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FlexDoc/Javadoc 2.0 Demo Java Doc |
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.
Nested Class Summary |
Nested classes/interfaces inherited from interface java.util.random.RandomGenerator |
Constructor Summary |
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Random()
Creates a new random number generator.
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Random(long seed)
Creates a new random number generator using a single long seed.
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Method Summary |
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doubles()
Returns an effectively unlimited stream of pseudorandom
double values, each between zero (inclusive) and one
(exclusive).
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doubles(double randomNumberOrigin, double randomNumberBound)
Returns an effectively unlimited stream of pseudorandom
double values, each conforming to the given origin (inclusive) and bound
(exclusive).
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doubles(long streamSize)
Returns a stream producing the given streamSize number of
pseudorandom double values, each between zero
(inclusive) and one (exclusive).
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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).
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ints()
Returns an effectively unlimited stream of pseudorandom int
values.
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ints(int randomNumberOrigin, int randomNumberBound)
Returns an effectively unlimited stream of pseudorandom
int values, each conforming to the given origin (inclusive) and bound
(exclusive).
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ints(long streamSize)
Returns a stream producing the given streamSize number of
pseudorandom int values.
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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).
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longs()
Returns an effectively unlimited stream of pseudorandom long
values.
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longs(long streamSize)
Returns a stream producing the given streamSize number of
pseudorandom long values.
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longs(long randomNumberOrigin, long randomNumberBound)
Returns an effectively unlimited stream of pseudorandom
long values, each conforming to the given origin (inclusive) and bound
(exclusive).
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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).
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protected int |
next(int bits)
Generates the next pseudorandom number.
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boolean |
Returns the next pseudorandom, uniformly distributed
boolean value from this random number generator's
sequence.
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void |
nextBytes(byte[] bytes)
Generates random bytes and places them into a user-supplied
byte array.
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double |
Returns the next pseudorandom, uniformly distributed
double value between 0.0 and
1.0 from this random number generator's sequence.
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float |
Returns the next pseudorandom, uniformly distributed float
value between 0.0 and 1.0 from this random
number generator's sequence.
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double |
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.
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int |
nextInt()
Returns the next pseudorandom, uniformly distributed int
value from this random number generator's sequence.
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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.
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long |
nextLong()
Returns the next pseudorandom, uniformly distributed long
value from this random number generator's sequence.
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void |
setSeed(long seed)
Sets the seed of this random number generator using a single
long seed.
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Methods inherited from class java.lang.Object |
Methods inherited from interface java.util.random.RandomGenerator |
public Random |
() |
public Random |
(long seed) |
Random rnd = new Random();
rnd.setSeed(seed);
public void setSeed |
(long seed) |
(seed ^ 0x5DEECE66DL) & ((1L << 48) - 1)
and clearing the haveNextNextGaussian flag used by RandomGenerator.nextGaussian(double, double).
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.
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.
public void nextBytes |
(byte[] bytes) |
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;
}
public int nextInt |
() |
public int nextInt() {
return next(32);
}
public int nextInt |
(int bound) |
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.
public long nextLong |
() |
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.public boolean nextBoolean |
() |
public boolean nextBoolean() {
return next(1) != 0;
}
public float nextFloat |
() |
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.
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.]public double nextDouble |
() |
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.
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.]public double nextGaussian |
() |
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.
private double nextNextGaussian;
private boolean haveNextNextGaussian = false;
public double nextGaussian() {
if (haveNextNextGaussian) {
haveNextNextGaussian = false;
return nextNextGaussian;
} else {
double v1, v2, s;
do {
v1 = 2 * nextDouble() - 1; // between -1.0 and 1.0
v2 = 2 * nextDouble() - 1; // between -1.0 and 1.0
s = v1 * v1 + v2 * v2;
} while (s >= 1 || s == 0);
double multiplier = StrictMath.sqrt(-2 * StrictMath.log(s)/s);
nextNextGaussian = v2 * multiplier;
haveNextNextGaussian = true;
return v1 * multiplier;
}
}
This uses the polar method of G. E. P. Box, M. E. Muller, and
G. Marsaglia, as described by Donald E. Knuth in The Art of
Computer Programming, Volume 2, 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 to StrictMath.log
and one call to StrictMath.sqrt.public IntStream ints |
(long streamSize) |
A pseudorandom int value is generated as if it's the result of calling the method nextInt().
public IntStream ints |
() |
A pseudorandom int value is generated as if it's the result of calling the method nextInt().
public IntStream ints |
(long streamSize, int randomNumberOrigin, int randomNumberBound) |
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;
}
}
public IntStream ints |
(int randomNumberOrigin, int randomNumberBound) |
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;
}
}
public LongStream longs |
(long streamSize) |
A pseudorandom long value is generated as if it's the result of calling the method nextLong().
public LongStream longs |
() |
A pseudorandom long value is generated as if it's the result of calling the method nextLong().
public LongStream longs |
(long streamSize, long randomNumberOrigin, long randomNumberBound) |
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;
}
public LongStream longs |
(long randomNumberOrigin, long randomNumberBound) |
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;
}
public DoubleStream doubles |
(long streamSize) |
A pseudorandom double value is generated as if it's the result of calling the method nextDouble().
public DoubleStream doubles |
() |
A pseudorandom double value is generated as if it's the result of calling the method nextDouble().
public DoubleStream doubles |
(long streamSize, double randomNumberOrigin, double randomNumberBound) |
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;
}
public DoubleStream doubles |
(double randomNumberOrigin, double randomNumberBound) |
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;
}
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FlexDoc/Javadoc 2.0 Demo Java Doc |