How does java do modulus calculations with negative numbers?

Am I doing modulus wrong? Because in Java -13 % 64 evaluates to -13 but I want to get 51.

Both definitions of modulus of negative numbers are in use - some languages use one definition and some the other.

If you want to get a negative number for negative inputs then you can use this:

int r = x % n;
if (r > 0 && x < 0)
{
    r -= n;
}

Likewise if you were using a language that returns a negative number on a negative input and you would prefer positive:

int r = x % n;
if (r < 0)
{
    r += n;
}

Since "mathematically" both are correct:

-13 % 64 = -13 (on modulus 64)  
-13 % 64 = 51 (on modulus 64)

One of the options had to be chosen by Java language developers and they chose:

the sign of the result equals the sign of the dividend.

Says it in Java specs:

https://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.17.3

Are you sure you are working in Java? 'cause Java gives -13 % 64 = -13 as expected. The sign of dividend!