Random
public
class
Random
extends Object
implements
RandomGenerator,
Serializable
java.lang.Object | |
↳ | java.util.Random |
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 E. Knuth, The Art of Computer Programming, Volume 2, Third edition: Seminumerical Algorithms, 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.
Summary
Public constructors | |
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Random()
Creates a new random number generator. |
|
Random(long seed)
Creates a new random number generator using a single |
Public methods | |
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DoubleStream
|
doubles(long streamSize)
Returns a stream producing the given |
DoubleStream
|
doubles()
Returns an effectively unlimited stream of pseudorandom |
DoubleStream
|
doubles(double randomNumberOrigin, double randomNumberBound)
Returns an effectively unlimited stream of pseudorandom |
DoubleStream
|
doubles(long streamSize, double randomNumberOrigin, double randomNumberBound)
Returns a stream producing the given |
IntStream
|
ints(long streamSize)
Returns a stream producing the given |
IntStream
|
ints(long streamSize, int randomNumberOrigin, int randomNumberBound)
Returns a stream producing the given |
IntStream
|
ints(int randomNumberOrigin, int randomNumberBound)
Returns an effectively unlimited stream of pseudorandom |
IntStream
|
ints()
Returns an effectively unlimited stream of pseudorandom |
LongStream
|
longs()
Returns an effectively unlimited stream of pseudorandom |
LongStream
|
longs(long streamSize)
Returns a stream producing the given |
LongStream
|
longs(long randomNumberOrigin, long randomNumberBound)
Returns an effectively unlimited stream of pseudorandom |
LongStream
|
longs(long streamSize, long randomNumberOrigin, long randomNumberBound)
Returns a stream producing the given |
boolean
|
nextBoolean()
Returns the next pseudorandom, uniformly distributed
|
void
|
nextBytes(byte[] bytes)
Generates random bytes and places them into a user-supplied byte array. |
double
|
nextDouble()
Returns the next pseudorandom, uniformly distributed
|
float
|
nextFloat()
Returns the next pseudorandom, uniformly distributed |
double
|
nextGaussian()
Returns the next pseudorandom, Gaussian ("normally") distributed
|
int
|
nextInt()
Returns the next pseudorandom, uniformly distributed |
int
|
nextInt(int bound)
Returns a pseudorandom, uniformly distributed |
long
|
nextLong()
Returns the next pseudorandom, uniformly distributed |
void
|
setSeed(long seed)
Sets the seed of this random number generator using a single
|
Protected methods | |
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int
|
next(int bits)
Generates the next pseudorandom number. |
Inherited methods | |
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Public constructors
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)
.
Implementation Requirements:
- The invocation
new Random(seed)
is equivalent to:Random rnd = new Random(); rnd.setSeed(seed);
Parameters | |
---|---|
seed |
long : the initial seed |
See also:
Public methods
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()
.
Parameters | |
---|---|
streamSize |
long : the number of values to generate |
Returns | |
---|---|
DoubleStream |
a stream of double values |
Throws | |
---|---|
IllegalArgumentException |
if streamSize is
less than zero |
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()
.
Implementation Note:
- This method is implemented to be equivalent to
doubles(Long.MAX_VALUE)
.
Returns | |
---|---|
DoubleStream |
a stream of pseudorandom double values |
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).
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;
}
Implementation Note:
- This method is implemented to be equivalent to
doubles(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)
.
Parameters | |
---|---|
randomNumberOrigin |
double : the origin (inclusive) of each random value |
randomNumberBound |
double : the bound (exclusive) of each random value |
Returns | |
---|---|
DoubleStream |
a stream of pseudorandom double values,
each with the given origin (inclusive) and bound (exclusive) |
Throws | |
---|---|
IllegalArgumentException |
if randomNumberOrigin
is greater than or equal to randomNumberBound |
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).
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;
}
Parameters | |
---|---|
streamSize |
long : the number of values to generate |
randomNumberOrigin |
double : the origin (inclusive) of each random value |
randomNumberBound |
double : the bound (exclusive) of each random value |
Returns | |
---|---|
DoubleStream |
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 |
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()
.
