- java.lang.Object
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- java.util.Random
 
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- 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 48-bit 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 Randomare 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 classRandom. Java implementations must use all the algorithms shown here for the classRandom, for the sake of absolute portability of Java code. However, subclasses of classRandomare permitted to use other algorithms, so long as they adhere to the general contracts for all the methods.The algorithms implemented by class Randomuse aprotectedutility 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.Randomare threadsafe. However, the concurrent use of the samejava.util.Randominstance across threads may encounter contention and consequent poor performance. Consider instead usingThreadLocalRandomin multithreaded designs.Instances of java.util.Randomare not cryptographically secure. Consider instead usingSecureRandomto get a cryptographically secure pseudo-random number generator for use by security-sensitive applications.- Since:
- 1.0
- See Also:
- Serialized Form
 
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Method SummaryAll Methods Instance Methods Concrete Methods Modifier and Type Method Description DoubleStreamdoubles()Returns an effectively unlimited stream of pseudorandomdoublevalues, each between zero (inclusive) and one (exclusive).DoubleStreamdoubles(double randomNumberOrigin, double randomNumberBound)Returns an effectively unlimited stream of pseudorandomdoublevalues, each conforming to the given origin (inclusive) and bound (exclusive).DoubleStreamdoubles(long streamSize)Returns a stream producing the givenstreamSizenumber of pseudorandomdoublevalues, each between zero (inclusive) and one (exclusive).DoubleStreamdoubles(long streamSize, double randomNumberOrigin, double randomNumberBound)Returns a stream producing the givenstreamSizenumber of pseudorandomdoublevalues, each conforming to the given origin (inclusive) and bound (exclusive).IntStreamints()Returns an effectively unlimited stream of pseudorandomintvalues.IntStreamints(int randomNumberOrigin, int randomNumberBound)Returns an effectively unlimited stream of pseudorandomintvalues, each conforming to the given origin (inclusive) and bound (exclusive).IntStreamints(long streamSize)Returns a stream producing the givenstreamSizenumber of pseudorandomintvalues.IntStreamints(long streamSize, int randomNumberOrigin, int randomNumberBound)Returns a stream producing the givenstreamSizenumber of pseudorandomintvalues, each conforming to the given origin (inclusive) and bound (exclusive).LongStreamlongs()Returns an effectively unlimited stream of pseudorandomlongvalues.LongStreamlongs(long streamSize)Returns a stream producing the givenstreamSizenumber of pseudorandomlongvalues.LongStreamlongs(long randomNumberOrigin, long randomNumberBound)Returns an effectively unlimited stream of pseudorandomlongvalues, each conforming to the given origin (inclusive) and bound (exclusive).LongStreamlongs(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.booleannextBoolean()Returns 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.doublenextDouble()Returns the next pseudorandom, uniformly distributeddoublevalue between0.0and1.0from this random number generator's sequence.floatnextFloat()Returns the next pseudorandom, uniformly distributedfloatvalue between0.0and1.0from this random number generator's sequence.doublenextGaussian()Returns 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 the seed of this random number generator using a singlelongseed.
 
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Constructor Detail- 
Randompublic 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.
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Randompublic 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).The invocation new Random(seed)is equivalent to:Random rnd = new Random(); rnd.setSeed(seed);- Parameters:
- seed- the initial seed
- See Also:
- setSeed(long)
 
 
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Method Detail- 
setSeedpublic void setSeed(long seed) Sets the seed of this random number generator using a singlelongseed. The general contract ofsetSeedis 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 argumentseedas a seed. The methodsetSeedis implemented by classRandomby atomically updating the seed to
 and clearing the(seed ^ 0x5DEECE66DL) & ((1L << 48) - 1)haveNextNextGaussianflag used bynextGaussian().The implementation of setSeedby classRandomhappens to use only 48 bits of the given seed. In general, however, an overriding method may use all 64 bits of thelongargument as a seed value.- Parameters:
- seed- the initial seed
 
