Random (Java Card API, Connected Edition)
Date: 2009-12-11

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## java.util
Class Random

    java.lang.Object
      java.util.Random

    @TransactionType(value=NOT_SUPPORTED)
    public class Random

extends Object

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.

Since:
  JDK1.0, CLDC 1.0, Java Card 3.0

Constructor Summary
`Random()` Creates a new random number generator.
`Random(long seed)` Creates a new random number generator using a single `long` seed:

Method Summary
`protected int` | `next(int bits)` Generates the next pseudorandom number.
`int` | `nextInt()` Returns the next pseudorandom, uniformly distributed `int` value from this random number generator's sequence.
`int` | `nextInt(int n)` 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(long seed)` Sets the seed of this random number generator using a single `long` seed.

Methods inherited from class java.lang.Object
`equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Constructor Detail

### Random

    public Random()

  Creates a new random number generator. Its seed is initialized to a value based on the current time:

  >      public Random() {
  >          this(System.currentTimeMillis());
  >      }

  See Also:
    `System.currentTimeMillis()`

### Random

    public Random(long seed)

  Creates a new random number generator using a single `long` seed:

  >      public Random(long seed) {
  >          setSeed(seed);
  >      }

  Used by method `next` to hold the state of the pseudorandom number generator.

  Parameters:
    `seed` - the initial seed.
  See Also:
    `setSeed(long)`

Method Detail

### 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 as follows:

  >      synchronized public void setSeed(long seed) {
  >          this.seed = (seed ˆ 0x5DEECE66DL) & ((1L << 48) - 1);
  >      }

  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. Subclass 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` as follows:

  >      synchronized protected int next(int bits) {
  >          seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
  >          return (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: Seminumerical Algorithms, section 3.2.1.

  Parameters:
    `bits` - random bits
  Returns:
    the next pseudorandom value from this random number generator's sequence.

### 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 follows:

  >      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 n)

  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 `n` possible `int` values are produced with (approximately) equal probability. The method `nextInt(int n)` is implemented by class `Random` as follows:

  >      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;
  >      }

  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 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:
    `n` - the bound on the random number to be returned. Must be positive.
  Returns:
    a pseudorandom, uniformly distributed `int` value between 0 (inclusive) and n (exclusive).
  Throws:
    `IllegalArgumentException` - n is not positive.
  Since:
    CLDC 1.1

### 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. All 264 possible `long` values are produced with (approximately) equal probability. The method `nextLong` is implemented by class `Random` as follows:

  >      public long nextLong() {
  >          return ((long) next(32) << 32) + next(32);
  >      }

  Returns:
    the next pseudorandom, uniformly distributed `long` value from this random number generator's sequence.

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Copyright (c) 2009 Sun Microsystems, Inc. All rights reserved.