Class Random
- All Implemented Interfaces:
Serializable,RandomGenerator
- Direct Known Subclasses:
SecureRandom,ThreadLocalRandom
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.
- Since:
- 1.0
- See Also:
-
Nested Class Summary
Nested classes/interfaces declared in interface java.util.random.RandomGenerator
RandomGenerator.ArbitrarilyJumpableGenerator, RandomGenerator.JumpableGenerator, RandomGenerator.LeapableGenerator, RandomGenerator.SplittableGenerator, RandomGenerator.StreamableGenerator -
Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptiondoubles()Returns an effectively unlimited stream of pseudorandomdoublevalues, each between zero (inclusive) and one (exclusive).doubles(double randomNumberOrigin, double randomNumberBound) Returns an effectively unlimited stream of pseudorandomdoublevalues, each conforming to the given origin (inclusive) and bound (exclusive).doubles(long streamSize) Returns a stream producing the givenstreamSizenumber of pseudorandomdoublevalues, each between zero (inclusive) and one (exclusive).doubles(long streamSize, double randomNumberOrigin, double randomNumberBound) Returns a stream producing the givenstreamSizenumber of pseudorandomdoublevalues, each conforming to the given origin (inclusive) and bound (exclusive).static Randomfrom(RandomGenerator generator) Returns an instance ofRandomthat delegates method calls to theRandomGeneratorargument.ints()Returns an effectively unlimited stream of pseudorandomintvalues.ints(int randomNumberOrigin, int randomNumberBound) Returns an effectively unlimited stream of pseudorandomintvalues, each conforming to the given origin (inclusive) and bound (exclusive).ints(long streamSize) Returns a stream producing the givenstreamSizenumber of pseudorandomintvalues.ints(long streamSize, int randomNumberOrigin, int randomNumberBound) Returns a stream producing the givenstreamSizenumber of pseudorandomintvalues, each conforming to the given origin (inclusive) and bound (exclusive).longs()Returns an effectively unlimited stream of pseudorandomlongvalues.longs(long streamSize) Returns a stream producing the givenstreamSizenumber of pseudorandomlongvalues.longs(long randomNumberOrigin, long randomNumberBound) Returns an effectively unlimited stream of pseudorandomlongvalues, each conforming to the given origin (inclusive) and bound (exclusive).longs(long streamSize, long randomNumberOrigin, long randomNumberBound) Returns a stream producing the givenstreamSizenumber of pseudorandomlong, each conforming to the given origin (inclusive) and bound (exclusive).protected intnext(int bits) Generates the next pseudorandom number.booleanReturns the next pseudorandom, uniformly distributedbooleanvalue from this random number generator's sequence.voidnextBytes(byte[] bytes) Generates random bytes and places them into a user-supplied byte array.doubleReturns the next pseudorandom, uniformly distributeddoublevalue between0.0and1.0from this random number generator's sequence.floatReturns the next pseudorandom, uniformly distributedfloatvalue between0.0and1.0from this random number generator's sequence.doubleReturns the next pseudorandom, Gaussian ("normally") distributeddoublevalue with mean0.0and standard deviation1.0from this random number generator's sequence.intnextInt()Returns the next pseudorandom, uniformly distributedintvalue from this random number generator's sequence.intnextInt(int bound) Returns a pseudorandom, uniformly distributedintvalue between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence.longnextLong()Returns the next pseudorandom, uniformly distributedlongvalue from this random number generator's sequence.voidsetSeed(long seed) Sets or updates the seed of this random number generator using the providedlongseed value (optional operation).Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, waitMethods declared in interface java.util.random.RandomGenerator
equiDoubles, isDeprecated, nextDouble, nextDouble, nextExponential, nextFloat, nextFloat, nextGaussian, nextInt, nextLong, nextLong
-
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 singlelongseed. The seed is the initial value of the internal state of the pseudorandom number generator which is maintained by methodnext(int).- Implementation Requirements:
- The invocation
new Random(seed)is equivalent to:Random rnd = new Random(); rnd.setSeed(seed); - Parameters:
seed- the initial seed- See Also:
-
-
Method Details
-
from
