java::util::Random Class Reference

Inheritance diagram for java::util::Random:

Inheritance graph
java::lang::Object
[legend]
Collaboration diagram for java::util::Random:

Collaboration graph
java::lang::Objectjava::lang::ObjectRefjava::lang::Interfacejava::lang::Comparable
[legend]

List of all members.


Detailed Description

An instance of this class is used to generate a stream of pseudorandom numbers.

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 random method in class Math simpler to use.

Author:
Frank Yellin
Version:
1.34, 02/02/00
See also:
java.lang.Math::random()
Since:
JDK1.0

Public Member Functions

 Random ()
 Creates a new random number generator.
 Random (jlong seed)
 Creates a new random number generator using a single long seed: <blockquote>.
virtual void setSeed (jlong seed)
 Sets the seed of this random number generator using a single long seed.
virtual void nextBytes (Array< jbyte > &bytes)
 Generates random bytes and places them into a user-supplied byte array.
virtual jint nextInt ()
 Returns the next pseudorandom, uniformly distributed int value from this random number generator's sequence.
virtual jint nextInt (jint n)
 Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence.
virtual jlong nextLong ()
 Returns the next pseudorandom, uniformly distributed long value from this random number generator's sequence.
virtual jboolean nextBoolean ()
 Returns the next pseudorandom, uniformly distributed boolean value from this random number generator's sequence.
virtual jfloat nextFloat ()
 Returns the next pseudorandom, uniformly distributed float value between 0.0 and 1.0 from this random number generator's sequence.
virtual jdouble nextDouble ()
 Returns the next pseudorandom, uniformly distributed double value between 0.0 and 1.0 from this random number generator's sequence.
virtual jdouble 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.

Protected Member Functions

virtual jint next (jint bits)
 Generates the next pseudorandom number.

Constructor & Destructor Documentation

java::util::Random::Random (  ) 

Creates a new random number generator.

Its seed is initialized to a value based on the current time: <blockquote>

 public Random() { this(System.currentTimeMillis()); }
</blockquote>

java::util::Random::Random ( jlong  seed  ) 

Creates a new random number generator using a single long seed: <blockquote>.

 public Random(long seed) { setSeed(seed); }
</blockquote> Used by method next to hold the state of the pseudorandom number generator.

Parameters:
seed the initial seed.


Member Function Documentation

virtual void java::util::Random::setSeed ( jlong  seed  )  [virtual]

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 as follows: <blockquote>

 synchronized public void setSeed(long seed) {
       this.seed = (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
       haveNextNextGaussian = false;
 }
</blockquote> 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.

virtual void java::util::Random::nextBytes ( Array< jbyte > &  bytes  )  [virtual]

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.

Parameters:
bytes the non-null byte array in which to put the random bytes.
Since:
JDK1.1

virtual jint java::util::Random::nextInt (  )  [virtual]

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 2<font size="-1">32 </font> possible int values are produced with (approximately) equal probability. The method nextInt is implemented by class Random as follows: <blockquote>

 public int nextInt() {  return next(32); }
</blockquote>

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

virtual jint java::util::Random::nextInt ( jint  n  )  [virtual]

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 n possible int values are produced with (approximately) equal probability. The method nextInt(int n) is implemented by class Random as follows: <blockquote>

 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;
 }
 
</blockquote>

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:
n the bound on the random number to be returned. Must be positive.
Returns:
a pseudorandom, uniformly distributed int value between 0 (inclusive) and n (exclusive).
Exceptions:
IllegalArgumentException n is not positive.
Since:
1.2

virtual jlong java::util::Random::nextLong (  )  [virtual]

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. All 2<font size="-1">64</font> possible long values are produced with (approximately) equal probability. The method nextLong is implemented by class Random as follows: <blockquote>

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

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

virtual jboolean java::util::Random::nextBoolean (  )  [virtual]

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 follows: <blockquote>

 public boolean nextBoolean() {return next(1) != 0;}
 
</blockquote>
Returns:
the next pseudorandom, uniformly distributed boolean value from this random number generator's sequence.
Since:
1.2

virtual jfloat java::util::Random::nextFloat (  )  [virtual]

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 2<font size="-1">24</font> possible float values of the form m x&nbsp2<font size="-1">-24</font>, where m is a positive integer less than 2<font size="-1">24 </font>, are produced with (approximately) equal probability. The method nextFloat is implemented by class Random as follows: <blockquote>

 public float nextFloat() {
      return next(24) / ((float)(1 << 24));
 }
</blockquote> 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 or 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: <blockquote>

 return next(30) / ((float)(1 << 30));
</blockquote> 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 0.0 and 1.0 from this random number generator's sequence.

virtual jdouble java::util::Random::nextDouble (  )  [virtual]

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. All 2<font size="-1">53</font> possible float values of the form m x 2<font size="-1">-53 </font>, where m is a positive integer less than 2<font size="-1">53</font>, are produced with (approximately) equal probability. The method nextDouble is implemented by class Random as follows: <blockquote>

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

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 or 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: <blockquote>

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

virtual jdouble java::util::Random::nextGaussian (  )  [virtual]

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 follows: <blockquote>

 synchronized 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 = Math.sqrt(-2 * Math.log(s)/s);
            nextNextGaussian = v2 * multiplier;
            haveNextNextGaussian = true;
            return v1 * multiplier;
    }
 }
</blockquote> 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 Math.log and one call to Math.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.

virtual jint java::util::Random::next ( jint  bits  )  [protected, virtual]

Generates the next pseudorandom number.

Subclass 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 as follows: <blockquote>

 synchronized protected int next(int bits) {
       seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
       return (int)(seed >>> (48 - bits));
 }
</blockquote> 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:
JDK1.1


The documentation for this class was generated from the following file:
Generated on Fri May 16 11:56:51 2008 for CrossPlatformJavaLikeC++API by  doxygen 1.5.3