Asked 2023-09-20 20:13:01 View
197,834

How do I generate a random `int`

value in a specific range?

The following methods have bugs related to integer overflow:

```
randomNum = minimum + (int)(Math.random() * maximum);
// Bug: `randomNum` can be bigger than `maximum`.
```

```
Random rn = new Random();
int n = maximum - minimum + 1;
int i = rn.nextInt() % n;
randomNum = minimum + i;
// Bug: `randomNum` can be smaller than `minimum`.
```

- Before you post a new answer, consider there are already 65+ answers for this question. Please, make sure that your answer contributes information that is not among existing answers. - anyone
- The corresponding one for JavaScript:
*Generating random whole numbers in JavaScript in a specific range*- anyone

In **Java 1.7 or later**, the standard way to do this is as follows:

```
import java.util.concurrent.ThreadLocalRandom;
// nextInt is normally exclusive of the top value,
// so add 1 to make it inclusive
int randomNum = ThreadLocalRandom.current().nextInt(min, max + 1);
```

See the relevant JavaDoc. This approach has the advantage of not needing to explicitly initialize a java.util.Random instance, which can be a source of confusion and error if used inappropriately.

However, conversely there is no way to explicitly set the seed so it can be difficult to reproduce results in situations where that is useful such as testing or saving game states or similar. In those situations, the pre-Java 1.7 technique shown below can be used.

**Before Java 1.7**, the standard way to do this is as follows:

```
import java.util.Random;
/**
* Returns a pseudo-random number between min and max, inclusive.
* The difference between min and max can be at most
* <code>Integer.MAX_VALUE - 1</code>.
*
* @param min Minimum value
* @param max Maximum value. Must be greater than min.
* @return Integer between min and max, inclusive.
* @see java.util.Random#nextInt(int)
*/
public static int randInt(int min, int max) {
// NOTE: This will (intentionally) not run as written so that folks
// copy-pasting have to think about how to initialize their
// Random instance. Initialization of the Random instance is outside
// the main scope of the question, but some decent options are to have
// a field that is initialized once and then re-used as needed or to
// use ThreadLocalRandom (if using at least Java 1.7).
//
// In particular, do NOT do 'Random rand = new Random()' here or you
// will get not very good / not very random results.
Random rand;
// nextInt is normally exclusive of the top value,
// so add 1 to make it inclusive
int randomNum = rand.nextInt((max - min) + 1) + min;
return randomNum;
}
```

See the relevant JavaDoc. In practice, the java.util.Random class is often preferable to java.lang.Math.random().

In particular, there is no need to reinvent the random integer generation wheel when there is a straightforward API within the standard library to accomplish the task.

Answered 2023-09-20 20:13:01

- For calls where
`max`

value is`Integer.MAX_VALUE`

it is possible to overflow ,resulting into a`java.lang.IllegalArgumentException`

. You can try with :`randInt(0, Integer.MAX_VALUE)`

. Also, if`nextInt((max-min) + 1)`

returns the most high value (quite rare, I assume) won't it overflow again( supposing min and max are high enough values)? How to deal with this kind of situations? - anyone - Now there is nextLong(long origin, long bound) available. Posting for reference. I am unaware of the fact if it was there when asnwer for posted. - anyone
- I prefer
`ThreadLocalRandom`

solution even if you can't specify the seed or you don't need a thread safe method, since the API is more intuitive if`min != 0`

. - anyone - I would argue that this is no longer the "standard answer" as of Java 17, though it works well if you want a single global multhreaded generator. Per
`RandomGenerator`

, "It is recommended that multithreaded applications use either`ThreadLocalRandom`

or (preferably) pseudorandom number generators that implement the`RandomGenerator.SplittableGenerator`

or`RandomGenerator.JumpableGenerator`

interface." See: stackoverflow.com/a/70529176/1108305. - anyone

Note that this approach is more biased and less efficient than a `nextInt`

approach, https://stackoverflow.com/a/738651/360211

One standard pattern for accomplishing this is:

```
Min + (int)(Math.random() * ((Max - Min) + 1))
```

The Java Math library function Math.random() generates a double value in the range `[0,1)`

. Notice this range does not include the 1.

