Class Random

Method Details

• from

  public static Random from(RandomGenerator generator)

  Returns an instance of Random that delegates method calls to the RandomGenerator argument. If the generator is an instance of Random, it is returned. Otherwise, this method returns an instance of Random that delegates all methods except setSeed to the generator. The returned instance's setSeed method always throws UnsupportedOperationException. The returned instance is not serializable.

  Parameters:

  generator - the RandomGenerator calls are delegated to

  Returns:

  the delegating Random instance

  Throws:

  NullPointerException - if generator is null

  Since:

  19

• setSeed

  public void setSeed(long seed)

  Sets or updates the seed of this random number generator using the provided long seed value (optional operation).

  Implementation Requirements:

  The implementation in this class alters the state of this random number generator so that it is in the same state as if it had just been created with new Random(seed). It atomically updates the seed to

  (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1)

  and clears the haveNextNextGaussian flag used by nextGaussian(). Note that this uses only 48 bits of the given seed value.

  Parameters:

  seed - the seed value

  Throws:

  UnsupportedOperationException - if the setSeed operation is not supported by this random number generator

• next

  protected int next(int bits)

  Generates the next pseudorandom number. This method returns an int value such that, if the argument bits is between 1 and 32 (inclusive), then that many low-order bits of the returned value will be (approximately) independently chosen bit values, each of which is (approximately) equally likely to be 0 or 1.

  API Note:

  The other random-producing methods in this class are implemented in terms of this method, so subclasses can override just this method to provide a different source of pseudorandom numbers for the entire class.

  Implementation Requirements:

  The implementation in this class atomically updates the seed to

  (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)

  and returns

  (int)(seed >>> (48 - bits)).

  This is a linear congruential pseudorandom number generator, as defined by D. H. Lehmer and described by Donald E. Knuth in The Art of Computer Programming, Volume 2, Third edition: Seminumerical Algorithms, section 3.2.1.

  Parameters:

  bits - random bits

  Returns:

  the next pseudorandom value from this random number generator's sequence

  Since:

  1.1

• nextBytes

  public void nextBytes(byte[] bytes)

  Generates random bytes and places them into a user-supplied byte array. The number of random bytes produced is equal to the length of the byte array.

  Specified by:

  nextBytes in interface RandomGenerator

  Implementation Requirements:

  The method nextBytes is implemented by class Random as if by:

  
   public void nextBytes(byte[] bytes) {
     for (int i = 0; i < bytes.length; )
       for (int rnd = nextInt(), n = Math.min(bytes.length - i, 4);
            n-- > 0; rnd >>= 8)
         bytes[i++] = (byte)rnd;
   }

  Parameters:

  bytes - the byte array to fill with random bytes

  Throws:

  NullPointerException - if the byte array is null

  Since:

  1.1

• nextInt

  public int nextInt()

  Returns the next pseudorandom, uniformly distributed int value from this random number generator's sequence. The general contract of nextInt is that one int value is pseudorandomly generated and returned. All 2³² possible int values are produced with (approximately) equal probability.

  Specified by:

  nextInt in interface RandomGenerator

  Implementation Requirements:

  The method nextInt is implemented by class Random as if by:

  
   public int nextInt() {
     return next(32);
   }

  Returns:

  the next pseudorandom, uniformly distributed int value from this random number generator's sequence

• nextInt

  public int nextInt(int bound)

  Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence. The general contract of nextInt is that one int value in the specified range is pseudorandomly generated and returned. All bound possible int values are produced with (approximately) equal probability.

  Specified by:

  nextInt in interface RandomGenerator

  Implementation Requirements:

  The method nextInt(int bound) is implemented by class Random as if by:

  
   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;
   }

  The hedge "approximately" is used in the foregoing description only because the next method is only approximately an unbiased source of independently chosen bits. If it were a perfect source of randomly chosen bits, then the algorithm shown would choose int values from the stated range with perfect uniformity.