Returns an instance ofRandomthat delegates method calls to theRandomGeneratorargument. If the generator is an instance ofRandom, it is returned. Otherwise, this method returns an instance ofRandomthat delegates all methods exceptsetSeedto the generator. The returned instance'ssetSeedmethod always throwsUnsupportedOperationException. The returned instance is not serializable.- Parameters:
generator- theRandomGeneratorcalls are delegated to- Returns:
- the delegating
Randominstance - Throws:
NullPointerException- if generator is null- Since:
- 19
-
setSeed
public void setSeed(long seed) Sets or updates the seed of this random number generator using the providedlongseed value (optional operation).- Implementation Requirements:
- The implementation in this class alters the state of this random number
generator so that it is in the same state as if it had just been created with
new Random(seed). It atomically updates the seed to
and clears the(seed ^ 0x5DEECE66DL) & ((1L << 48) - 1)haveNextNextGaussianflag used bynextGaussian(). Note that this uses only 48 bits of the given seed value. - Parameters:
seed- the seed value- Throws:
UnsupportedOperationException- if thesetSeedoperation is not supported by this random number generator
-
next
protected int next(int bits) Generates the next pseudorandom number. This method returns anintvalue such that, if the argumentbitsis between1and32(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 be0or1.- API Note:
- The other random-producing methods in this class are implemented in terms of this method, so subclasses can override just this method to provide a different source of pseudorandom numbers for the entire class.
- Implementation Requirements:
- The implementation in this class atomically updates the seed to
and returns(seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1)(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- 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 user-supplied byte array. The number of random bytes produced is equal to the length of the byte array.- Specified by:
nextBytesin interfaceRandomGenerator- Implementation Requirements:
- The method
nextBytesis implemented by classRandomas 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 distributedintvalue from this random number generator's sequence. The general contract ofnextIntis that oneintvalue is pseudorandomly generated and returned. All 232 possibleintvalues are produced with (approximately) equal probability.- Specified by:
nextIntin interfaceRandomGenerator- Implementation Requirements:
- The method
nextIntis implemented by classRandomas if by:public int nextInt() { return next(32); } - Returns:
- the next pseudorandom, uniformly distributed
intvalue from this random number generator's sequence
-
nextInt
public int nextInt(int bound) Returns a pseudorandom, uniformly distributedintvalue between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence. The general contract ofnextIntis that oneintvalue in the specified range is pseudorandomly generated and returned. Allboundpossibleintvalues are produced with (approximately) equal probability.- Specified by:
nextIntin interfaceRandomGenerator- Implementation Requirements:
- The method
nextInt(int bound)is implemented by classRandomas 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
intvalues 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- the upper bound (exclusive). Must be positive.- Returns:
- the next pseudorandom, uniformly distributed
intvalue 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 distributedlongvalue from this random number generator's sequence. The general contract ofnextLongis that onelongvalue is pseudorandomly generated and returned.- Specified by:
nextLongin interfaceRandomGenerator- Implementation Requirements:
- The method
nextLongis implemented by classRandomas if by:
Because classpublic long nextLong() { return ((long)next(32) << 32) + next(32); }Randomuses a seed with only 48 bits, this algorithm will not return all possiblelongvalues. - Returns:
- the next pseudorandom, uniformly distributed
longvalue from this random number generator's sequence
-
nextBoolean
public boolean nextBoolean()Returns the next pseudorandom, uniformly distributedbooleanvalue from this random number generator's sequence. The general contract ofnextBooleanis that onebooleanvalue is pseudorandomly generated and returned. The valuestrueandfalseare produced with (approximately) equal probability.- Specified by:
nextBooleanin interfaceRandomGenerator- Implementation Requirements:
- The method
nextBooleanis implemented by classRandomas if by:public boolean nextBoolean() { return next(1) != 0; } - Returns:
- the next pseudorandom, uniformly distributed
booleanvalue from this random number generator's sequence - Since:
- 1.2
-
nextFloat
public float nextFloat()Returns the next pseudorandom, uniformly distributedfloatvalue between0.0and1.0from this random number generator's sequence.The general contract of
nextFloatis that onefloatvalue, chosen (approximately) uniformly from the range0.0f(inclusive) to1.0f(exclusive), is pseudorandomly generated and returned. All 224 possiblefloatvalues of the form m x 2-24, where m is a positive integer less than 224, are produced with (approximately) equal probability.- Specified by:
nextFloatin interfaceRandomGenerator- Implementation Requirements:
- The method
nextFloatis implemented by classRandomas 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
floatvalues from the stated range with perfect uniformity.[In early versions of Java, the result was incorrectly calculated as:
This might seem to be equivalent, if not better, but in fact it introduced a slight nonuniformity because of the bias in the rounding of floating-point numbers: it was slightly more likely that the low-order bit of the significand would be 0 than that it would be 1.]return next(30) / ((float)(1 << 30)); - Returns:
- the next pseudorandom, uniformly distributed
floatvalue between0.0fand1.0ffrom this random number generator's sequence
-
nextDouble
public double nextDouble()Returns the next pseudorandom, uniformly distributeddoublevalue between0.0and1.0from this random number generator's sequence.The general contract of
nextDoubleis that onedoublevalue, chosen (approximately) uniformly from the range0.0d(inclusive) to1.0d(exclusive), is pseudorandomly generated and returned.- Specified by:
nextDoublein interfaceRandomGenerator- Implementation Requirements:
- The method
nextDoubleis implemented by classRandomas 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
nextmethod 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 choosedoublevalues from the stated range with perfect uniformity.[In early versions of Java, the result was incorrectly calculated as:
This might seem to be equivalent, if not better, but in fact it introduced a large nonuniformity because of the bias in the rounding of floating-point numbers: it was three times as likely that the low-order bit of the significand would be 0 than that it would be 1! This nonuniformity probably doesn't matter much in practice, but we strive for perfection.]return (((long)next(27) << 27) + next(27)) / (double)(1L << 54); - Returns:
- the next pseudorandom, uniformly distributed
doublevalue between0.0and1.0from this random number generator's sequence - See Also:
-
nextGaussian
public double nextGaussian()Returns the next pseudorandom, Gaussian ("normally") distributeddoublevalue with mean0.0and standard deviation1.0from this random number generator's sequence.The general contract of
nextGaussianis that onedoublevalue, chosen from (approximately) the usual normal distribution with mean0.0and standard deviation1.0, is pseudorandomly generated and returned.- Specified by:
nextGaussianin interfaceRandomGenerator- Implementation Requirements:
- The method
nextGaussianis implemented by classRandomas 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.logand one call toStrictMath.sqrt. - Returns:
- the next pseudorandom, Gaussian ("normally") distributed
doublevalue with mean0.0and standard deviation1.0from this random number generator's sequence
-
ints
Returns a stream producing the givenstreamSizenumber of pseudorandomintvalues.A pseudorandom
intvalue is generated as if it's the result of calling the methodnextInt().- Specified by:
intsin interfaceRandomGenerator- Parameters:
streamSize- the number of values to generate- Returns:
- a stream of pseudorandom
intvalues - Throws:
IllegalArgumentException- ifstreamSizeis less than zero- Since:
- 1.8
-
ints
Returns an effectively unlimited stream of pseudorandomintvalues.A pseudorandom
intvalue is generated as if it's the result of calling the methodnextInt().- Specified by:
intsin interfaceRandomGenerator- Implementation Note:
- This method is implemented to be equivalent to
ints(Long.MAX_VALUE). - Returns:
- a stream of pseudorandom
intvalues - Since:
- 1.8
-
ints
Returns a stream producing the givenstreamSizenumber of pseudorandomintvalues, each conforming to the given origin (inclusive) and bound (exclusive).A pseudorandom
intvalue 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; } }- Specified by:
intsin interfaceRandomGenerator- 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
