|
||||||||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | |||||||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |
java.lang.Objectants.p2p.utils.misc.Random
public class Random
This class generates pseudorandom numbers. It uses the same
algorithm as the original JDK-class, so that your programs behave
exactly the same way, if started with the same seed.
The algorithm is described in The Art of Computer Programming,
Volume 2 by Donald Knuth in Section 3.2.1. It is a 48-bit seed,
linear congruential formula.
If two instances of this class are created with the same seed and
the same calls to these classes are made, they behave exactly the
same way. This should be even true for foreign implementations
(like this), so every port must use the same algorithm as described
here.
If you want to implement your own pseudorandom algorithm, you
should extend this class and overload the next()
and
setSeed(long)
method. In that case the above
paragraph doesn't apply to you.
This class shouldn't be used for security sensitive purposes (like
generating passwords or encryption keys. See SecureRandom
in package java.security
for this purpose.
For simple random doubles between 0.0 and 1.0, you may consider using
Math.random instead.
SecureRandom
,
Math.random()
,
Serialized FormConstructor Summary | |
---|---|
Random()
Creates a new pseudorandom number generator. |
|
Random(long seed)
Creates a new pseudorandom number generator, starting with the specified seed, using setSeed(seed); . |
Method Summary | |
---|---|
long |
getSeed()
Gets the seed for this pseudorandom number generator. |
protected int |
next(int bits)
Generates the next pseudorandom number. |
boolean |
nextBoolean()
Generates the next pseudorandom boolean. |
void |
nextBytes(byte[] bytes)
Fills an array of bytes with random numbers. |
double |
nextDouble()
Generates the next pseudorandom double uniformly distributed between 0.0 (inclusive) and 1.0 (exclusive). |
float |
nextFloat()
Generates the next pseudorandom float uniformly distributed between 0.0f (inclusive) and 1.0f (exclusive). |
double |
nextGaussian()
Generates the next pseudorandom, Gaussian (normally) distributed double value, with mean 0.0 and standard deviation 1.0. |
int |
nextInt()
Generates the next pseudorandom number. |
int |
nextInt(int n)
Generates the next pseudorandom number. |
long |
nextLong()
Generates the next pseudorandom long number. |
void |
setSeed(long seed)
Sets the seed for this pseudorandom number generator. |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Constructor Detail |
---|
public Random()
setSeed(System.currentTimeMillis());
.
System.currentTimeMillis()
public Random(long seed)
setSeed(seed);
.
seed
- the initial seedMethod Detail |
---|
public void setSeed(long seed)
public synchronized void setSeed(long seed) { this.seed = (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1); haveNextNextGaussian = false; }
seed
- the new seedpublic long getSeed()
public synchronized long getSeed() { return this.seed; }
seed
- the new seedprotected int next(int bits)
bits
low order bits are
independent chosen random bits (0 and 1 are equally likely).
The implementation for java.util.Random is:
protected synchronized int next(int bits) { seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1); return (int) (seed >>> (48 - bits)); }
bits
- the number of random bits to generate, in the range 1..32
public void nextBytes(byte[] bytes)
public void nextBytes(byte[] bytes) { for (int i = 0; i < bytes.length; i += 4) { int random = next(32); for (int j = 0; i + j < bytes.length && j < 4; j++) { bytes[i+j] = (byte) (random & 0xff) random >>= 8; } } }
bytes
- the byte array that should be filled
java.lang.NullPointerException
- if bytes is nullpublic int nextInt()
public int nextInt() { return next(32); }
public int nextInt(int n)
n
(exclusive), and
each value has the same likelihodd (1/n
).
(0 and 1 are equally likely). The implementation for
java.util.Random is:
public int nextInt(int n) { if (n <= 0) throw new IllegalArgumentException("n must be positive"); if ((n & -n) == n) // i.e., n is a power of 2 return (int)((n * (long) next(31)) >> 31); int bits, val; do { bits = next(31); val = bits % n; } while(bits - val + (n-1) < 0); return val; }
This algorithm would return every value with exactly the same probability, if the next()-method would be a perfect random number generator. The loop at the bottom only accepts a value, if the random number was between 0 and the highest number less then 1<<31, which is divisible by n. The probability for this is high for small n, and the worst case is 1/2 (for n=(1<<30)+1). The special treatment for n = power of 2, selects the high bits of the random number (the loop at the bottom would select the low order bits). This is done, because the low order bits of linear congruential number generators (like the one used in this class) are known to be ``less random'' than the high order bits.
n
- the upper bound
java.lang.IllegalArgumentException
- if the given upper bound is negativepublic long nextLong()
public long nextLong() { return ((long) next(32) << 32) + next(32); }
public boolean nextBoolean()
public boolean nextBoolean() { return next(1) != 0; }
public float nextFloat()
public float nextFloat() { return next(24) / ((float)(1 << 24)); }
public double nextDouble()
public double nextDouble() { return (((long) next(26) << 27) + next(27)) / (double)(1L << 53); }
public double nextGaussian()
public synchronized 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); double norm = Math.sqrt(-2 * Math.log(s) / s); nextNextGaussian = v2 * norm; haveNextNextGaussian = true; return v1 * norm; } }
This is described in section 3.4.1 of The Art of Computer Programming, Volume 2 by Donald Knuth.
|
||||||||||
PREV CLASS NEXT CLASS | FRAMES NO FRAMES | |||||||||
SUMMARY: NESTED | FIELD | CONSTR | METHOD | DETAIL: FIELD | CONSTR | METHOD |