  The algorithm is slightly tricky. It rejects values that would result in an uneven distribution (due to the fact that 2^31 is not divisible by n). The probability of a value being rejected depends on n. The worst case is n=2^30+1, for which the probability of a reject is 1/2, and the expected number of iterations before the loop terminates is 2.

  The algorithm treats the case where n is a power of two specially: it returns the correct number of high-order bits from the underlying pseudo-random number generator. In the absence of special treatment, the correct number of low-order bits would be returned. Linear congruential pseudo-random number generators such as the one implemented by this class are known to have short periods in the sequence of values of their low-order bits. Thus, this special case greatly increases the length of the sequence of values returned by successive calls to this method if n is a small power of two.

  Parameters:

  bound - the upper bound (exclusive). Must be positive.

  Returns:

  the next pseudorandom, uniformly distributed int value between zero (inclusive) and bound (exclusive) from this random number generator's sequence

  Throws:

  IllegalArgumentException - if bound is not positive

  Since:

  1.2

• nextLong

  public long nextLong()

  Returns the next pseudorandom, uniformly distributed long value from this random number generator's sequence. The general contract of nextLong is that one long value is pseudorandomly generated and returned.

  Specified by:

  nextLong in interface RandomGenerator

  Implementation Requirements:

  The method nextLong is implemented by class Random as if by:

  
   public long nextLong() {
     return ((long)next(32) << 32) + next(32);
   }

  Because class Random uses a seed with only 48 bits, this algorithm will not return all possible long values.

  Returns:

  the next pseudorandom, uniformly distributed long value from this random number generator's sequence

• nextBoolean

  public boolean nextBoolean()

  Returns the next pseudorandom, uniformly distributed boolean value from this random number generator's sequence. The general contract of nextBoolean is that one boolean value is pseudorandomly generated and returned. The values true and false are produced with (approximately) equal probability.

  Specified by:

  nextBoolean in interface RandomGenerator

  Implementation Requirements:

  The method nextBoolean is implemented by class Random as if by:

  
   public boolean nextBoolean() {
     return next(1) != 0;
   }

  Returns:

  the next pseudorandom, uniformly distributed boolean value from this random number generator's sequence

  Since:

  1.2

• nextFloat

  public float nextFloat()

  Returns the next pseudorandom, uniformly distributed float value between 0.0 and 1.0 from this random number generator's sequence.

  The general contract of nextFloat is that one float value, chosen (approximately) uniformly from the range 0.0f (inclusive) to 1.0f (exclusive), is pseudorandomly generated and returned. All 2²⁴ possible float values of the form m x 2^-24, where m is a positive integer less than 2²⁴, are produced with (approximately) equal probability.

  Specified by:

  nextFloat in interface RandomGenerator

  Implementation Requirements:

  The method nextFloat is implemented by class Random as if by:

  
   public float nextFloat() {
     return next(24) / ((float)(1 << 24));
   }

  The hedge "approximately" is used in the foregoing description only because the next method is only approximately an unbiased source of independently chosen bits. If it were a perfect source of randomly chosen bits, then the algorithm shown would choose float values from the stated range with perfect uniformity.

  [In early versions of Java, the result was incorrectly calculated as:

   return next(30) / ((float)(1 << 30));

  This might seem to be equivalent, if not better, but in fact it introduced a slight nonuniformity because of the bias in the rounding of floating-point numbers: it was slightly more likely that the low-order bit of the significand would be 0 than that it would be 1.]

  Returns:

  the next pseudorandom, uniformly distributed float value between 0.0f and 1.0f from this random number generator's sequence

• nextDouble

  public double nextDouble()

  Returns the next pseudorandom, uniformly distributed double value between 0.0 and 1.0 from this random number generator's sequence.

  The general contract of nextDouble is that one double value, chosen (approximately) uniformly from the range 0.0d (inclusive) to 1.0d (exclusive), is pseudorandomly generated and returned.

