[Java] 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);
}

• math.random() 方法生成[0, 1)范围内的double类型随机数；random类中的nextxxxx系列方法生成0－n的随机数；
  
• math.random() 线程安全，多线程环境能被调用；
  
• 如无特殊需求，则使用(int)(math.random()*n)的方式生成随机数即可。
  

@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);
  }
}