在Java中生成随机整数列表（以类似Scala的方式）

Question

Scala中是否存在Java 8或更高版本中的等效内容？

Seq.fill(n)(Random.nextInt)

Answer 1

You can use Random.ints(n) to generate n random numbers. To avoid boxing, you probably want to put them into an array:

int[] seq = rnd.ints(n).toArray();

If you want a list and you don't mind boxing, you can do this:

List<Integer> list = rnd.ints(n).boxed().collect(toList());

The pseudorandom number generator defined by java.util.Random is used if rnd is an instance of that class. This should be fine for most purposes. If you want to use a different source of randomness, you can override Random.next(bits) to supply bits from a custom source instead of from the built-in generator. For example, you can do this:

WARNING: THIS IS A TERRIBLE SOURCE OF PSEUDORANDOM NUMBERS. DON'T USE IT. THIS IS JUST AN EXAMPLE.

class MyRandom extends Random {
    int state = 0;
    protected int next(int bits) {
        return state ^= 1;
    }
}

Random rnd = new MyRandom();
int[] seq = rnd.ints(n).toArray();

This lets you use your own source of random bits using the API of java.util.Random , such as the ability to get values of various primitives, in different distributions, and streams of different types, ranges, and lengths.

Answer 2

Maybe somewhat closer approximation than Random.ints() would be something like this:

Stream
  .generate(rnd::nextInt)
  .limit(n)
  .collect(toList());

It is "closer" in the sense that, if one ignores minor syntactic differences and Seq vs. List distinction, both the Scala version and this version unify with a function of signature

(Int, Unit => T) => Seq[T]

for any T :

(n, f) => Stream.generate(f).limit(n).collect(toList())
(n, f) => Seq.fill(n)(f)

Unlike Random.ints() , this version works for more complex distributions, and it actually limits the length to some number n , just like the Scala version does. Depending on what you want to do with it, you can collect it to some other data structure, or just leave it as Stream .

---

Full demo with all imports:

import java.util.stream.*;
import java.util.Random;
import java.util.List;
import static java.util.stream.Collectors.toList;

public class RandomListGeneration {
    public static void main(String args[]) {
        Random rnd = new Random();
        int n = 42;
        List<Integer> result = Stream
            .generate(rnd::nextInt)
            .limit(n)
            .collect(toList());
        System.out.println(result);
    }
}