  Specified by:

  nextDouble in interface RandomGenerator

  Implementation Requirements:

  The method nextDouble is implemented by class Random as if by:

  
   public double nextDouble() {
     return (((long)next(26) << 27) + next(27))
       / (double)(1L << 53);
   }

  The hedge "approximately" is used in the foregoing description only because the next method is only approximately an unbiased source of independently chosen bits. If it were a perfect source of randomly chosen bits, then the algorithm shown would choose double values from the stated range with perfect uniformity.

  [In early versions of Java, the result was incorrectly calculated as:

   return (((long)next(27) << 27) + next(27)) / (double)(1L << 54);

  This might seem to be equivalent, if not better, but in fact it introduced a large nonuniformity because of the bias in the rounding of floating-point numbers: it was three times as likely that the low-order bit of the significand would be 0 than that it would be 1! This nonuniformity probably doesn't matter much in practice, but we strive for perfection.]

  Returns:

  the next pseudorandom, uniformly distributed double value between 0.0 and 1.0 from this random number generator's sequence

  See Also:

  • Math.random()

• nextGaussian

  public double nextGaussian()

  Returns the next pseudorandom, Gaussian ("normally") distributed double value with mean 0.0 and standard deviation 1.0 from this random number generator's sequence.

  The general contract of nextGaussian is that one double value, chosen from (approximately) the usual normal distribution with mean 0.0 and standard deviation 1.0, is pseudorandomly generated and returned.

  Specified by:

  nextGaussian in interface RandomGenerator

  Implementation Requirements:

  The method nextGaussian is implemented by class Random as if by a threadsafe version of the following:

  
   private double nextNextGaussian;
   private boolean haveNextNextGaussian = false;
  
   public double nextGaussian() {
     if (haveNextNextGaussian) {
       haveNextNextGaussian = false;
       return nextNextGaussian;
     } else {
       double v1, v2, s;
       do {
         v1 = 2 * nextDouble() - 1;   // between -1.0 and 1.0
         v2 = 2 * nextDouble() - 1;   // between -1.0 and 1.0
         s = v1 * v1 + v2 * v2;
       } while (s >= 1 || s == 0);
       double multiplier = StrictMath.sqrt(-2 * StrictMath.log(s)/s);
       nextNextGaussian = v2 * multiplier;
       haveNextNextGaussian = true;
       return v1 * multiplier;
     }
   }

  This uses the polar method of G. E. P. Box, M. E. Muller, and G. Marsaglia, as described by Donald E. Knuth in The Art of Computer Programming, Volume 2, third edition: Seminumerical Algorithms, section 3.4.1, subsection C, algorithm P. Note that it generates two independent values at the cost of only one call to StrictMath.log and one call to StrictMath.sqrt.

  Returns:

  the next pseudorandom, Gaussian ("normally") distributed double value with mean 0.0 and standard deviation 1.0 from this random number generator's sequence

• ints

  public IntStream ints(long streamSize)

  Returns a stream producing the given streamSize number of pseudorandom int values.

  A pseudorandom int value is generated as if it's the result of calling the method nextInt().

  Specified by:

  ints in interface RandomGenerator

  Parameters:

  streamSize - the number of values to generate

  Returns:

  a stream of pseudorandom int values

  Throws:

  IllegalArgumentException - if streamSize is less than zero

  Since:

  1.8

• ints

  public IntStream ints()

  Returns an effectively unlimited stream of pseudorandom int values.

  A pseudorandom int value is generated as if it's the result of calling the method nextInt().

  Specified by:

  ints in interface RandomGenerator

  Implementation Note:

  This method is implemented to be equivalent to ints(Long.MAX_VALUE).

  Returns:

  a stream of pseudorandom int values

  Since:

  1.8

• ints

  public IntStream ints(long streamSize, int randomNumberOrigin, int randomNumberBound)

  Returns a stream producing the given streamSize number of pseudorandom int values, each conforming to the given origin (inclusive) and bound (exclusive).

