 All Implemented Interfaces:
Serializable
 Direct Known Subclasses:
SecureRandom
,ThreadLocalRandom
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 pseudorandom number generator for use
by securitysensitive applications.
 Since:
 1.0
 See Also:
 Serialized Form

Constructor Summary

Method Summary
Modifier and TypeMethodDescriptiondoubles()
Returns an effectively unlimited stream of pseudorandomdouble
values, each between zero (inclusive) and one (exclusive).doubles(double randomNumberOrigin, double randomNumberBound)
Returns an effectively unlimited stream of pseudorandomdouble
values, each conforming to the given origin (inclusive) and bound (exclusive).doubles(long streamSize)
Returns a stream producing the givenstreamSize
number of pseudorandomdouble
values, each between zero (inclusive) and one (exclusive).doubles(long streamSize, double randomNumberOrigin, double randomNumberBound)
Returns a stream producing the givenstreamSize
number of pseudorandomdouble
values, each conforming to the given origin (inclusive) and bound (exclusive).ints()
Returns an effectively unlimited stream of pseudorandomint
values.ints(int randomNumberOrigin, int randomNumberBound)
Returns an effectively unlimited stream of pseudorandomint
values, each conforming to the given origin (inclusive) and bound (exclusive).ints(long streamSize)
Returns a stream producing the givenstreamSize
number of pseudorandomint
values.ints(long streamSize, int randomNumberOrigin, int randomNumberBound)
Returns a stream producing the givenstreamSize
number of pseudorandomint
values, each conforming to the given origin (inclusive) and bound (exclusive).longs()
Returns an effectively unlimited stream of pseudorandomlong
values.longs(long streamSize)
Returns a stream producing the givenstreamSize
number of pseudorandomlong
values.longs(long randomNumberOrigin, long randomNumberBound)
Returns an effectively unlimited stream of pseudorandomlong
values, each conforming to the given origin (inclusive) and bound (exclusive).longs(long streamSize, long randomNumberOrigin, long randomNumberBound)
Returns a stream producing the givenstreamSize
number of pseudorandomlong
, each conforming to the given origin (inclusive) and bound (exclusive).protected int
next(int bits)
Generates the next pseudorandom number.boolean
Returns the next pseudorandom, uniformly distributedboolean
value from this random number generator's sequence.void
nextBytes(byte[] bytes)
Generates random bytes and places them into a usersupplied byte array.double
Returns the next pseudorandom, uniformly distributeddouble
value between0.0
and1.0
from this random number generator's sequence.float
Returns the next pseudorandom, uniformly distributedfloat
value between0.0
and1.0
from this random number generator's sequence.double
Returns the next pseudorandom, Gaussian ("normally") distributeddouble
value with mean0.0
and standard deviation1.0
from this random number generator's sequence.int
nextInt()
Returns the next pseudorandom, uniformly distributedint
value from this random number generator's sequence.int
nextInt(int bound)
Returns a pseudorandom, uniformly distributedint
value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence.long
nextLong()
Returns the next pseudorandom, uniformly distributedlong
value from this random number generator's sequence.void
setSeed(long seed)
Sets the seed of this random number generator using a singlelong
seed.

Constructor Details

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 singlelong
seed. The seed is the initial value of the internal state of the pseudorandom number generator which is maintained by methodnext(int)
.The invocation
new Random(seed)
is equivalent to:Random rnd = new Random(); rnd.setSeed(seed);
 Parameters:
seed
 the initial seed See Also:
setSeed(long)


Method Details

setSeed
public void setSeed(long seed)Sets the seed of this random number generator using a singlelong
seed. The general contract ofsetSeed
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 argumentseed
as a seed. The methodsetSeed
is implemented by classRandom
by atomically updating the seed to
and clearing the(seed ^ 0x5DEECE66DL) & ((1L << 48)  1)
haveNextNextGaussian
flag used bynextGaussian()
.The implementation of
setSeed
by classRandom
happens to use only 48 bits of the given seed. In general, however, an overriding method may use all 64 bits of thelong
argument as a seed value. Parameters:
seed
 the initial seed

