实例讲解Java中random.nextInt()与Math.random()的基础用法

// 两种生成对象方式：带种子和不带种子(两种方式的区别见注解)
Random random = new Random();
Integer res = random.nextInt(n);

Integer res = (int)(Math.random() * n);

// random.nextInt(n)
public int nextInt(int n) {
  if (n <= 0)
    throw new IllegalArgumentException("n must be positive");
  if ((n & -n) == n) // i.e., n is a power of 2
    return (int)((n * (long)next(31)) >> 31);
  int bits, val;
  do {
    bits = next(31);
    val = bits % n;
  } while (bits - val + (n-1) < 0);
  return val;
}

// Math.random()
public static double random() {
  Random rnd = randomNumberGenerator;
  // 第一次调用，生成一个随机数生成器
  if (rnd == null) rnd = initRNG();
  return rnd.nextDouble();
}
// 生成的方法为同步的，线程安全
private static synchronized Random initRNG() {
 Random rnd = randomNumberGenerator;
   return (rnd == null) ? (randomNumberGenerator = new Random()) : rnd;
 }
// 该方法为 Random 类中的方法
public double nextDouble() {
  return (((long)(next(26)) << 27) + next(27))
    / (double)(1L << 53);
}

@Test
public void createProjectNo() {
   Random r1 = new Random(100);
   Random r2 = new Random(100);
   for (int i = 0; i < 100; i ++) {
     System.out.println(r1.nextInt(10)+", "+r2.nextInt(10));
   }
 }

public Random() {
 this(seedUniquifier() ^ System.nanoTime());
}
public Random(long seed) {
  if (getClass() == Random.class)
    this.seed = new AtomicLong(initialScramble(seed));
  else {
    // subclass might have overriden setSeed
    this.seed = new AtomicLong();
    setSeed(seed);
  }
}