Class Random
- All Implemented Interfaces:
-
Serializable,RandomGenerator
- Direct Known Subclasses:
-
SecureRandom,ThreadLocalRandom
public class Random extends Object implements RandomGenerator, 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 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
| Constructor | Description |
|---|---|
Random() |
Creates a new random number generator. |
Random |
Creates a new random number generator using a single long seed. |
Method Summary
| Modifier and Type | Method | Description |
|---|---|---|
DoubleStream |
doubles() |
Returns an effectively unlimited stream of pseudorandom
double values, each between zero (inclusive) and one (exclusive). |
DoubleStream |
doubles |
Returns an effectively unlimited stream of pseudorandom
double values, each conforming to the given origin (inclusive) and bound (exclusive). |
DoubleStream |
doubles |
Returns a stream producing the given streamSize number of pseudorandom double values, each between zero (inclusive) and one (exclusive). |
DoubleStream |
doubles |
Returns a stream producing the given streamSize number of pseudorandom double values, each conforming to the given origin (inclusive) and bound (exclusive). |
IntStream |
ints() |
Returns an effectively unlimited stream of pseudorandom int values. |
IntStream |
ints |
Returns an effectively unlimited stream of pseudorandom
int values, each conforming to the given origin (inclusive) and bound (exclusive). |
IntStream |
ints |
Returns a stream producing the given streamSize number of pseudorandom int values. |
IntStream |
ints |
Returns a stream producing the given streamSize number of pseudorandom int values, each conforming to the given origin (inclusive) and bound (exclusive). |
LongStream |
longs() |
Returns an effectively unlimited stream of pseudorandom long values. |
LongStream |
longs |
Returns a stream producing the given streamSize number of pseudorandom long values. |
LongStream |
longs |
Returns an effectively unlimited stream of pseudorandom
long values, each conforming to the given origin (inclusive) and bound (exclusive). |
LongStream |
longs |
Returns a stream producing the given streamSize number of pseudorandom long, each conforming to the given origin (inclusive) and bound (exclusive). |
protected int |
next |
Generates the next pseudorandom number. |
boolean |
nextBoolean() |
Returns the next pseudorandom, uniformly distributed boolean value from this random number generator's sequence. |
void |
nextBytes |
Generates random bytes and places them into a user-supplied byte array. |
double |
nextDouble() |
Returns the next pseudorandom, uniformly distributed double value between 0.0 and 1.0 from this random number generator's sequence. |
float |
nextFloat() |
Returns the next pseudorandom, uniformly distributed float value between 0.0 and 1.0 from this random number generator's sequence. |
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. |
int |
nextInt() |
Returns the next pseudorandom, uniformly distributed int value from this random number generator's sequence. |
int |
nextInt |
Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive), drawn from this random number generator's sequence. |
long |
nextLong() |
Returns the next pseudorandom, uniformly distributed long value from this random number generator's sequence. |
void |
setSeed |
Sets the seed of this random number generator using a single long seed. |
Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Methods declared in interface java.util.random.RandomGenerator
isDeprecated, nextDouble, nextDouble, nextExponential, nextFloat, nextFloat, nextGaussian, nextInt, nextLong, nextLong
Constructor Details
Random
public Random()
Random
public Random(long seed)
long seed. The seed is the initial value of the internal state of the pseudorandom number generator which is maintained by method next(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
setSeed
public void setSeed(long seed)
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)
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 low-order 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, 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)
- 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()
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 232 possible int values 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)
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.- 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()
long value from this random number generator's sequence. The general contract of nextLong is that one long value 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()
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.- 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()
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 224 possible float values 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 between0.0and1.0from this random number generator's sequence
nextDouble
public double nextDouble()
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.
- 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()
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.
- 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
public IntStream ints(long streamSize)
streamSize number of pseudorandom int values. A pseudorandom int value is generated as if it's the result of calling the method nextInt().
- 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
public IntStream ints()
int values. A pseudorandom int value is generated as if it's the result of calling the method nextInt().
- 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
public IntStream ints(long streamSize, int randomNumberOrigin, int randomNumberBound)
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;
}
}
- Specified by:
-
intsin interfaceRandomGenerator - 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- ifstreamSizeis less than zero, orrandomNumberOriginis greater than or equal torandomNumberBound - Since:
- 1.8
ints
public IntStream ints(int randomNumberOrigin, int randomNumberBound)
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;
}
}
- 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 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- ifrandomNumberOriginis greater than or equal torandomNumberBound - Since:
- 1.8
longs
public LongStream longs(long streamSize)
streamSize number of pseudorandom long values. A pseudorandom long value is generated as if it's the result of calling the method nextLong().
- 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
public LongStream longs()
long values. A pseudorandom long value is generated as if it's the result of calling the method nextLong().
- 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
public LongStream longs(long streamSize, long randomNumberOrigin, long randomNumberBound)
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 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 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- ifstreamSizeis less than zero, orrandomNumberOriginis greater than or equal torandomNumberBound - Since:
- 1.8
longs
public LongStream longs(long randomNumberOrigin, long randomNumberBound)
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 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 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- ifrandomNumberOriginis greater than or equal torandomNumberBound - Since:
- 1.8
doubles
public DoubleStream doubles(long streamSize)
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().
- 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
public DoubleStream doubles()
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().
- 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
public DoubleStream doubles(long streamSize, double randomNumberOrigin, double randomNumberBound)
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;
}
- Specified by:
-
doublesin interfaceRandomGenerator - 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- ifstreamSizeis less than zero, orrandomNumberOriginis not finite, orrandomNumberBoundis not finite, orrandomNumberOriginis greater than or equal torandomNumberBound - Since:
- 1.8
doubles
public DoubleStream doubles(double randomNumberOrigin, double randomNumberBound)
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;
}
- 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 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- ifrandomNumberOriginis greater than or equal torandomNumberBound - Since:
- 1.8
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