I need to represent a value of 0xFF00 as two bytes (in Java). I am trying to do it like this:
int val = 0xFF00;
bytearray[0] = (byte)((val >> 8) & 0xFF);
bytearray[1] = (byte)((val >> 0) & 0xFF);
I know that byte in Java can hold values 0-255. So I expect the first array element to have a value of 255 and the second element to be zero. But what I am getting instead is -1 and 0. What I am doing wrong? What this -1 value mean?
Byte in java is from -128 to 127, not from 0 to 255
-1 is 1111 1111 in two's complement binary, equal to 255 in unsigned byte.
You aren't doing anything wrong, you just need to know that if you see -1, it means the byte is representing the bits 1111 1111.
Related
I don't understand why there's a difference between this code:
byte b = (byte) (0xff >> 1);
(so now b = 01111111),
and this code:
byte b = (byte) 0xff;
b >>= 1;
(but now b = 11111111).
Thanks in advance for your help!
In the first code, (0xff >> 1) is 255 >> 1, which is 127. That is calculated with ints and then you cast it to a byte. 127 as a byte is 01111111 bin.
In the second code, you start with (byte) 0xff, which is 11111111 bin, which is the two's complement representation of -1 in 8 bits. So (byte) 0xff is -1.
When you perform shifting, the byte value -1 is promoted to the int value -1. That's 11111111 11111111 11111111 11111111 bin.
Shifting it right one place with the arithmetic right shift operator, (-1) >> 1 gives you 11111111 11111111 11111111 11111111 again, because the >> operator on a negative number moves the bits to the right and fills in the left with ones instead of zeroes.
Then, since you're using >>=, the result is cast back to a byte to be stored in b. That only retains the last 8 bits, which are 11111111.
Alternatively, if you used the logical right shift operator, (-1) >>> 1 would give you 01111111 11111111 11111111 11111111 in binary (a zero followed by 31 ones). Since the last 8 bits are the same, this would still give you 11111111 when it is cast back to a byte.
I'm coding a personal project in Java right now and have recently been using bit operations for the first time. I was trying to convert two bytes into a short, with one byte being the upper 8 bits and the other being the lower 8 bits.
I ran into an error when running the first line of code below.
Incorrect Results
short regPair = (short) ( (byte1 << 8) + (byte2) );
Correct Results
short regPair = (short) ( (byte1 << 8) + (byte2 & 0xFF) );
The expected results were: AAAAAAAABBBBBBBB, where A represents bits from byte1 and B represents bits from byte2.
Using the 1st line of code I would get the typical addition between a bit-shifted byte1 with byte 2 added to it.
Example of incorrect results
byte1 = 11, byte2 = -72
result = 2816 -72
= 2744
When using the line of code which produces the expected results I can get the proper answer of 3000. I am curious as to why the bit-masking is needed for byte2. My thoughts are that it converts byte2 into binary before the addition and then performs binary addition with both bytes.
In the incorrect case, byte2 is promoted to an int because of the + operator. This doesn't just mean adding some zeros to the start of the binary representation of byte2. Since integer types are represented in two's complement in Java, 1s will be added. After the promotion, byte2 becomes:
1111 1111 1111 1111 1111 1111 1011 1000
By doing & 0xFF, you force the promotion to int first, then you keep the least significant 8 bits: 1011 1000 and make everything else 0.
Print the intermediate value directly to see what is going on. Like,
System.out.printf("%d %s%n", ((byte) -72) & 0xFF, Integer.toBinaryString(((byte) -72) & 0xFF));
I get
184 10111000
So the correct code is actually adding 184 (not subtracting 72).
So I totally forgot that byte is singed in Java, therefore when performing math with a variable of this data type it will take the signed interpretation and not the direct value of the bits. By performing byte2 & 0xFF, Java converts the signed byte value into an unsigned int with all but the first 8 bits set as 0's. Therefore you can perform binary addition correctly.
signed byte value 0x11111111 = -1
unsigned byte value 0x11111111 = 255
In both cases byte values are promoted to int in the expression when
it is evaluated.
byte byte1 = 11, byte2 = -72;
short regPair = (short) ( (byte1 << 8) + (byte2) );
(2816) + (-72) = 2744
And even in below expression byte is promoted to int
short regPair = (short) ( (byte1 << 8) + (byte2 & 0xFF) );
2816 + 184 = 3000
Here in this expression there is no concatenation of two bytes like it has been expressed in the above question- AAAAAAAABBBBBBBB, where A represents bits from byte1 and B represents bits from byte2.
Actually -7 & 255 gives 184 which is added to 2816 to give the output 3000.
I'm new to Java and I am really confused about signed byte in Java.
byte a = -128;
byte b = 126;
System.out.println((byte)(a ^ b));
The output is -2. Can someone please explain why we get this? Is -2 the correct result for -128 XOR 126?
Another question is I have a byte b and I want it to XOR all possible bytes, my code is
byte i = -128
while (i <= 127) {
byte c = (byte) b ^ i;
i++;
}
Is it correct?
This about the representation of signed numbers in computers. They are represented as 2s-complement. This means:
126 = 0111 1110
-128 = 1000 0000
-2 = 1111 1110
Negative values in 2s-complement are formed by taking the absolute value as binary number, inverting all bits except the MSB, adding one to the result and setting the MSB which is used as sign-bit to 1, eg.:
-3:
0000 0011 absolute value (3)
0111 1100 invert all bits except MSB
0111 1101 add 1
1111 1101 set MSB to 1
binary math is pretty straight forward...