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nextprotected 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 nextis that it returns anintvalue and 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. The methodnextis implemented by classRandomby 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
 
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nextBytespublic 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.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
 
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nextIntpublic 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.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
 
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nextIntpublic 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. The methodnextInt(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
 
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nextLongpublic 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.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
 
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nextBooleanpublic 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.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
 
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nextFloatpublic 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.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 between0.0and1.0from this random number generator's sequence
 
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nextDoublepublic 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.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:
- Math.random()
 
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nextGaussianpublic 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.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: 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
 
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intspublic IntStream ints(long streamSize) Returns a stream producing the givenstreamSizenumber of pseudorandomintvalues.A pseudorandom intvalue 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 intvalues
- Throws:
- IllegalArgumentException- if- streamSizeis less than zero
- Since:
- 1.8
 
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intspublic IntStream ints() Returns an effectively unlimited stream of pseudorandomintvalues.A pseudorandom intvalue 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 intvalues
- Since:
- 1.8
 
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intspublic IntStream 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).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; } }- 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 intvalues, each with the given origin (inclusive) and bound (exclusive)
- Throws:
- IllegalArgumentException- if- streamSizeis less than zero, or- randomNumberOriginis greater than or equal to- randomNumberBound
- Since:
- 1.8
 
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intspublic IntStream ints(int randomNumberOrigin, int randomNumberBound) 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; } }- 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 intvalues, each with the given origin (inclusive) and bound (exclusive)
- Throws:
- IllegalArgumentException- if- randomNumberOriginis greater than or equal to- randomNumberBound
- Since:
- 1.8
 
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longspublic LongStream longs(long streamSize) Returns a stream producing the givenstreamSizenumber of pseudorandomlongvalues.A pseudorandom longvalue 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 longvalues
- Throws:
- IllegalArgumentException- if- streamSizeis less than zero
- Since:
- 1.8
 
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longspublic LongStream longs() Returns an effectively unlimited stream of pseudorandomlongvalues.A pseudorandom longvalue 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 longvalues
- Since:
- 1.8
 
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longspublic LongStream 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).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; }- 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 longvalues, each with the given origin (inclusive) and bound (exclusive)
- Throws:
- IllegalArgumentException- if- streamSizeis less than zero, or- randomNumberOriginis greater than or equal to- randomNumberBound
- Since:
- 1.8
 
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longspublic LongStream longs(long randomNumberOrigin, long randomNumberBound) 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; }- 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 longvalues, each with the given origin (inclusive) and bound (exclusive)
- Throws:
- IllegalArgumentException- if- randomNumberOriginis greater than or equal to- randomNumberBound
- Since:
- 1.8
 
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doublespublic DoubleStream doubles(long streamSize) 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().- Parameters:
- streamSize- the number of values to generate
- Returns:
- a stream of doublevalues
- Throws:
- IllegalArgumentException- if- streamSizeis less than zero
- Since:
- 1.8
 
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doublespublic DoubleStream 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().- Implementation Note:
- This method is implemented to be equivalent to doubles(Long.MAX_VALUE).
- Returns:
- a stream of pseudorandom doublevalues
- Since:
- 1.8
 
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doublespublic DoubleStream 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).A pseudorandom doublevalue 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 doublevalues, each with the given origin (inclusive) and bound (exclusive)
- Throws:
- IllegalArgumentException- if- streamSizeis less than zero
- IllegalArgumentException- if- randomNumberOriginis greater than or equal to- randomNumberBound
- Since:
- 1.8
 
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doublespublic DoubleStream doubles(double randomNumberOrigin, double randomNumberBound) Returns an effectively unlimited stream of pseudorandomdoublevalues, each conforming to the given origin (inclusive) and bound (exclusive).A pseudorandom doublevalue 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 doublevalues, each with the given origin (inclusive) and bound (exclusive)
- Throws:
- IllegalArgumentException- if- randomNumberOriginis greater than or equal to- randomNumberBound
- Since:
- 1.8
 
 
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