Recipe 5.13 Generating Random Numbers

Problem

You need to generate random numbers in a hurry.

Solution

Use java.lang.Math.random( ) to generate random numbers. There is no claim that the random values it returns are very good random numbers, however. This code exercises the random( ) method:

// Random1.java  // java.lang.Math.random( ) is static, don't need to construct Math  System.out.println("A random from java.lang.Math is " + Math.random( ));

Note that this method only generates double values. If you need integers, you need to scale and round:

/** Generate random ints by asking Random( ) for  * a series of random integers from 1 to 10, inclusive.  *  * @author Ian Darwin, http://www.darwinsys.com/  * @version $Id: ch05.xml,v 1.5 2004/05/04 20:1:35 ian Exp $  */ public class RandomInt {     public static void main(String[] a) {         Random r = new Random( );         for (int i=0; i<1000; i++)             // nextInt(10) goes from 0-9; add 1 for 1-10;             System.out.println(1+Math.round(r.nextInt(10)));     } }

To see if it was really working well, I used the Unix tools sort and uniq, which, together, give a count of how many times each value was chosen. For 1,000 integers, each of 10 values should be chosen about 100 times:

C:> java RandomInt | sort -n | uniq -c  110 1  106 2   98 3  109 4  108 5   99 6   94 7   91 8   94 9   91 10 C:>

See Also

Recipe 5.14 shows easier and better ways to get random integers and doubles. Also see the Javadoc documentation for java.lang.Math and the warning in this chapter's Introduction about pseudo-randomness versus real randomness.