In order to get a specific range of values first, you need to multiply by the magnitude of the range of values you want covered.

```
Math.random() * ( Max - Min )
```

This returns a value in the range `[0,Max-Min)`

, where 'Max-Min' is not included.

For example, if you want `[5,10)`

, you need to cover five integer values so you use

```
Math.random() * 5
```

This would return a value in the range `[0,5)`

, where 5 is not included.

Now you need to shift this range up to the range that you are targeting. You do this by adding the Min value.

```
Min + (Math.random() * (Max - Min))
```

You now will get a value in the range `[Min,Max)`

. Following our example, that means `[5,10)`

:

```
5 + (Math.random() * (10 - 5))
```

But, this still doesn't include `Max`

and you are getting a double value. In order to get the `Max`

value included, you need to add 1 to your range parameter `(Max - Min)`

and then truncate the decimal part by casting to an int. This is accomplished via:

```
Min + (int)(Math.random() * ((Max - Min) + 1))
```

And there you have it. A random integer value in the range `[Min,Max]`

, or per the example `[5,10]`

:

```
5 + (int)(Math.random() * ((10 - 5) + 1))
```

Answered 2023-09-20 20:13:01

- The Sun documentation explicitly says that you should better use Random() if you need an int instead of Math.random() which produces a double. - anyone
- This is actually biased compared to nextInt methods stackoverflow.com/a/738651/360211 - anyone
- "Biased" in this case means that after 2^53 executions, some numbers will have had one extra occourance, on average. - anyone
- Even thought i use this too, i want to point out that this is not a true random number. Thats why it should not be used for any security functionality. But for any casual cases it is the most straight forwrd method. - anyone
- Also, the nextInt() methods in Random do not include the upper bound. - anyone

Use:

```
Random ran = new Random();
int x = ran.nextInt(6) + 5;
```

The integer `x`

is now the random number that has a possible outcome of `5-10`

.

Answered 2023-09-20 20:13:01

With java-8 they introduced the method `ints(int randomNumberOrigin, int randomNumberBound)`

in the `Random`

class.

For example if you want to generate five random integers (or a single one) in the range [0, 10], just do:

```
Random r = new Random();
int[] fiveRandomNumbers = r.ints(5, 0, 11).toArray();
int randomNumber = r.ints(1, 0, 11).findFirst().getAsInt();
```

The first parameter indicates just the size of the `IntStream`

generated (which is the overloaded method of the one that produces an unlimited `IntStream`

).

If you need to do multiple separate calls, you can create an infinite primitive iterator from the stream:

```
public final class IntRandomNumberGenerator {
private PrimitiveIterator.OfInt randomIterator;
/**
* Initialize a new random number generator that generates
* random numbers in the range [min, max]
* @param min - the min value (inclusive)
* @param max - the max value (inclusive)
*/
public IntRandomNumberGenerator(int min, int max) {
randomIterator = new Random().ints(min, max + 1).iterator();
}
/**
* Returns a random number in the range (min, max)
* @return a random number in the range (min, max)
*/
public int nextInt() {
return randomIterator.nextInt();
}
}
```

You can also do it for `double`

and `long`

values.

Answered 2023-09-20 20:13:01

- I would suggest that you instantiate the randomIterator only once. See Greg Case comment on his own answer. - anyone

You can edit your second code example to:

```
Random rn = new Random();
int range = maximum - minimum + 1;
int randomNum = rn.nextInt(range) + minimum;
```

Answered 2023-09-20 20:13:01

Just a small modification of your first solution would suffice.

```
Random rand = new Random();
randomNum = minimum + rand.nextInt((maximum - minimum) + 1);
```

See more here for implementation of `Random`

Answered 2023-09-20 20:13:01

- For minimum <= value < maximum, I did the same with Math :
**randomNum = minimum + (int)(Math.random() * (maximum-minimum));**but the casting isn't really nice to see ;) - anyone

`ThreadLocalRandom`

equivalent of class `java.util.Random`

for multithreaded environment. Generating a random number is carried out locally in each of the threads. So we have a better performance by reducing the conflicts.

```
int rand = ThreadLocalRandom.current().nextInt(x,y);
```

`x`

,`y`

- intervals e.g. (1,10)

Answered 2023-09-20 20:13:01

The `Math.Random`

class in Java is 0-based. So, if you write something like this:

```
Random rand = new Random();
int x = rand.nextInt(10);
```

`x`

will be between `0-9`

inclusive.

