Class Random
 java.lang.Object

 java.util.Random
 All Implemented Interfaces:
Serializable
 Direct Known Subclasses:

SecureRandom
,ThreadLocalRandom
public class Random extends Object implements Serializable
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 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
Constructor  Description 

Random()  Creates a new random number generator. 
Random(long seed)  Creates a new random number generator using a single 
Method Summary
Modifier and Type  Method  Description 

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)  Returns a stream producing the given 
DoubleStream  doubles(long streamSize,
double randomNumberOrigin,
double randomNumberBound)  Returns a stream producing the given 
IntStream  ints()  Returns an effectively unlimited stream of pseudorandom 
IntStream  ints(int randomNumberOrigin,
int randomNumberBound)  Returns an effectively unlimited stream of pseudorandom 
IntStream  ints(long streamSize)  Returns a stream producing the given 
IntStream  ints(long streamSize,
int randomNumberOrigin,
int randomNumberBound)  Returns a stream producing the given 
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 
protected int  next(int bits)  Generates the next pseudorandom number. 
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 
Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Constructor Detail
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)
.
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 Detail
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
 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 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 2: Seminumerical Algorithms, section 3.2.1.
 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 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
 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 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:
 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. 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
 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 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:
 the next pseudorandom, uniformly distributed
long
value from this random number generator's sequence
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.
The method nextBoolean
is implemented by class Random
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 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:
 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 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:
 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") 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 2: 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:
 the next pseudorandom, Gaussian ("normally") distributed
double
value with mean0.0
and standard deviation1.0
from this random number generator's sequence
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
 the number of values to generate  Returns:
 a stream of pseudorandom
int
values  Throws:

IllegalArgumentException
 ifstreamSize
is less than zero  Since:
 1.8
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:
 a stream of pseudorandom
int
values  Since:
 1.8
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
 the number of values to generate 
randomNumberOrigin
 the origin (inclusive) of each random value 
randomNumberBound
 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
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:

randomNumberOrigin
 the origin (inclusive) of each random value 
randomNumberBound
 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
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
 the number of values to generate  Returns:
 a stream of pseudorandom
long
values  Throws:

IllegalArgumentException
 ifstreamSize
is less than zero  Since:
 1.8
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:
 a stream of pseudorandom
long
values  Since:
 1.8
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 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 generate 
randomNumberOrigin
 the origin (inclusive) of each random value 
randomNumberBound
 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
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 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 value 
randomNumberBound
 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
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
 the number of values to generate  Returns:
 a stream of
double
values  Throws:

IllegalArgumentException
 ifstreamSize
is less than zero  Since:
 1.8
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:
 a stream of pseudorandom
double
values  Since:
 1.8
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
 the number of values to generate 
randomNumberOrigin
 the origin (inclusive) of each random value 
randomNumberBound
 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 zero 
IllegalArgumentException
 ifrandomNumberOrigin
is greater than or equal torandomNumberBound
 Since:
 1.8
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
 the origin (inclusive) of each random value 
randomNumberBound
 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