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 anint
value and if the argumentbits
is between1
and32
(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 be0
or1
. The methodnext
is implemented by classRandom
by atomically updating the seed to
and returning(seed * 0x5DEECE66DL + 0xBL) & ((1L << 48)  1)
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.(int)(seed >>> (48  bits))
. Parameters:
bits
 random bits Returns:
 the next pseudorandom value from this random number generator's sequence
 Since:
 1.1

nextBytes
public 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 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
 the byte array to fill with random bytes Throws:
NullPointerException
 if the byte array is null Since:
 1.1

nextInt
public int nextInt()Returns the next pseudorandom, uniformly distributedint
value from this random number generator's sequence. The general contract ofnextInt
is that oneint
value is pseudorandomly generated and returned. All 2^{32} possibleint
values are produced with (approximately) equal probability.The method
nextInt
is implemented by classRandom
as if by:public int nextInt() { return next(32); }
 Returns:
 the next pseudorandom, uniformly distributed
int
value from this random number generator's sequence

nextInt
public int nextInt(int bound)Returns a pseudorandom, uniformly distributedint
value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence. The general contract ofnextInt
is that oneint
value in the specified range is pseudorandomly generated and returned. Allbound
possibleint
values are produced with (approximately) equal probability. The methodnextInt(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 + (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
 the upper bound (exclusive). Must be positive. Returns:
 the next pseudorandom, uniformly distributed
int
value between zero (inclusive) andbound
(exclusive) from this random number generator's sequence  Throws:
IllegalArgumentException
 if bound is not positive Since:
 1.2

nextLong
public long nextLong()Returns the next pseudorandom, uniformly distributedlong
value from this random number generator's sequence. The general contract ofnextLong
is that onelong
value is pseudorandomly generated and returned.The method
nextLong
is implemented by classRandom
as if by:public 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:
 the next pseudorandom, uniformly distributed
long
value from this random number generator's sequence

nextBoolean
public boolean nextBoolean()Returns the next pseudorandom, uniformly distributedboolean
value from this random number generator's sequence. The general contract ofnextBoolean
is that oneboolean
value is pseudorandomly generated and returned. The valuestrue
andfalse
are produced with (approximately) equal probability.The method
nextBoolean
is implemented by classRandom
as if by:public boolean nextBoolean() { return next(1) != 0; }
 Returns:
 the next pseudorandom, uniformly distributed
boolean
value from this random number generator's sequence  Since:
 1.2

nextFloat
public float nextFloat()Returns the next pseudorandom, uniformly distributedfloat
value between0.0
and1.0
from this random number generator's sequence.The general contract of
nextFloat
is that onefloat
value, chosen (approximately) uniformly from the range0.0f
(inclusive) to1.0f
(exclusive), is pseudorandomly generated and returned. All 2^{24} possiblefloat
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 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:
 the next pseudorandom, uniformly distributed
float
value between0.0
and1.0
from this random number generator's sequence

nextDouble
public double nextDouble()Returns the next pseudorandom, uniformly distributeddouble
value between0.0
and1.0
from this random number generator's sequence.The general contract of
nextDouble
is that onedouble
value, chosen (approximately) uniformly from the range0.0d
(inclusive) to1.0d
(exclusive), is pseudorandomly generated and returned.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:
 the next pseudorandom, uniformly distributed
double
value between0.0
and1.0
from this random number generator's sequence  See Also:
Math.random()

nextGaussian
public double nextGaussian()Returns the next pseudorandom, Gaussian ("normally") distributeddouble
value with mean0.0
and standard deviation1.0
from this random number generator's sequence.The general contract of
nextGaussian
is that onedouble
value, chosen from (approximately) the usual normal distribution with mean0.0
and standard deviation1.0
, is pseudorandomly generated and returned.The method
nextGaussian
is implemented by classRandom
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; } }
StrictMath.log
and one call toStrictMath.sqrt
. Returns:
 the next pseudorandom, Gaussian ("normally") distributed
double
value with mean0.0
and standard deviation1.0
from this random number generator's sequence

ints
Returns a stream producing the givenstreamSize
number of pseudorandomint
values.A pseudorandom
int
value is generated as if it's the result of calling the methodnextInt()
. Parameters:
streamSize
 the number of values to generate Returns:
 a stream of pseudorandom
int
values  Throws:
IllegalArgumentException
 ifstreamSize
is less than zero Since:
 1.8

ints
Returns an effectively unlimited stream of pseudorandomint
values.A pseudorandom
int
value is generated as if it's the result of calling the methodnextInt()
. Implementation Note:
 This method is implemented to be equivalent to
ints(Long.MAX_VALUE)
.  Returns:
 a stream of pseudorandom
int
values  Since:
 1.8

