Random class has a method to generate random int in a given range. For example:
Random r = new Random();
int x = r.nextInt(100);
This would generate an int number more or equal to 0 and less than 100. I'd like to do exactly the same with long number.
long y = magicRandomLongGenerator(100);
Random class has only nextLong(), but it doesn't allow to set range.
ベストアンサー1
Starting from Java 7 (or Android API Level 21 = 5.0+) you could directly use ThreadLocalRandom.current().nextLong(n) (for 0 ≤ x < n) and ThreadLocalRandom.current().nextLong(m, n) (for m ≤ x < n). See @Alex's answer for detail.
If you are stuck with Java 6 (or Android 4.x) you need to use an external library (e.g. org.apache.commons.math3.random.RandomDataGenerator.getRandomGenerator().nextLong(0, n-1), see @mawaldne's answer), or implement your own nextLong(n).
According to Random documentation, nextInt is implemented as
public int nextInt(int bound) {
if (bound <= 0)
throw new IllegalArgumentException("bound must be positive");
if ((bound & -bound) == bound) // i.e., bound is a power of 2
return (int)((bound * (long)next(31)) >> 31);
int bits, val;
do {
bits = next(31);
val = bits % bound;
} while (bits - val + (bound-1) < 0);
return val;
}
So we may modify this to perform nextLong:
long nextLong(Random rng, long bound) {
// error checking and 2^x checking removed for simplicity.
long bits, val;
do {
bits = (rng.nextLong() << 1) >>> 1;
val = bits % bound;
} while (bits-val+(bound-1) < 0L);
return val;
}