  A pseudorandom int value is generated as if it's the result of calling the following method with the origin and bound:

   
   int nextInt(int origin, int bound) {
     int n = bound - origin;
     if (n > 0) {
       return nextInt(n) + origin;
     }
     else {  // range not representable as int
       int r;
       do {
         r = nextInt();
       } while (r < origin || r >= bound);
       return r;
     }
   }

  Specified by:

  ints in interface RandomGenerator

  Parameters:

  streamSize - the number of values to generate

  randomNumberOrigin - the origin (inclusive) of each random value

  randomNumberBound - the bound (exclusive) of each random value

  Returns:

  a stream of pseudorandom int values, each with the given origin (inclusive) and bound (exclusive)

  Throws:

  IllegalArgumentException - if streamSize is less than zero, or randomNumberOrigin is greater than or equal to randomNumberBound

  Since:

  1.8

• ints

  public IntStream ints(int randomNumberOrigin, int randomNumberBound)

  Returns an effectively unlimited stream of pseudorandom int values, each conforming to the given origin (inclusive) and bound (exclusive).

  A pseudorandom int value is generated as if it's the result of calling the following method with the origin and bound:

   
   int nextInt(int origin, int bound) {
     int n = bound - origin;
     if (n > 0) {
       return nextInt(n) + origin;
     }
     else {  // range not representable as int
       int r;
       do {
         r = nextInt();
       } while (r < origin || r >= bound);
       return r;
     }
   }

  Specified by:

  ints in interface RandomGenerator

  Implementation Note:

  This method is implemented to be equivalent to ints(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound).

  Parameters:

  randomNumberOrigin - the origin (inclusive) of each random value

  randomNumberBound - the bound (exclusive) of each random value

  Returns:

  a stream of pseudorandom int values, each with the given origin (inclusive) and bound (exclusive)

  Throws:

  IllegalArgumentException - if randomNumberOrigin is greater than or equal to randomNumberBound

  Since:

  1.8

• longs

  public LongStream longs(long streamSize)

  Returns a stream producing the given streamSize number of pseudorandom long values.

  A pseudorandom long value is generated as if it's the result of calling the method nextLong().

  Specified by:

  longs in interface RandomGenerator

  Parameters:

  streamSize - the number of values to generate

  Returns:

  a stream of pseudorandom long values

  Throws:

  IllegalArgumentException - if streamSize is less than zero

  Since:

  1.8

• longs

  public LongStream longs()

  Returns an effectively unlimited stream of pseudorandom long values.

  A pseudorandom long value is generated as if it's the result of calling the method nextLong().

  Specified by:

  longs in interface RandomGenerator

  Implementation Note:

  This method is implemented to be equivalent to longs(Long.MAX_VALUE).

  Returns:

  a stream of pseudorandom long values

  Since:

  1.8

• longs

  public LongStream longs(long streamSize, long randomNumberOrigin, long randomNumberBound)

  Returns a stream producing the given streamSize number of pseudorandom long, each conforming to the given origin (inclusive) and bound (exclusive).

  A pseudorandom long value is generated as if it's the result of calling the following method with the origin and bound:

   
   long nextLong(long origin, long bound) {
     long r = nextLong();
     long n = bound - origin, m = n - 1;
     if ((n & m) == 0L)  // power of two
       r = (r & m) + origin;
     else if (n > 0L) {  // reject over-represented candidates
       for (long u = r >>> 1;            // ensure nonnegative
            u + m - (r = u % n) < 0L;    // rejection check
            u = nextLong() >>> 1) // retry
           ;
       r += origin;
     }
     else {              // range not representable as long
       while (r < origin || r >= bound)
         r = nextLong();
     }
     return r;
   }

  Specified by:

  longs in interface RandomGenerator

  Parameters:

  streamSize - the number of values to generate

  randomNumberOrigin - the origin (inclusive) of each random value

  randomNumberBound - the bound (exclusive) of each random value

  Returns:

  a stream of pseudorandom long values, each with the given origin (inclusive) and bound (exclusive)

  Throws:

  IllegalArgumentException - if streamSize is less than zero, or randomNumberOrigin is greater than or equal to randomNumberBound

  Since:

  1.8

• longs

  public LongStream longs(long randomNumberOrigin, long randomNumberBound)

  Returns an effectively unlimited stream of pseudorandom long values, each conforming to the given origin (inclusive) and bound (exclusive).

