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java.util.Random Class Reference

This class generates pseudorandom numbers. More...

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Public Member Functions

 Random ()
 Creates a new pseudorandom number generator.
 Random (long seed)
 Creates a new pseudorandom number generator, starting with the specified seed, using setSeed(seed);.
synchronized void setSeed (long seed)
 Sets the seed for this pseudorandom number generator.
void nextBytes (byte[] bytes)
 Fills an array of bytes with random numbers.
int nextInt ()
 Generates the next pseudorandom number.
int nextInt (int n)
 Generates the next pseudorandom number.
long nextLong ()
 Generates the next pseudorandom long number.
boolean nextBoolean ()
 Generates the next pseudorandom boolean.
float nextFloat ()
 Generates the next pseudorandom float uniformly distributed between 0.0f (inclusive) and 1.0f (exclusive).
double nextDouble ()
 Generates the next pseudorandom double uniformly distributed between 0.0 (inclusive) and 1.0 (exclusive).
synchronized double nextGaussian ()
 Generates the next pseudorandom, Gaussian (normally) distributed double value, with mean 0.0 and standard deviation 1.0.

Protected Member Functions

synchronized int next (int bits)
 Generates the next pseudorandom number.

Private Attributes

boolean haveNextNextGaussian
 True if the next nextGaussian is available.
double nextNextGaussian
 The next nextGaussian, when available.
long seed
 The seed.

Static Private Attributes

static final long serialVersionUID = 3905348978240129619L
 Compatible with JDK 1.0+.

Detailed Description

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.

See also:
java.security.SecureRandom
Math::random()
Author:
Jochen Hoenicke
Eric Blake (ebb9@.nosp@m.emai.nosp@m.l.byu.nosp@m..edu) updated to 1.4

Definition at line 76 of file Random.java.


Constructor & Destructor Documentation

Creates a new pseudorandom number generator.

The seed is initialized to the current time, as if by setSeed(System.currentTimeMillis());.

See also:
System::currentTimeMillis()

Definition at line 121 of file Random.java.

  {
    this(System.currentTimeMillis());
  }
java.util.Random.Random ( long  seed) [inline]

Creates a new pseudorandom number generator, starting with the specified seed, using setSeed(seed);.

Parameters:
seedthe initial seed

Definition at line 132 of file Random.java.

  {
    setSeed(seed);
  }

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Member Function Documentation

synchronized int java.util.Random.next ( int  bits) [inline, protected]

Generates the next pseudorandom number.

This returns an int value whose 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));
}
Parameters:
bitsthe number of random bits to generate, in the range 1..32
Returns:
the next pseudorandom value
Since:
1.1

Definition at line 173 of file Random.java.

  {
    seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
    return (int) (seed >>> (48 - bits));
  }

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Generates the next pseudorandom boolean.

True and false have the same probability. The implementation is:

public boolean nextBoolean()
{
  return next(1) != 0;
}
Returns:
the next pseudorandom boolean
Since:
1.2

Definition at line 335 of file Random.java.

  {
    return next(1) != 0;
  }

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void java.util.Random.nextBytes ( byte[]  bytes) [inline]

Fills an array of bytes with random numbers.

All possible values are (approximately) equally likely. The JDK documentation gives no implementation, but it seems to be:

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;
    }
  }
}
Parameters:
bytesthe byte array that should be filled
Exceptions:
NullPointerExceptionif bytes is null
Since:
1.1

Definition at line 201 of file Random.java.

  {
    int random;
    // Do a little bit unrolling of the above algorithm.
    int max = bytes.length & ~0x3;
    for (int i = 0; i < max; i += 4)
      {
        random = next(32);
        bytes[i] = (byte) random;
        bytes[i + 1] = (byte) (random >> 8);
        bytes[i + 2] = (byte) (random >> 16);
        bytes[i + 3] = (byte) (random >> 24);
      }
    if (max < bytes.length)
      {
        random = next(32);
        for (int j = max; j < bytes.length; j++)
          {
            bytes[j] = (byte) random;
            random >>= 8;
          }
      }
  }

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double java.util.Random.nextDouble ( ) [inline]

Generates the next pseudorandom double uniformly distributed between 0.0 (inclusive) and 1.0 (exclusive).

