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 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 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:
- Serialized Form
Constructors
Constructor and Description |
---|
Random() Creates a new random number generator. |
Random(long seed) Creates a new random number generator using a single |
Methods
Modifier and Type | Method and 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 user-supplied 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 inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Constructors
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)
Methods
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 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 3: 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.
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 232 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 + (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 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 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
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 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.
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 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.]
- 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 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.]
- 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 3: 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 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
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 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
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
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