Java Math.abs random vs. nextInt()

Question

Creating a random number generator in Java - looking at num1 and num2, in which instances would I use either of these methods for creating a random number between 1 and 8? Is one more efficient than the other, any other benefits?

import java.util.*;

public class Chpt3 {
  public static void main(String[] args) {
    Random rand = new Random();
    int num1 = Math.abs(rand.nextInt()) % 8 + 1;
    int num2 = rand.nextInt(8) + 1;
    System.out.println(num1);
    System.out.println(num2);

  }
}

Answer 1

nextInt(n) returns between 0 and n-1.

So to get between 1 to 8,

int num2 = rand.nextInt(8) + 1;

Which means your second one is the one you need.

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Update:

Math.abs(Integer.MIN_VALUE) returns negative value.

According to this answer on SO:

Integer.MIN_VALUE is -2147483648, but the highest value a 32 bit integer can contain is +2147483647. Attempting to represent +2147483648 in a 32 bit int will effectively "roll over" to -2147483648. This is because, when using signed integers, the two's complement binary representations of +2147483648 and -2147483648 are identical. This is not a problem, however, as +2147483648 is considered out of range.

For a little more reading on this matter, you might want to check out the Wikipedia article on Two's complement.

First method just contains a rare corner case. That's why it's better to use second one as it's safe.

Answer 2

Math.abs(rand.nextInt()) % 8 + 1;

This has a subtle bug. Math.abs(Integer.MIN_VALUE) returns MIN_VALUE which is a negative number. A simpler solution is

(rand.nextInt() & 7) + 1;

This will always be non-negative and be slightly faster.

rand.nextInt(8) + 1

This is both faster, but more importantly, clearer as to what you are trying to achieve. For the later reason, rather than speed alone, it is the better choice.

Note: You could use abs if you really wanted to like this. However this is not as clean as the nextInt call.

Math.abs(rand.nextInt() % 8) + 1;