-128 in 8 bits is 10000000 (dont forget is 2 complement)
126 in 8 bits is 01111110
xor those then you get 1111 1110: which is -2 in 8 bits 2 complement
I am trying to construct an IP header.
An IP header has the following fields: Version, IHL, DSCP etc. I would like to populate a Byte Array such that I can store the information in bytes.
Where I get confused however is that the Version field is only 4 bits wide. IHL is also only 4 bits wide. How do I fit the values of both of those fields to be represented as a byte? Do I need to do bitshifting?
E.g. Version = 4, IHL = 5. I would need to create a byte that would equal 0100 0101 = 45h or 69 decimal.
(byte) (4 << 4) | 5
This shifts the value 4 to the left, then sets lower 4 bits to the value 5.
00000100 A value (4)
01000000 After shifting left 4 bits (<< 4)
00000101 Another value (5)
01000101 The result of a bitwise OR (|) of #2 and #3
Because the operands are int types (and even if they were byte values, they'd be promoted to int when operators like | act on them), the final result needs a cast to be stored in a byte.
If you are using byte values as operands in any bitwise operations, the implicit conversion to int can cause unexpected results. If you want to treat a byte as if it were unsigned in that conversion, use a bitwise AND (&):
byte b = -128; // The byte value 0x80, -128d
int uint8 = b & 0xFF; // The int value 0x00000080, 128d
int i = b; // The int value 0xFFFFFF80, -128d
int uintr = (b & 0xFF) | 0x04; // 0x00000084
int sintr = b | 0x04; // 0xFFFFFF84
You can do something like this:
int a = 0x04;
a <<= 4;
a |= 0x05;
System.out.println(a);
which essentially turns 0b00000100 into 0b01000000, then into 0b01000101.
https://docs.oracle.com/javase/tutorial/java/nutsandbolts/op3.html
To make a compact field containing both Version and IHL in one byte, try doing
byte b = (byte)((Version << 4) + IHL);
This will only work if Version and IHL are numbers from 0 to 15
Just because a byte is 8 bits and your values can only be a maximum of 4 is not a problem. The extra 4 bits will just always be zeroes.
So if you were storing 1 for example:
0000 0001
or 15 (which is the maximum value right?):
0000 1111
Byte shifting is not possible in Java.
How does bitshifting work in Java?
However, as far as the logic is concerned, if you want the version and IHL in one byte, you could do it using the following
byte value = (byte) (IHL | VERSION << 4);
I receive datas, from a RS422 communication, in a byte tab (byte[]).
Some of my data are in two's complement binary with the following rules :
Significant bits Two's complement
MSB LSB
00000000 0
00000001 + LSB
01111111 + MSB - LSB
10000000 - MSB
10000001 - MSB + LSB
11111111 - LSB
To convert byte[] data to decimal, in pure binary, I use the following code :
Byte b05 = new Byte(new Integer(0x7A).byteValue()); // I use those bytes for my test
Byte b06 = new Byte(new Integer(0x00).byteValue());
Byte[] byteTabDay = new Byte[2] ;
byteTabDay[0] = b05 ;
byteTabDay[1] = b06 ;
int valueDay = byteTabDay[1] << 8 | byteTabDay[0] ;
System.out.println("day :" + valueDay); // print 122
But I don't know how to convert, like previously, byte[] that contain two's complement binary data like that:
Byte b20 = new Byte(new Integer(0x00).byteValue());
Byte b21 = new Byte(new Integer(0xFF).byteValue());
Byte b22 = new Byte(new Integer(0x3C).byteValue());
Those data contain, in theory, the value (more or less) : 1176
So I need help cause I don't understand how I can convert my byte data which contains two's complement binary to decimal.
Two's complement binary is the standard for representing numbers with negative numbers included.
For three-bit numbers:
Base 2 (One's Two's
complement) complement
000 = 0
001 = 1
010 = 2
011 = 3
100 = 4 -3 -4
101 = 5 -2 -3
110 = 6 -1 -2
111 = 7 -0 -1
Java assumes two's complement: the most significant bit being 1 means negative..
Also in java byte, short, int, long are signed.
As an aside, you used the Object wrappers for the primitive types. Primitive types are more immediate.
byte b05 = (byte) 0x7A;
byte b06 = (byte) 0x00; // MSB, positive as < 0x80
nyte[] byteTabDay = new byte[2];
byteTabDay[0] = b05;
byteTabDay[1] = b06;
int valueDay = ((int) byteTabDay[1]) << 8) | (0xFF & byteTabDay[0]);
System.out.println("day :" + valueDay); // print 122
What one has to do: keep the sign extension of the most significant byte, but for other bytes keep them to 8 bits by masking them with 0xFF.
The easiest way to convert an arbitrary byte array containing a two’s complement value to a decimal representation is new BigInteger(bytearray).toString().
This, however, expects the data to be in the big endian byte order. If the data is in little endian order as it seems in your question, you have to reverse it for the use with BigInteger.
byte[] bytearray={ 0x7A, 0x00 };
ByteBuffer b=ByteBuffer.allocate(bytearray.length);
for(int ix=bytearray.length; ix>0; ) b.put(bytearray[--ix]);
System.out.println(new BigInteger(b.array()).toString()); // print 122
If the length of the array matches a standard primitive value type size, you can use ByteBuffer to get the value directly:
byte[] bytearray={ 0x7A, 0x00 };
System.out.println(ByteBuffer.wrap(bytearray)
.order(ByteOrder.LITTLE_ENDIAN).getShort()); // print 122
However, don’t expect the value 1176.61254 as a result. That’s impossible.
If you think that this is the encoded value you will have to adapt your specification. There is no standard three-byte format for floating point values.