java.util
Class Random

java.lang.Object

  |

  +--java.util.Random

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

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.
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 ^ 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.

Since:

JDK1.1

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 follows:

 public int nextInt() {  return next(32); }

Returns:

the next pseudorandom, uniformly distributed int value from this random number generator's sequence.

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.