ints
Returns a stream producing the givenstreamSize
number of pseudorandomint
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
 the number of values to generaterandomNumberOrigin
 the origin (inclusive) of each random valuerandomNumberBound
 the bound (exclusive) of each random value Returns:
 a stream of pseudorandom
int
values, each with the given origin (inclusive) and bound (exclusive)  Throws:
IllegalArgumentException
 ifstreamSize
is less than zero, orrandomNumberOrigin
is greater than or equal torandomNumberBound
 Since:
 1.8

ints
Returns an effectively unlimited stream of pseudorandomint
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
 the origin (inclusive) of each random valuerandomNumberBound
 the bound (exclusive) of each random value Returns:
 a stream of pseudorandom
int
values, each with the given origin (inclusive) and bound (exclusive)  Throws:
IllegalArgumentException
 ifrandomNumberOrigin
is greater than or equal torandomNumberBound
 Since:
 1.8

longs
Returns a stream producing the givenstreamSize
number of pseudorandomlong
values.A pseudorandom
long
value is generated as if it's the result of calling the methodnextLong()
. Parameters:
streamSize
 the number of values to generate Returns:
 a stream of pseudorandom
long
values  Throws:
IllegalArgumentException
 ifstreamSize
is less than zero Since:
 1.8

longs
Returns an effectively unlimited stream of pseudorandomlong
values.A pseudorandom
long
value is generated as if it's the result of calling the methodnextLong()
. Implementation Note:
 This method is implemented to be equivalent to
longs(Long.MAX_VALUE)
.  Returns:
 a stream of pseudorandom
long
values  Since:
 1.8

longs
Returns a stream producing the givenstreamSize
number of pseudorandomlong
, 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
 the number of values to generaterandomNumberOrigin
 the origin (inclusive) of each random valuerandomNumberBound
 the bound (exclusive) of each random value Returns:
 a stream of pseudorandom
long
values, each with the given origin (inclusive) and bound (exclusive)  Throws:
IllegalArgumentException
 ifstreamSize
is less than zero, orrandomNumberOrigin
is greater than or equal torandomNumberBound
 Since:
 1.8

longs
Returns an effectively unlimited stream of pseudorandomlong
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
 the origin (inclusive) of each random valuerandomNumberBound
 the bound (exclusive) of each random value Returns:
 a stream of pseudorandom
long
values, each with the given origin (inclusive) and bound (exclusive)  Throws:
IllegalArgumentException
 ifrandomNumberOrigin
is greater than or equal torandomNumberBound
 Since:
 1.8

doubles
Returns a stream producing the givenstreamSize
number of pseudorandomdouble
values, each between zero (inclusive) and one (exclusive).A pseudorandom
double
value is generated as if it's the result of calling the methodnextDouble()
. Parameters:
streamSize
 the number of values to generate Returns:
 a stream of
double
values  Throws:
IllegalArgumentException
 ifstreamSize
is less than zero Since:
 1.8

doubles
Returns an effectively unlimited stream of pseudorandomdouble
values, each between zero (inclusive) and one (exclusive).A pseudorandom
double
value is generated as if it's the result of calling the methodnextDouble()
. Implementation Note:
 This method is implemented to be equivalent to
doubles(Long.MAX_VALUE)
.  Returns:
 a stream of pseudorandom
double
values  Since:
 1.8

doubles
Returns a stream producing the givenstreamSize
number of pseudorandomdouble
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
 the number of values to generaterandomNumberOrigin
 the origin (inclusive) of each random valuerandomNumberBound
 the bound (exclusive) of each random value Returns:
 a stream of pseudorandom
double
values, each with the given origin (inclusive) and bound (exclusive)  Throws:
IllegalArgumentException
 ifstreamSize
is less than zeroIllegalArgumentException
 ifrandomNumberOrigin
is greater than or equal torandomNumberBound
 Since:
 1.8

doubles
Returns an effectively unlimited stream of pseudorandomdouble
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
 the origin (inclusive) of each random valuerandomNumberBound
 the bound (exclusive) of each random value Returns:
 a stream of pseudorandom
double
values, each with the given origin (inclusive) and bound (exclusive)  Throws:
IllegalArgumentException
 ifrandomNumberOrigin
is greater than or equal torandomNumberBound
 Since:
 1.8