  A pseudorandom long value is generated as if it's the result of calling the following method with the origin and bound:

   
   long nextLong(long origin, long bound) {
     long r = nextLong();
     long n = bound - origin, m = n - 1;
     if ((n & m) == 0L)  // power of two
       r = (r & m) + origin;
     else if (n > 0L) {  // reject over-represented candidates
       for (long u = r >>> 1;            // ensure nonnegative
            u + m - (r = u % n) < 0L;    // rejection check
            u = nextLong() >>> 1) // retry
           ;
       r += origin;
     }
     else {              // range not representable as long
       while (r < origin || r >= bound)
         r = nextLong();
     }
     return r;
   }

  Specified by:

  longs in interface RandomGenerator

  Implementation Note:

  This method is implemented to be equivalent to longs(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound).

  Parameters:

  randomNumberOrigin - the origin (inclusive) of each random value

  randomNumberBound - the bound (exclusive) of each random value

  Returns:

  a stream of pseudorandom long values, each with the given origin (inclusive) and bound (exclusive)

  Throws:

  IllegalArgumentException - if randomNumberOrigin is greater than or equal to randomNumberBound

  Since:

  1.8

• doubles

  public DoubleStream doubles(long streamSize)

  Returns a stream producing the given streamSize number of pseudorandom double values, each between zero (inclusive) and one (exclusive).

  A pseudorandom double value is generated as if it's the result of calling the method nextDouble().

  Specified by:

  doubles in interface RandomGenerator

  Parameters:

  streamSize - the number of values to generate

  Returns:

  a stream of double values

  Throws:

  IllegalArgumentException - if streamSize is less than zero

  Since:

  1.8

• doubles

  public DoubleStream doubles()

  Returns an effectively unlimited stream of pseudorandom double values, each between zero (inclusive) and one (exclusive).

  A pseudorandom double value is generated as if it's the result of calling the method nextDouble().

  Specified by:

  doubles in interface RandomGenerator

  Implementation Note:

  This method is implemented to be equivalent to doubles(Long.MAX_VALUE).

  Returns:

  a stream of pseudorandom double values

  Since:

  1.8

• doubles

  public DoubleStream doubles(long streamSize, double randomNumberOrigin, double randomNumberBound)

  Returns a stream producing the given streamSize number of pseudorandom double values, each conforming to the given origin (inclusive) and bound (exclusive).

  Specified by:

  doubles in interface RandomGenerator

  Parameters:

  streamSize - the number of values to generate

  randomNumberOrigin - the origin (inclusive) of each random value

  randomNumberBound - the bound (exclusive) of each random value

  Returns:

  a stream of pseudorandom double values, each with the given origin (inclusive) and bound (exclusive)

  Throws:

  IllegalArgumentException - if streamSize is less than zero, or randomNumberOrigin is not finite, or randomNumberBound is not finite, or randomNumberOrigin is greater than or equal to randomNumberBound

  Since:

  1.8

• doubles

  public DoubleStream doubles(double randomNumberOrigin, double randomNumberBound)

  Returns an effectively unlimited stream of pseudorandom double values, each conforming to the given origin (inclusive) and bound (exclusive).

  Specified by:

  doubles in interface RandomGenerator

  Implementation Note:

  This method is implemented to be equivalent to doubles(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound).

  Parameters:

  randomNumberOrigin - the origin (inclusive) of each random value

  randomNumberBound - the bound (exclusive) of each random value

  Returns:

  a stream of pseudorandom double values, each with the given origin (inclusive) and bound (exclusive)

  Throws:

  IllegalArgumentException - if randomNumberOrigin is not finite, or randomNumberBound is not finite, or randomNumberOrigin is greater than or equal to randomNumberBound

  Since:

  1.8