public class Random extends Object implements Serializable
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.
| Constructor | Description |
|---|---|
Random () |
Creates a new random number generator.
|
Random (long seed) |
Creates a new random number generator using a single
long seed. |
| Modifier and Type | Method | Description |
|---|---|---|
DoubleStream |
doubles () |
Returns an effectively unlimited stream of pseudorandom
double values, each between zero (inclusive) and one
(exclusive). |
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). |
DoubleStream |
doubles (long streamSize) |
Returns a stream producing the given
streamSize number of
pseudorandom double values, each between zero
(inclusive) and one (exclusive). |
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). |
IntStream |
ints () |
Returns an effectively unlimited stream of pseudorandom
int
values. |
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). |
IntStream |
ints (long streamSize) |
Returns a stream producing the given
streamSize number of
pseudorandom int values. |
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). |
LongStream |
longs () |
Returns an effectively unlimited stream of pseudorandom
long
values. |
LongStream |
longs (long streamSize) |
Returns a stream producing the given
streamSize number of
pseudorandom long values. |
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). |
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). |
protected int |
next (int bits) |
Generates the next pseudorandom number.
|
boolean |
nextBoolean () |
Returns the next pseudorandom, uniformly distributed
boolean value from this random number generator's
sequence. |
void |
nextBytes (byte[] bytes) |
Generates random bytes and places them into a user-supplied
byte array.
|
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. |
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. |
int |
nextInt () |
Returns the next pseudorandom, uniformly distributed
int
value from this random number generator's sequence. |
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. |
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. |
public Random()
public Random(long seed)
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);seed - the initial seedsetSeed(long) public void setSeed(long seed)
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)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.
seed - the initial seedprotected int next(int bits)
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) (int)(seed >>> (48 - bits)).bits - random bitspublic void nextBytes(byte[] bytes)
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;
}bytes - the byte array to fill with random bytesNullPointerException - if the byte array is nullpublic int nextInt()
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);
}int
value from this random number generator's sequencepublic int nextInt(int bound)
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. 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.
bound - the upper bound (exclusive). Must be positive.int
value between zero (inclusive) and bound (exclusive)
from this random number generator's sequenceIllegalArgumentException - if bound is not positivepublic long nextLong()
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.long
value from this random number generator's sequencepublic boolean nextBoolean()
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;
}boolean value from this random number generator's
sequencepublic float nextFloat()
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.]float
value between 0.0 and 1.0 from this
random number generator's sequencepublic double nextDouble()
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.]double
value between 0.0 and 1.0 from this
random number generator's sequenceMath.random() public double nextGaussian()
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 3: 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.double value with mean 0.0 and
standard deviation 1.0 from this random number
generator's sequencepublic IntStream ints(long streamSize)
streamSize number of
pseudorandom int values.
A pseudorandom int value is generated as if it's the result of
calling the method nextInt().
streamSize - the number of values to generateint valuesIllegalArgumentException - if streamSize is
less than zeropublic IntStream ints()
int
values.
A pseudorandom int value is generated as if it's the result of
calling the method nextInt().
ints(Long.MAX_VALUE).int valuespublic IntStream ints(long streamSize, int randomNumberOrigin, int randomNumberBound)
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;
}
}streamSize - the number of values to generaterandomNumberOrigin - the origin (inclusive) of each random valuerandomNumberBound - the bound (exclusive) of each random valueint values,
each with the given origin (inclusive) and bound (exclusive)IllegalArgumentException - if streamSize is
less than zero, or randomNumberOrigin
is greater than or equal to randomNumberBoundpublic IntStream ints(int randomNumberOrigin, int randomNumberBound)
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;
}
}ints(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound).randomNumberOrigin - the origin (inclusive) of each random valuerandomNumberBound - the bound (exclusive) of each random valueint values,
each with the given origin (inclusive) and bound (exclusive)IllegalArgumentException - if randomNumberOrigin
is greater than or equal to randomNumberBoundpublic LongStream longs(long streamSize)
streamSize number of
pseudorandom long values.
A pseudorandom long value is generated as if it's the result
of calling the method nextLong().
streamSize - the number of values to generatelong valuesIllegalArgumentException - if streamSize is
less than zeropublic LongStream longs()
long
values.
A pseudorandom long value is generated as if it's the result
of calling the method nextLong().
longs(Long.MAX_VALUE).long valuespublic LongStream longs(long streamSize, long randomNumberOrigin, long randomNumberBound)
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;
}streamSize - the number of values to generaterandomNumberOrigin - the origin (inclusive) of each random valuerandomNumberBound - the bound (exclusive) of each random valuelong values,
each with the given origin (inclusive) and bound (exclusive)IllegalArgumentException - if streamSize is
less than zero, or randomNumberOrigin
is greater than or equal to randomNumberBoundpublic LongStream longs(long randomNumberOrigin, long randomNumberBound)
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;
}longs(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound).randomNumberOrigin - the origin (inclusive) of each random valuerandomNumberBound - the bound (exclusive) of each random valuelong values,
each with the given origin (inclusive) and bound (exclusive)IllegalArgumentException - if randomNumberOrigin
is greater than or equal to randomNumberBoundpublic DoubleStream doubles(long streamSize)
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().
streamSize - the number of values to generatedouble valuesIllegalArgumentException - if streamSize is
less than zeropublic DoubleStream doubles()
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().
doubles(Long.MAX_VALUE).double valuespublic DoubleStream doubles(long streamSize, double randomNumberOrigin, double randomNumberBound)
streamSize number of
pseudorandom double values, each conforming to the given origin
(inclusive) and bound (exclusive).
A pseudorandom double value is generated as if it's the result
of calling the following method with the origin and bound:
double nextDouble(double origin, double bound) {
double r = nextDouble();
r = r * (bound - origin) + origin;
if (r >= bound) // correct for rounding
r = Math.nextDown(bound);
return r;
}streamSize - the number of values to generaterandomNumberOrigin - the origin (inclusive) of each random valuerandomNumberBound - the bound (exclusive) of each random valuedouble values,
each with the given origin (inclusive) and bound (exclusive)IllegalArgumentException - if streamSize is
less than zeroIllegalArgumentException - if randomNumberOrigin
is greater than or equal to randomNumberBoundpublic DoubleStream doubles(double randomNumberOrigin, double randomNumberBound)
double values, each conforming to the given origin (inclusive) and bound
(exclusive).
A pseudorandom double value is generated as if it's the result
of calling the following method with the origin and bound:
double nextDouble(double origin, double bound) {
double r = nextDouble();
r = r * (bound - origin) + origin;
if (r >= bound) // correct for rounding
r = Math.nextDown(bound);
return r;
}doubles(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound).randomNumberOrigin - the origin (inclusive) of each random valuerandomNumberBound - the bound (exclusive) of each random valuedouble values,
each with the given origin (inclusive) and bound (exclusive)IllegalArgumentException - if randomNumberOrigin
is greater than or equal to randomNumberBound Submit a bug or feature
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