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To generate random numbers the Math.random
method can be used, which returns a double
, greater than or equal to 0.0 and less than 1.0.
The following code returns a random integer between n and m (where n <= randomNumber < m):
Code section 3.30: A random integer.
1 int randomNumber = n + (int)(Math.random * ( m  n ));

Alternatively, the java.util.Random
class provides methods for generating random boolean
s, byte
s, float
s, int
s, long
s and 'Gaussians' (double
s from a normal distribution with mean 0.0 and standard deviation 1.0). For example, the following code is equivalent to that above:
Code section 3.31: A random integer with Gaussian.
1 Random random = new Random;
2 int randomNumber = n + random.nextInt(m  n);

As an example using random numbers, we can make a program that uses a Random object to simulate flipping a coin 20 times:


Of course, if you run the program you will probably get different results.
Both Math.random
and the Random
class produce pseudorandom numbers. This is good enough for a lot of applications, but remember that it is not truly random. If you want a more secure random number generator, Java provides the java.security.SecureRandom
package. What happens with Math.random
and the Random
class is that a 'seed' is chosen from which the pseudorandom numbers are generated. SecureRandom
increases the security to ensure that the seed which is used by the pseudorandom number generator is nondeterministic  that is, you cannot simply put the machine in the same state to get the same set of results. Once you have created a SecureRandom
instance, you can use it in the same way as you can the Random
class.
If you want truly random numbers, you can get a hardware random number generator or use a randomness generation service.
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