Java basics - random numbers

Math.random()

This is as simple as it gets:
Returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0.

Usage:

double d = Math.random();

Random class methods

An instance of this class is used to generate a stream of pseudorandom numbers.

Here are some of the methods:

Method	Return type	Return value
Get the next value in the stream:
nextBoolean()	boolean	true or false
nextDouble()	double	between 0.0d (inclusive) and 1.0d (exclusive)
nextFloat()	float	between 0.0f (inclusive) and 1.0f (exclusive)
nextInt()	int	any int within int range
nextLong()	long	any long within long range
Generate a random integer in a given range:
nextInt(int n)	int	between 0 (inclusive) and the specified value (exclusive)

Usage:

Random r = new Random();
double d = r.nextDouble();

ThreadLocalRandom class methods

A random number generator isolated to the current thread.
It's a subclass of java.util.Random, so all methods that are in Random can be used here too.
Get the instance with ThreadLocalRandom.current()

Method	Return type	Return value
nextDouble(double n)	double	between 0 (inclusive) and the specified value (exclusive)
nextDouble(double least, double bound)	double	between the given least value (inclusive) and bound (exclusive)
nextInt(int least, int bound)	int	between the given least value (inclusive) and bound (exclusive)
nextLong(long n)	long	between 0 (inclusive) and the specified value (exclusive)
nextLong(long least, long bound)	long	between the given least value (inclusive) and bound (exclusive)

Usage:

double d = ThreadLocalRandom.current().nextDouble(0.0, 1.0);