Why does ThreadLocalRandom.current().nextInt(1, 255); return values outside of the range [1, 254]?

Question

I am trying to generate a random positive 8-bit integer. The documentation for ThreadLocalRandom.current().nextInt(min, max) specifically says that min is the minimum inclusive bound, and max is the maximum exclusive bound. However, if I set max to any value > 128, I will occasionally get a negative value.

This seems to be associated with the bits of an integer, but the documentation says nothing about bits for nextInt(). I really need as many positive 8-bit numbers as I can get. Why is nextInt() returning negative values when I only specify a positive range of (1, 255)?

byte aa = 1;
    do {
            aa = (byte) ThreadLocalRandom.current().nextInt(1, 128);
    } while (aa > 0);
    System.out.println(aa);

Can you show us a simple program to test this? – Louis Wasserman Apr 20 '17 at 00:22 — Louis Wasserman, Apr 20 '17 at 00:22

score 9 · Accepted Answer · answered Apr 20 '17 at 00:27

9

You cast the result to byte. byte only holds numbers between -128 and 127, and when nextInt returns a larger number than 127, it overflows and rolls over.

This isn't the fault of nextInt, it's the fault of the cast to byte.

answered Apr 20 '17 at 00:27

Louis Wasserman

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Oh. Thanks for that. – so8857 Apr 20 '17 at 00:29

Why does ThreadLocalRandom.current().nextInt(1, 255); return values outside of the range [1, 254]?

1 Answers1