ArrayIndexOutOfBoundsException: Index 256 out of bounds for length 256 in java

Question

I want output an array of integers in java which has a index 5120, but in java only up to 256 index , while in C index to 5120 there is output not 0, I tried changing the loop from <255 to <65536 but instead got an error like the title of this question

Code in C :

    int Tbl0[256],Tbl1[256],Tbl2[256],Tbl3[256];
    int i;
    char SBox={ 0xab,0x88 .....};

unsigned char x, y;

for( i = 0; i < 256; i++ )
{
x = (unsigned char) SBox[i]; 

        y = (x << 1 ^ ((x & 0x80) != 0 ? 0x1B : 0x00));
Tbl0[i] = (int) ( x ^ y ) ^
( (int) x << 8 ) ^
( (int) x << 16 ) ^
( (int) y << 24 );

Tbl0[i] &= 0xFFFFFFFF;

    Tbl1[i] = ( ( ( Tbl0[i] << 24 ) & 0xFFFFFFFF ) | ( ( Tbl0[i] & 0xFFFFFFFF ) >>> 8 ) );





}
        printf("%d\n" , Tbl1[(char)257]);

Output :

1154899012

Java :

    int Tbl0[65536],Tbl1[65536],Tbl2[65536],Tbl3[65536];
        int i;
        char SBox={ 0xab,0x88 .....};
    
    char x;
byte y;
    
    for( i = 0; i < 256; i++ )
    {
    x = (char) SBox[i]; 
    
            y = (byte)(x << 1 ^ ((x & 0x80) != 0 ? 0x1B : 0x00));
    Tbl0[i] = (int) ( x ^ y ) ^
    ( (int) x << 8 ) ^
    ( (int) x << 16 ) ^
    ( (int) y << 24 );
    
    Tbl0[i] &= 0xFFFFFFFF;
    
        Tbl1[i] = ( ( ( Tbl0[i] << 24 ) & 0xFFFFFFFF ) | ( ( Tbl0[i] & 0xFFFFFFFF ) >>> 8 ) );
    
    
    
    
    }

        System.out.println(Tbl1[(char)257]);

Output :

I tried to print Tb11 [255] and the results are the same as those in C but after index > 256 the results are always 0, I tried changing the looping to :

for( i = 0; i< 65536; i++)

but I got an error : Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 256 out of bounds for length 256

The C code's `printf("%d\n" , Tbl1[257])` is *undefined behaviour*. How do expect to replicate the output `1154899012` in java? Also, in the C code `Tbl0[i] &= 0xFFFFFFFF;` does nothing useful. — Weather Vane, Jul 22 '20 at 15:09
It is unclear how big the `SBox` array is but that may be your problem when you change your loop. — WJS, Jul 22 '20 at 15:11
@WeatherVane but why in c there a result and in java no? i confuse.. — Atrs, Jul 22 '20 at 15:18
It is unlikely that people will want to read much of the code when it contains typos, the dialog going 'What about this' - 'that is a typo' and 'what about that' - 'that is another typo.' It's best to copy/paste *actual code*, and preferably the [Minimal Reproducible Example](https://stackoverflow.com/help/minimal-reproducible-example) with complete code that demonstrates the problem. — Weather Vane, Jul 22 '20 at 15:27
Please show properly formatted code. Nobody likes to read poorly formatted code and if people don't read your code you get fewer answers, or none at all. — Jabberwocky, Jul 22 '20 at 15:29
Your error is shown when accessing index 256 of a 256-length array, because Java arrays start at zero. — NomadMaker, Jul 22 '20 at 15:34

WJS · Answer 1 · 2020-07-22T15:51:56.617

0

A few things.

You are only indexing up to 255 in your for loop so there will be nothing in any of the tables at 257.
When you change your for loop to 65536 your SBox will throw an exception because it is only of size 256.
array indices in Java are integers so why are you casting one to a char?
Your tables are not properly declared for Java so everything else is suspect.

edited Jul 22 '20 at 15:51

answered Jul 22 '20 at 15:29

WJS

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ArrayIndexOutOfBoundsException: Index 256 out of bounds for length 256 in java

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