The implementation is as follows.

public double nextDouble()
{
  return (((long) next(26) << 27) + next(27)) / (double)(1L << 53);
}
Returns:
the next pseudorandom double

Definition at line 369 of file Random.java.

  {
    return (((long) next(26) << 27) + next(27)) / (double) (1L << 53);
  }

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float java.util.Random.nextFloat ( ) [inline]

Generates the next pseudorandom float uniformly distributed between 0.0f (inclusive) and 1.0f (exclusive).

The implementation is as follows.

public float nextFloat()
{
  return next(24) / ((float)(1 << 24));
}
Returns:
the next pseudorandom float

Definition at line 352 of file Random.java.

  {
    return next(24) / (float) (1 << 24);
  }

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synchronized double java.util.Random.nextGaussian ( ) [inline]

Generates the next pseudorandom, Gaussian (normally) distributed double value, with mean 0.0 and standard deviation 1.0.

The algorithm is as follows.

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.

Returns:
the next pseudorandom Gaussian distributed double

Definition at line 409 of file Random.java.

  {
    if (haveNextNextGaussian)
      {
        haveNextNextGaussian = false;
        return nextNextGaussian;
      }
    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;
  }

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Generates the next pseudorandom number.

This returns an int value whose 32 bits are independent chosen random bits (0 and 1 are equally likely). The implementation for java.util.Random is:

public int nextInt()
{
  return next(32);
}
Returns:
the next pseudorandom value

Definition at line 238 of file Random.java.

  {
    return next(32);
  }

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int java.util.Random.nextInt ( int  n) [inline]

Generates the next pseudorandom number.

This returns a value between 0(inclusive) and 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.

Parameters:
nthe upper bound
Exceptions:
IllegalArgumentExceptionif the given upper bound is negative
Returns:
the next pseudorandom value
Since:
1.2

Definition at line 290 of file Random.java.

  {
    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;
  }

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long java.util.Random.nextLong ( ) [inline]

Generates the next pseudorandom long number.

All bits of this long are independently chosen and 0 and 1 have equal likelihood. The implementation for java.util.Random is:

public long nextLong()
{
  return ((long) next(32) << 32) + next(32);
}
Returns:
the next pseudorandom value

Definition at line 318 of file Random.java.

  {
    return ((long) next(32) << 32) + next(32);
  }

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synchronized void java.util.Random.setSeed ( long  seed) [inline]

Sets the seed for this pseudorandom number generator.

As described above, two instances of the same random class, starting with the same seed, should produce the same results, if the same methods are called. The implementation for java.util.Random is:

public synchronized void setSeed(long seed)
{
  this.seed = (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
  haveNextNextGaussian = false;
}
Parameters:
seedthe new seed

Definition at line 151 of file Random.java.

  {
    this.seed = (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
    haveNextNextGaussian = false;
  }

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Member Data Documentation

True if the next nextGaussian is available.

This is used by nextGaussian, which generates two gaussian numbers by one call, and returns the second on the second call.

whether nextNextGaussian is available

See also:
nextGaussian()
nextNextGaussian

Definition at line 87 of file Random.java.

The next nextGaussian, when available.

This is used by nextGaussian, which generates two gaussian numbers by one call, and returns the second on the second call.

the second gaussian of a pair

See also:
nextGaussian()
haveNextNextGaussian

Definition at line 98 of file Random.java.

long java.util.Random.seed [private]

The seed.

This is the number set by setSeed and which is used in next.

the internal state of this generator

See also:
next()

Definition at line 107 of file Random.java.

final long java.util.Random.serialVersionUID = 3905348978240129619L [static, private]

Compatible with JDK 1.0+.

Definition at line 112 of file Random.java.


The documentation for this class was generated from the following file: