How to get a random between 1 - 100 from randDouble in Java?

Question

Okay, I'm still fairly new to Java. We've been given an assisgnment to create a game where you have to guess a random integer that the computer had generated. The problem is that our lecturer is insisting that we use:

double randNumber = Math.random();

And then translate that into an random integer that accepts 1 - 100 inclusive. I'm a bit at a loss. What I have so far is this:

//Create random number 0 - 99
double randNumber = Math.random();
d = randNumber * 100;

//Type cast double to int
int randomInt = (int)d;

However, the random the lingering problem of the random double is that 0 is a possibility while 100 is not. I want to alter that so that 0 is not a possible answer and 100 is. Help?

Answer 1

or

Random r = new Random();
int randomInt = r.nextInt(100) + 1;

Answer 2

The ThreadLocalRandom class provides the int nextInt(int origin, int bound) method to get a random integer in a range:

// Returns a random int between 1 (inclusive) & 101 (exclusive)
int randomInt = ThreadLocalRandom.current().nextInt(1, 101)

ThreadLocalRandom is one of several ways to generate random numbers in Java, including the older Math.random() method and java.util.Random class. The advantage of ThreadLocalRandom is that it is specifically designed be used within a single thread, avoiding the additional thread synchronization costs imposed by the other implementations. Therefore, it is usually the best built-in random implementation to use outside of a security-sensitive context.

When applicable, use of ThreadLocalRandom rather than shared Random objects in concurrent programs will typically encounter much less overhead and contention.