[java] Can we make unsigned byte in Java

I am trying to convert a signed byte in unsigned. The problem is the data I am receiving is unsigned and Java does not support unsigned byte, so when it reads the data it treats it as signed.

I tried it to convert it by the following solution I got from Stack Overflow.

public static int unsignedToBytes(byte a)
{
    int b = a & 0xFF;
    return b;
}

But when again it's converted in byte, I get the same signed data. I am trying to use this data as a parameter to a function of Java that accepts only a byte as parameter, so I can't use any other data type. How can I fix this problem?

There is no unsigned byte in Java, but if you want to display a byte, you can do,

int myInt = 144;

byte myByte = (byte) myInt;

char myChar = (char) (myByte & 0xFF);

System.out.println("myChar :" + Integer.toHexString(myChar));

Output:

myChar : 90

For more information, please check, How to display a hex/byte value in Java.

If you want unsigned bytes in Java, just subtract 256 from the number you're interested in. It will produce two's complement with a negative value, which is the desired number in unsigned bytes.

Example:

int speed = 255; //Integer with the desired byte value
byte speed_unsigned = (byte)(speed-256);
//This will be represented in two's complement so its binary value will be 1111 1111
//which is the unsigned byte we desire.

You need to use such dirty hacks when using leJOS to program the NXT brick.

If you have a function which must be passed a signed byte, what do you expect it to do if you pass an unsigned byte?

Why can't you use any other data type?

Unsually you can use a byte as an unsigned byte with simple or no translations. It all depends on how it is used. You would need to clarify what you indend to do with it.

Adamski provided the best answer, but it is not quite complete, so read his reply, as it explains the details I'm not.

If you have a system function that requires an unsigned byte to be passed to it, you can pass a signed byte as it will automatically treat it as an unsigned byte.

So if a system function requires four bytes, for example, 192 168 0 1 as unsigned bytes you can pass -64 -88 0 1, and the function will still work, because the act of passing them to the function will un-sign them.

However you are unlikely to have this problem as system functions are hidden behind classes for cross-platform compatibility, though some of the java.io read methods return unsighed bytes as an int.

If you want to see this working, try writing signed bytes to a file and read them back as unsigned bytes.

Yes and no. Ive been digging around with this problem. Like i understand this:

The fact is that java has signed interger -128 to 127.. It is possible to present a unsigned in java with:

public static int toUnsignedInt(byte x) {
    return ((int) x) & 0xff;
}

If you for example add -12 signed number to be unsigned you get 244. But you can use that number again in signed, it has to be shifted back to signed and it´ll be again -12.

If you try to add 244 to java byte you'll get outOfIndexException.

Cheers..

I think the other answers have covered memory representation and how you handle these depends on the context of how you plan on using it. I'll add that Java 8 added some support for dealing with unsigned types. In this case, you could use Byte.toUnsignedInt

int unsignedInt = Byte.toUnsignedInt(myByte);

You can also:

public static int unsignedToBytes(byte a)
{
    return (int) ( ( a << 24) >>> 24);
}    

Explanation:

let's say a = (byte) 133;

In memory it's stored as: "1000 0101" (0x85 in hex)

So its representation translates unsigned=133, signed=-123 (as 2's complement)

a << 24

When left shift is performed 24 bits to the left, the result is now a 4 byte integer which is represented as:

"10000101 00000000 00000000 00000000" (or "0x85000000" in hex)

then we have

( a << 24) >>> 24

and it shifts again on the right 24 bits but fills with leading zeros. So it results to:

"00000000 00000000 00000000 10000101" (or "0x00000085" in hex)

and that is the unsigned representation which equals to 133.

If you tried to cast a = (int) a; then what would happen is it keeps the 2's complement representation of byte and stores it as int also as 2's complement:

(int) "10000101" ---> "11111111 11111111 11111111 10000101"

And that translates as: -123

I am trying to use this data as a parameter to a function of Java that accepts only a byte as parameter

This is not substantially different from a function accepting an integer to which you want to pass a value larger than 2^32-1.

That sounds like it depends on how the function is defined and documented; I can see three possibilities:

1. It may explicitly document that the function treats the byte as an unsigned value, in which case the function probably should do what you expect but would seem to be implemented wrong. For the integer case, the function would probably declare the parameter as an unsigned integer, but that is not possible for the byte case.

2. It may document that the value for this argument must be greater than (or perhaps equal to) zero, in which case you are misusing the function (passing an out-of-range parameter), expecting it to do more than it was designed to do. With some level of debugging support you might expect the function to throw an exception or fail an assertion.

3. The documentation may say nothing, in which case a negative parameter is, well, a negative parameter and whether that has any meaning depends on what the function does. If this is meaningless then perhaps the function should really be defined/documented as (2). If this is meaningful in an nonobvious manner (e.g. non-negative values are used to index into an array, and negative values are used to index back from the end of the array so -1 means the last element) the documentation should say what it means and I would expect that it isn't what you want it to do anyway.