Parameters | |
---|---|
streamSize |
long : the number of values to generate |
Returns | |
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IntStream |
a stream of pseudorandom int values |
Throws | |
---|---|
IllegalArgumentException |
if streamSize is
less than zero |
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;
}
}
Parameters | |
---|---|
streamSize |
long : the number of values to generate |
randomNumberOrigin |
int : the origin (inclusive) of each random value |
randomNumberBound |
int : the bound (exclusive) of each random value |
Returns | |
---|---|
IntStream |
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 |
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;
}
}
Implementation Note:
- This method is implemented to be equivalent to
ints(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)
.
Parameters | |
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randomNumberOrigin |
int : the origin (inclusive) of each random value |
randomNumberBound |
int : the bound (exclusive) of each random value |
Returns | |
---|---|
IntStream |
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 |
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()
.
Implementation Note:
- This method is implemented to be equivalent to
ints(Long.MAX_VALUE)
.
Returns | |
---|---|
IntStream |
a stream of pseudorandom int values |
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()
.
Implementation Note:
- This method is implemented to be equivalent to
longs(Long.MAX_VALUE)
.
Returns | |
---|---|
LongStream |
a stream of pseudorandom long values |
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()
.
Parameters | |
---|---|
streamSize |
long : the number of values to generate |
Returns | |
---|---|
LongStream |
a stream of pseudorandom long values |
Throws | |
---|---|
IllegalArgumentException |
if streamSize is
less than zero |
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;
}
Implementation Note:
- This method is implemented to be equivalent to
longs(Long.MAX_VALUE, randomNumberOrigin, randomNumberBound)
.
Parameters | |
---|---|
randomNumberOrigin |
long : the origin (inclusive) of each random value |
randomNumberBound |
long : the bound (exclusive) of each random value |
Returns | |
---|---|
LongStream |
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 |
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;
}
Parameters | |
---|---|
streamSize |
long : the number of values to generate |
randomNumberOrigin |
long : the origin (inclusive) of each random value |
randomNumberBound |
long : the bound (exclusive) of each random value |
Returns | |
---|---|
LongStream |
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 |
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.
Implementation Requirements:
- The method
nextBoolean
is implemented by classRandom
as if by:public boolean nextBoolean() { return next(1) != 0; }
Returns | |
---|---|
boolean |
the next pseudorandom, uniformly distributed
boolean value from this random number generator's
sequence |
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.
Implementation Requirements:
- The method
nextBytes
is implemented by classRandom
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 |
byte : the byte array to fill with random bytes |
Throws | |
---|---|
NullPointerException |
if the byte array is null |
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.
Implementation Requirements:
- The method
nextDouble
is implemented by classRandom
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 choosedouble
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);
Returns | |
---|---|
double |
the next pseudorandom, uniformly distributed double
value between 0.0 and 1.0 from this
random number generator's sequence |
See also:
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.
Implementation Requirements:
- The method
nextFloat
is implemented by classRandom
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));
Returns | |
---|---|
float |
the next pseudorandom, uniformly distributed float
value between 0.0 and 1.0 from this
random number generator's sequence |
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.
Implementation Requirements:
- The method
nextGaussian
is implemented by classRandom
as if by a threadsafe version of the following:
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 toprivate 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; } }
StrictMath.log
and one call toStrictMath.sqrt
.
Returns | |
---|---|
double |
the next pseudorandom, Gaussian ("normally") distributed
double value with mean 0.0 and
standard deviation 1.0 from this random number
generator's sequence |
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.
Implementation Requirements:
- The method
nextInt
is implemented by classRandom
as if by:public int nextInt() { return next(32); }
Returns | |
---|---|
int |
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.
Implementation Requirements:
- The method
nextInt(int bound)
is implemented by classRandom
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 |
int : the upper bound (exclusive). Must be positive. |
Returns | |
---|---|
int |
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 |
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.
Implementation Requirements:
- The method
nextLong
is implemented by classRandom
as if by:
Because classpublic long nextLong() { return ((long)next(32) << 32) + next(32); }
Random
uses a seed with only 48 bits, this algorithm will not return all possiblelong
values.
Returns | |
---|---|
long |
the next pseudorandom, uniformly distributed long
value from this random number generator's sequence |
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 |
long : the initial seed |
Protected methods
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, Third edition:
Seminumerical Algorithms, section 3.2.1.
Parameters | |
---|---|
bits |
int : random bits |
Returns | |
---|---|
int |
the next pseudorandom value from this random number generator's sequence |