Random
public
class
Random
extends Object
implements
Serializable
java.lang.Object  
↳  java.util.Random 
An instance of this class is used to generate a stream of pseudorandom numbers. The class uses a 48bit 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 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 pseudorandom number generator for use
by securitysensitive applications.
Summary
Public constructors  

Random()
Creates a new random number generator. 

Random(long seed)
Creates a new random number generator using a single 
Public methods  

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 usersupplied 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  

int

next(int bits)
Generates the next pseudorandom number. 
Inherited methods  

Public constructors
Random
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
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 
long : the initial seed 
See also:
Public methods
doubles
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
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
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
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 
IllegalArgumentException 
if randomNumberOrigin
is greater than or equal to randomNumberBound 
ints
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  

IntStream 
a stream of pseudorandom int values 
Throws  

IllegalArgumentException 
if streamSize is
less than zero 
ints
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
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  

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
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
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
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
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 overrepresented 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
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 overrepresented 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
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;
}
Returns  

boolean 
the next pseudorandom, uniformly distributed
boolean value from this random number generator's
sequence 
nextBytes
void nextBytes (byte[] bytes)
Generates random bytes and places them into a usersupplied 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;
}
Parameters  

bytes 
byte : the byte array to fill with random bytes 
Throws  

NullPointerException 
if the byte array is null 
nextDouble
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 floatingpoint numbers: it was three times as likely that the
loworder 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  

double 
the next pseudorandom, uniformly distributed double
value between 0.0 and 1.0 from this
random number generator's sequence 
See also:
nextFloat
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 2^{24} possible
float
values of the form m x 2^{24},
where m is a positive integer less than 2^{24}, 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 floatingpoint numbers: it was slightly more likely that the
loworder bit of the significand would be 0 than that it would be 1.]
Returns  

float 
the next pseudorandom, uniformly distributed float
value between 0.0 and 1.0 from this
random number generator's sequence

nextGaussian
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 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
.
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
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 2^{32} 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);
}
Returns  

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

nextInt
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. 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 + (bound1) < 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 highorder bits from the underlying pseudorandom number generator. In the absence of special treatment, the correct number of loworder bits would be returned. Linear congruential pseudorandom number generators such as the one implemented by this class are known to have short periods in the sequence of values of their loworder 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
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.
Returns  

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

setSeed
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
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 loworder
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 3:
Seminumerical Algorithms, section 3.2.1.
Parameters  

bits 
int : random bits 
Returns  

int 
the next pseudorandom value from this random number generator's sequence 