Java - Can't edit byte array contents?

Question

    byte[] stuffA = {69,96,13,37,-69,-96,-13,-37};
    for(int x = 0; x < stuffA.length; x++){
        if(stuffA[x] < 0){
            System.out.println("Somethin be up yo! ");
            System.out.println("This number be negative! " + (int)stuffA[x]);
            stuffA[x] = (byte)((int)stuffA[x] + 256);
            System.out.println("I added 256 and now stuff is positive yo! " + stuffA[x]);
        }
    }
    return;

When I run this, my output is:

Somethin be up yo! 
This number be negative! -69
I added 256 and now stuff is positive yo! -69
Somethin be up yo! 
This number be negative! -96
I added 256 and now stuff is positive yo! -96
Somethin be up yo! 
This number be negative! -13
I added 256 and now stuff is positive yo! -13
Somethin be up yo! 
This number be negative! -37
I added 256 and now stuff is positive yo! -37

What is happening?

What do you think the range of values you can store in a byte is? — Andy Turner, Sep 29 '15 at 20:18

score 1 · Answer 1 · answered Sep 29 '15 at 20:25

1

Quoting Java Language Specification, section 4.2.1:

The values of the integral types are integers in the following ranges: For byte, from -128 to 127, inclusive.

So, -69 + 256 = 187 = 0x000000BB cast to byte is 0xBB = -69.

answered Sep 29 '15 at 20:25

Andreas

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score 1 · Answer 2 · answered Sep 29 '15 at 20:32

1

A byte has a range between -128 and 127 (-2^7 to 2^7-1) Adding 256 its like doing a 360 degree turn . Change 256 to 128 in your code and in will show different results

answered Sep 29 '15 at 20:32

Armando SM

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score 0 · Answer 3 · answered Sep 29 '15 at 20:26

0

byte is only 8 bits, so can only hold 2^8 = 256 values. It is signed in java so the values are in the range of [-128,127] inclusive.

answered Sep 29 '15 at 20:26

James Wierzba

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score 0 · Answer 4 · answered Sep 29 '15 at 20:26

0

A byte can hold 256 differnet values. If you add 256 the byte overflows, and the result stays the same. In java the range is [-128, 127].

answered Sep 29 '15 at 20:26

AlexWien

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Johnny Willer · Answer 5 · 2015-09-29T23:30:03.283

A byte can hold 256 different values, so when you add 256 you are making a Rol (Rotate left) 8 times, which returns the same value as previous.

If your desire is to transform -69 in 69, why you simply don't make a stuffA[x] *=-1?

If you really want to reverse the bits, you can use complement operator (~), stuffA[x] = (byte) ~stuffA[x];, but due to the two's complement form, to get the same number as previos you need to add 1,

like stuffA[x] = (byte) ((~stuffA[x]) + 1);

abhish_gl · Answer 6 · 2015-09-29T21:03:19.623

0

byte is 8 bits and integer is 32 bits

basically looking at bits:

byte b = -37 is equivalent to 219(int) whose binary is:

0000 0000 0000 1101 1011

int 256:

0000 0000 0001 0000 0000

adding both will give int value of 475:

0000 0000 0001 1101 1011

now convert it to byte i.e. take LSB 8 bits will be:

1101 1011

which is -37

Hope this explains

edited Sep 29 '15 at 21:03

answered Sep 29 '15 at 20:58

abhish_gl

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Java - Can't edit byte array contents?

6 Answers6