java.util
Class Random
java.lang.Object
java.util.Random
public class Random
- extends Object
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.
|
Constructor Summary |
Random()
Creates a new random number generator. |
Random(long seed)
Creates a new random number generator using a single long seed:
public Random(long seed) { setSeed(seed); }
Used by method next to hold the state of the pseudorandom number
generator. |
|
Method Summary |
protected int |
next(int bits)
Generates the next pseudorandom number. |
double |
nextDouble()
Returns the next pseudorandom, uniformly distributed double value
between 0.0 and 1.0 from this random number generator's sequence. |
float |
nextFloat()
Returns the next pseudorandom, uniformly distributed float value between 0.0
and 1.0 from this random number generator's sequence. |
int |
nextInt()
Returns the next pseudorandom, uniformly distributed int value from this
random number generator's sequence. |
int |
nextInt(int n)
Returns a pseudorandom, uniformly distributed int value between 0
(inclusive) and the specified value (exclusive), drawn from this random
number generator's sequence. |
long |
nextLong()
Returns the next pseudorandom, uniformly distributed long value from this
random number generator's sequence. |
void |
setSeed(long seed)
Sets the seed of this random number generator using a single long seed. |
Random
public Random()
- Creates a new random number generator. Its seed is initialized to a
value based on the current time:
public Random() { this(System.currentTimeMillis()); }
Random
public Random(long seed)
- Creates a new random number generator using a single long seed:
public Random(long seed) { setSeed(seed); }
Used by method next to hold the state of the pseudorandom number
generator.
next
protected int next(int bits)
- 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:
synchronized protected int next(int bits) {
0x5DEECE66DL + 0xBL) & ((1L < < 48) - 1);
return (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.
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. All 253 possible
float values of the form m x 2-53 , where m is a positive integer less
than 253, are produced with (approximately) equal probability. The
method nextDouble is implemented by class Random as follows:
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 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:
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.]
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 follows:
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 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:
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.]
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 follows:
public int nextInt() { return next(32); }
nextInt
public int nextInt(int n)
- 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:
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;
}
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.
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. All 264 possible
long values are produced with (approximately) equal probability. The method
nextLong is implemented by class Random as follows:
public long nextLong() {
return ((long)next(32) < < 32) + next(32);
}
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 as follows:
synchronized public void setSeed(long seed) {
this.seed = (seed ^ 0x5DEECE66DL) & ((1L < < 48) - 1);
}
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.