THE PACKAGE JAVA.UTIL

java.util.Random

21.9

21.9.10

public float nextFloat()

The general contract of

nextFloat

 is that one

float

 value, chosen (approxi 

mately) uniformly from the range

0.0f

 (inclusive) to

1.0f

 (exclusive), is pseudo 

randomly generated and returned. All

24

2

possible

float

 values of the form

m

24

2

 24

, where

m

 is a positive integer less than 2 , are produced with (approxi 

mately) equal probability.

The method

setSeed

 is implemented by class

Random

 as follows:

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 or 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));

This might seem to be equivalent, if not better, but in fact it introduced a slight

nonuniformity because of the bias in the rounding of floating point numbers: it

was slightly more likely that the low order bit of the significand would be

0

 than

that it would be

1

.]

21.9.11

public double nextDouble()

The general contract of

nextDouble

 is that one

double

 value, chosen (approxi 

mately) uniformly from the range

0.0d

 (inclusive) to

1.0d

 (exclusive), is pseudo 

randomly generated and returned. All

53

2

possible

float

 values of the form

 53

53

m

2

, where

m

 is a positive integer less than 2 , are produced with (approxi 

mately) equal probability.

The method

setSeed

 is implemented by class

Random

 as follows:

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 or 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:

649