A side note, if you want to print it out, you can just say

byte b = 255;
System.out.println((b < 0 ? 256 + b : b));

Complete guide for working with unsigned bytes in Java:

Unsigned byte in Java

(Source for this answer.)

The Java Language does not provide anything like the unsigned keyword. A byte according to the language spec represents a value between -128 - 127. For instance, if a byte is cast to an int Java will interpret the first bit as the sign and use sign extension.

That being said, nothing prevents you from viewing a byte simply as 8 bits and interpret those bits as a value between 0 and 255. Just keep in mind that there's nothing you can do to force your interpretation upon someone else's method. If a method accepts a byte, then that method accepts a value between -128 and 127 unless explicitly stated otherwise.

Here are a couple of useful conversions / manipulations for your convenience:

Conversions to / from int

// From int to unsigned byte
int i = 200;                    // some value between 0 and 255
byte b = (byte) i;              // 8 bits representing that value

// From unsigned byte to int
byte b = 123;                   // 8 bits representing a value between 0 and 255
int i = b & 0xFF;               // an int representing the same value

(Or, if you're on Java 8+, use Byte.toUnsignedInt.)

Parsing / formatting

Best way is to use the above conversions:

// Parse an unsigned byte
byte b = (byte) Integer.parseInt("200");

// Print an unsigned byte
System.out.println("Value of my unsigned byte: " + (b & 0xFF));

Arithmetics

The 2-complement representation "just works" for addition, subtraction and multiplication:

// two unsigned bytes
byte b1 = (byte) 200;
byte b2 = (byte) 15;

byte sum  = (byte) (b1 + b2);  // 215
byte diff = (byte) (b1 - b2);  // 185
byte prod = (byte) (b2 * b2);  // 225

Division requires manual conversion of operands:

byte ratio = (byte) ((b1 & 0xFF) / (b2 & 0xFF));

As per limitations in Java, unsigned byte is almost impossible in the current data-type format. You can go for some other libraries of another language for what you are implementing and then you can call them using JNI.

If think you are looking for something like this.

public static char toUnsigned(byte b) {
    return (char) (b >= 0 ? b : 256 + b);
}

There are no primitive unsigned bytes in Java. The usual thing is to cast it to bigger type:

int anUnsignedByte = (int) aSignedByte & 0xff;

Although it may seem annoying (coming from C) that Java did not include unsigned byte in the language it really is no big deal since a simple "b & 0xFF" operation yields the unsigned value for (signed) byte b in the (rare) situations that it is actually needed. The bits don't actually change -- just the interpretation (which is important only when doing for example some math operations on the values).

The fact that primitives are signed in Java is irrelevant to how they're represented in memory / transit - a byte is merely 8 bits and whether you interpret that as a signed range or not is up to you. There is no magic flag to say "this is signed" or "this is unsigned".

As primitives are signed the Java compiler will prevent you from assigning a value higher than +127 to a byte (or lower than -128). However, there's nothing to stop you downcasting an int (or short) in order to achieve this:

int i = 200; // 0000 0000 0000 0000 0000 0000 1100 1000 (200)
byte b = (byte) 200; // 1100 1000 (-56 by Java specification, 200 by convention)

/*
 * Will print a negative int -56 because upcasting byte to int does
 * so called "sign extension" which yields those bits:
 * 1111 1111 1111 1111 1111 1111 1100 1000 (-56)
 *
 * But you could still choose to interpret this as +200.
 */
System.out.println(b); // "-56"

/*
 * Will print a positive int 200 because bitwise AND with 0xFF will
 * zero all the 24 most significant bits that:
 * a) were added during upcasting to int which took place silently
 *    just before evaluating the bitwise AND operator.
 *    So the `b & 0xFF` is equivalent with `((int) b) & 0xFF`.
 * b) were set to 1s because of "sign extension" during the upcasting
 *
 * 1111 1111 1111 1111 1111 1111 1100 1000 (the int)
 * &
 * 0000 0000 0000 0000 0000 0000 1111 1111 (the 0xFF)
 * =======================================
 * 0000 0000 0000 0000 0000 0000 1100 1000 (200)
 */
System.out.println(b & 0xFF); // "200"

/*
 * You would typically do this *within* the method that expected an 
 * unsigned byte and the advantage is you apply `0xFF` only once
 * and than you use the `unsignedByte` variable in all your bitwise
 * operations.
 *
 * You could use any integer type longer than `byte` for the `unsignedByte` variable,
 * i.e. `short`, `int`, `long` and even `char`, but during bitwise operations
 * it would get casted to `int` anyway.
 */
void printUnsignedByte(byte b) {
    int unsignedByte = b & 0xFF;
    System.out.println(unsignedByte); // "200"
}