Module java.base
Package java.util

Class Random

All Implemented Interfaces:
Serializable, RandomGenerator
Direct Known Subclasses:
SecureRandom, ThreadLocalRandom

                                                        public class Random
extends RandomSupport.AbstractSpliteratorGenerator
implements Serializable
                        
                        
An instance of this class is used to generate a stream of pseudorandom numbers; its period is only 248. The class uses a 48-bit seed, which is modified using a linear congruential formula. (See Donald Knuth, The Art of Computer Programming, Volume 2 , Section 3.2.1.)

If two instances of Random are created with the same seed, and the same sequence of method calls is made for each, they will generate and return identical sequences of numbers. In order to guarantee this property, particular algorithms are specified for the class Random. Java implementations must use all the algorithms shown here for the class Random, for the sake of absolute portability of Java code. However, subclasses of class Random are permitted to use other algorithms, so long as they adhere to the general contracts for all the methods.

The algorithms implemented by class Random use a protected utility method that on each invocation can supply up to 32 pseudorandomly generated bits.

Many applications will find the method Math.random() simpler to use.

Instances of java.util.Random are threadsafe. However, the concurrent use of the same java.util.Random instance across threads may encounter contention and consequent poor performance. Consider instead using ThreadLocalRandom in multithreaded designs.

Instances of java.util.Random are not cryptographically secure. Consider instead using SecureRandom to get a cryptographically secure pseudo-random number generator for use by security-sensitive applications.

Since:
1.0
See Also:
Serialized Form
  • Constructor Details

    • Random

      public Random()
      Creates a new random number generator. This constructor sets the seed of the random number generator to a value very likely to be distinct from any other invocation of this constructor.

    • Random

      public Random(long seed)
      Creates a new random number generator using a single long seed. The seed is the initial value of the internal state of the pseudorandom number generator which is maintained by method next(int).

      The invocation new Random(seed) is equivalent to:

                                                                                                                               
       Random rnd = new Random();
       rnd.setSeed(seed);
                                                              
                                                              

      Parameters:
      seed - the initial seed
      See Also:
      setSeed(long)

  • Method Details

    • setSeed

      public void setSeed(long seed)
      Sets the seed of this random number generator using a single long seed. The general contract of setSeed is that it alters the state of this random number generator object so as to be in exactly the same state as if it had just been created with the argument seed as a seed. The method setSeed is implemented by class Random by atomically updating the seed to
                                                                                                                      (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1)
                                                          
                                                          
      and clearing the haveNextNextGaussian flag used by nextGaussian().

      The implementation of setSeed by class Random happens to use only 48 bits of the given seed. In general, however, an overriding method may use all 64 bits of the long argument as a seed value.

      Parameters:
      seed - the initial seed

    • next

      protected int next(int bits)
      Generates the next pseudorandom number. Subclasses should override this, as this is used by all other methods.

      The general contract of next is that it returns an int value and 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. The method next is implemented by class Random by atomically updating the seed to

                                                                                                                              (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)
                                                              
                                                              
      and returning
                                                                                                                              (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: 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.

      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;
       }
                                                              
                                                              

      Specified by:
      nextBytes in interface RandomGenerator
      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.

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

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

      Specified by:
      nextInt in interface RandomGenerator
      Returns:
      the next pseudorandom, uniformly distributed int value from this random number generator's sequence

    • nextInt

      public int nextInt(int bound)
      Returns a pseudorandom int value between zero (inclusive) and the specified bound (exclusive).
      Specified by:
      nextInt in interface RandomGenerator
      Parameters:
      bound - the upper bound (exclusive). Must be positive.
      Returns:
      a pseudorandom int value between zero (inclusive) and the bound (exclusive)
      Throws:
      IllegalArgumentException - if bound is not positive

    • 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.

      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.

      Specified by:
      nextLong in interface RandomGenerator
      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.

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

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

      Specified by:
      nextBoolean in interface RandomGenerator
      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.

      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.]

      Specified by:
      nextFloat in interface RandomGenerator
      Returns:
      the next pseudorandom, uniformly distributed float value between 0.0 and 1.0 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.

      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.]

      Specified by:
      nextDouble in interface RandomGenerator
      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.

      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: 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.

      Specified by:
      nextGaussian in interface RandomGenerator
      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