Random

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

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

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

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

Returns the next pseudorandom, uniformly distributed double value between 0.0 and 1.0 from this random number generator's sequence.

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.

The method nextDouble is implemented by class Random as if by:

 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);
 

Returns the next pseudorandom, uniformly distributed float value between 0.0 and 1.0 from this random number generator's sequence.

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.

The method nextFloat is implemented by class Random as if by:

 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));
 

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 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 if by:

 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 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.

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);
 }
 

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);
 }
 

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));
 }