intvalues, each with the given origin (inclusive) and bound (exclusive) - Throws:
IllegalArgumentException- ifstreamSizeis less than zero, orrandomNumberOriginis greater than or equal torandomNumberBound- Since:
- 1.8
-
ints
Returns an effectively unlimited stream of pseudorandomintvalues, each conforming to the given origin (inclusive) and bound (exclusive).A pseudorandom
intvalue 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; } }- Specified by:
intsin interfaceRandomGenerator- 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
intvalues, each with the given origin (inclusive) and bound (exclusive) - Throws:
IllegalArgumentException- ifrandomNumberOriginis greater than or equal torandomNumberBound- Since:
- 1.8
-
longs
Returns a stream producing the givenstreamSizenumber of pseudorandomlongvalues.A pseudorandom
longvalue is generated as if it's the result of calling the methodnextLong().- Specified by:
longsin interfaceRandomGenerator- Parameters:
streamSize- the number of values to generate- Returns:
- a stream of pseudorandom
longvalues - Throws:
IllegalArgumentException- ifstreamSizeis less than zero- Since:
- 1.8
-
longs
Returns an effectively unlimited stream of pseudorandomlongvalues.A pseudorandom
longvalue is generated as if it's the result of calling the methodnextLong().- Specified by:
longsin interfaceRandomGenerator- Implementation Note:
- This method is implemented to be equivalent to
longs(Long.MAX_VALUE). - Returns:
- a stream of pseudorandom
longvalues - Since:
- 1.8
-
longs
Returns a stream producing the givenstreamSizenumber of pseudorandomlong, each conforming to the given origin (inclusive) and bound (exclusive).A pseudorandom
longvalue 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; }- Specified by:
longsin interfaceRandomGenerator- 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
longvalues, each with the given origin (inclusive) and bound (exclusive) - Throws:
IllegalArgumentException- ifstreamSizeis less than zero, orrandomNumberOriginis greater than or equal torandomNumberBound- Since:
- 1.8
-
longs
Returns an effectively unlimited stream of pseudorandomlongvalues, each conforming to the given origin (inclusive) and bound (exclusive).A pseudorandom
longvalue 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; }- Specified by:
longsin interfaceRandomGenerator- 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
longvalues, each with the given origin (inclusive) and bound (exclusive) - Throws:
IllegalArgumentException- ifrandomNumberOriginis greater than or equal torandomNumberBound- Since:
- 1.8
-
doubles
Returns a stream producing the givenstreamSizenumber of pseudorandomdoublevalues, each between zero (inclusive) and one (exclusive).A pseudorandom
doublevalue is generated as if it's the result of calling the methodnextDouble().- Specified by:
doublesin interfaceRandomGenerator- Parameters:
streamSize- the number of values to generate- Returns:
- a stream of
doublevalues - Throws:
IllegalArgumentException- ifstreamSizeis less than zero- Since:
- 1.8
-
doubles
Returns an effectively unlimited stream of pseudorandomdoublevalues, each between zero (inclusive) and one (exclusive).A pseudorandom
doublevalue is generated as if it's the result of calling the methodnextDouble().- Specified by:
doublesin interfaceRandomGenerator- Implementation Note:
- This method is implemented to be equivalent to
doubles(Long.MAX_VALUE). - Returns:
- a stream of pseudorandom
doublevalues - Since:
- 1.8
-
doubles
Returns a stream producing the givenstreamSizenumber of pseudorandomdoublevalues, each conforming to the given origin (inclusive) and bound (exclusive).- Specified by:
doublesin interfaceRandomGenerator- 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
doublevalues, each with the given origin (inclusive) and bound (exclusive) - Throws:
IllegalArgumentException- ifstreamSizeis less than zero, orrandomNumberOriginis not finite, orrandomNumberBoundis not finite, orrandomNumberOriginis greater than or equal torandomNumberBound- Since:
- 1.8
-
doubles
Returns an effectively unlimited stream of pseudorandomdoublevalues, each conforming to the given origin (inclusive) and bound (exclusive).- Specified by:
doublesin interfaceRandomGenerator- 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
doublevalues, each with the given origin (inclusive) and bound (exclusive) - Throws:
IllegalArgumentException- ifrandomNumberOriginis not finite, orrandomNumberBoundis not finite, orrandomNumberOriginis greater than or equal torandomNumberBound- Since:
- 1.8
-