How to get a random number in a specific range

Question

I know it is asked many times, but all of them are mixed with another questions that make it hard to understand. However I want to use Math.random to get a random number in 0-10.

In fact the random number will be used as index number of a list.

What I have done so far:

int random = Math.random(0,9); // Obviously does not work

It's not a BAD question... it's just been asked too many times... mainly because the API is confusing in that Math exposes a partial solution to generating random numbers which noobs (including myself at one time) find and therefore stop looking for a GOOD solution to this problem... so they ask an expert, again, and again, and again. Blame the highly-paid expert API designers, not the noob! — corlettk, Aug 17 '13 at 01:08

score 5 · Answer 1 · answered Aug 17 '13 at 00:26

5

You can create a Random object and call nextInt to get an int between 0 and the inputted number (exclusive).

Random rnd = new Random();
int random = rnd.nextInt(11);  // returns range 0-10

Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive)

answered Aug 17 '13 at 00:26

rgettman

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Note: In practice there's no need to get a new instance of Random EVERY time you get a random value... and it can lead to "pointless excessive garbage". Just saying. – corlettk Aug 17 '13 at 00:36
1

Not just "no need", it's absolutely wrong to do so. If you recreate the object for every call, your numbers won't be random at all. – Lee Daniel Crocker Aug 17 '13 at 07:26

nanofarad · Answer 2 · 2013-08-17T12:34:02.910

1

Don't use Math.random() Use Random#nextInt() instead:

int myInt=new Random().nextInt(11);

Or, as per comments, use

Random r=new Random();
int myInt=r.nextInt(11);

and reuse r as needed.

edited Aug 17 '13 at 12:34

answered Aug 17 '13 at 00:25

nanofarad

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Creating the random object and only getting one number from it won't generate truly random numbers. – Lee Daniel Crocker Aug 17 '13 at 07:27
@LeeDanielCrocker If only one number is being generated it should be sufficient. – nanofarad Aug 17 '13 at 12:33

score 1 · Answer 3 · answered Aug 17 '13 at 01:02

To get a random item from any java.util.List (using generics) I get:

WARNING: The following code is NOT tested.

package forums;

import java.util.List;
import java.util.Random;

public final class ListHelper<T>
{
    private static final Random RANDOM = new Random();

    public T getRandom(List<T> list) {
        return list.get(RANDOM.nextInt(list.size()));
    }
}

... or you may prefer the non-generics solution, which can be static, but requires you to typecast the result of getRandom every time (everywhere) you call it

package forums;

import java.util.List;
import java.util.Random;

public final class Lists
{
    private static final Random RANDOM = new Random();

    public static Object getRandom(List list) {
        return list.get(RANDOM.nextInt(list.size()));
    }
}

Cheers. Keith.

How to get a random number in a specific range

3 Answers3