正面和另一个负面）组合成短

Question

I am trying to port my C++ project in java. Down the line I have to read some bytes from serial port and combine two bytes into short. Things are working elegantly with below code.

               ByteBuffer bb = ByteBuffer.allocate(2);
                bb.order(ByteOrder.LITTLE_ENDIAN);

                bb.put(b1);
                bb.put(b2);
                  result = bb.getShort(0);

In c++ project instead of short they have unsigned short (in java we don't have unsigned).

So my above logic result does not align with C++ result for below case:-
 b1 = 106 ,  b2 = -1     c++ result = 150   and java = -150


 b1 =-6, b2 =   -1     506 in VC++ but -6 in java

However, if only first byte is negative then my result are similar :
b1 = -12 ,  b2 = 1     c++ result = 500   and java = 500

I want to align my result with c++. any suggestions and help would be highly appreciable .

Answer 1

Don't use getShort() , use getInt() instead.

Your problem is that the most significant bit is being interpreted as the sign bit due to the fact that this is the case for a signed short. If you extend the data type so that bit is no longer considered the sign bit, you should get the behaviour you want.

You may need to pad out your buffer to do this:

ByteBuffer bb = ByteBuffer.allocate(4);
bb.order(ByteOrder.LITTLE_ENDIAN);

bb.put(b1);
bb.put(b2);
bb.put(0);
bb.put(0);

result = bb.getInt(0);

EDIT: This would work if only the C++ code was doing the right thing, but as pointed out in the comments above, it isn't (ie it is NOT simply taking the two bytes and combining them into a short. It performs some other transform on them as well).

Answer 2

java -6 (0xFFFA) is correct, C++ 506 (0x01FA) is not. 0xFA. Seems you mistyped -1: 1 or - -1.