So, given the following array of `25`

items, the code to generate a random number between `0`

(the base of the array) and `array.length`

would be:

```
String[] i = new String[25];
Random rand = new Random();
int index = 0;
index = rand.nextInt( i.length );
```

Since `i.length`

will return `25`

, the `nextInt( i.length )`

will return a number between the range of `0-24`

. The other option is going with `Math.Random`

which works in the same way.

```
index = (int) Math.floor(Math.random() * i.length);
```

For a better understanding, check out forum post *Random Intervals (archive.org)*.

Answered 2023-09-20 20:13:01

- It baffles me why you instantiate index to 0. - anyone
- @CodeConfident The
`index`

variable will not affect the result of the random number. You can choose to initialize it any way you would like without having to worry about changing the outcome. Hope this helps. - anyone - Exactly... it's completely unused. I would initialise it directly to the rand:
`int index = rand.nextInt(i.Length);`

- anyone - Or not at all.
`int index; \n index = rand...`

if one is fond of declarations and assignments on different lines. Some coding standards are more stringent (and without apparent purpose) than others. - anyone

It can be done by simply doing the statement:

```
Randomizer.generate(0, 10); // Minimum of zero and maximum of ten
```

Below is its source code.

```
public class Randomizer {
public static int generate(int min, int max) {
return min + (int)(Math.random() * ((max - min) + 1));
}
}
```

It is just clean and simple.

Answered 2023-09-20 20:13:01

- This could have been an edit of the other example, now it is just a blatant copy for reputation. Pointing out the "answer with the most votes" is also not very direct, it can change. - anyone
- That is right @MaartenBodewes. When I wrote this answer, the answer with the most votes above was still written as an algorithm-like solution. Now, the solution above has changed a lot and now this answer looked like a copy-cat. - anyone
- I really don't get why such fundamental bits of code are not part of Java standard libraries. Why do I have to implement this? - anyone
- @MaartenBodewes When two answers provide the same solution, and both have been edited, it may not be clear and obvious which answer provided a given solution first. In that scenario please be very cautious about accusing anyone of
*"blatant copy for reputation"*. (I'm all in favor of calling out copycats, but just make sure that you don't make a false accusation, which is what seems to have happened here.) - anyone - @skomisa What the hell are you talking about, it explicitly pointed towards the answer with the same solution and then wrote a function header in front of it. - anyone

Forgive me for being fastidious, but the solution suggested by the majority, i.e., `min + rng.nextInt(max - min + 1))`

, seems perilous due to the fact that:

`rng.nextInt(n)`

cannot reach`Integer.MAX_VALUE`

.`(max - min)`

may cause overflow when`min`

is negative.

A foolproof solution would return correct results for any `min <= max`

within [`Integer.MIN_VALUE`

, `Integer.MAX_VALUE`

]. Consider the following naive implementation:

```
int nextIntInRange(int min, int max, Random rng) {
if (min > max) {
throw new IllegalArgumentException("Cannot draw random int from invalid range [" + min + ", " + max + "].");
}
int diff = max - min;
if (diff >= 0 && diff != Integer.MAX_VALUE) {
return (min + rng.nextInt(diff + 1));
}
int i;
do {
i = rng.nextInt();
} while (i < min || i > max);
return i;
}
```

Although inefficient, note that the probability of success in the `while`

loop will always be 50% or higher.

Answered 2023-09-20 20:13:01

- Why not throw an IllegalArgumentException when the difference = Integer.MAX_VALUE? Then you don't need the while loop. - anyone
- @mpkorstanje This implementation is designed to work with any values of min <= max, even when their difference is equal to or even larger than MAX_VALUE. Running a loop until success is a common pattern in this case, to guarantee uniform distribution (if the underlying source of randomness is uniform). Random.nextInt(int) does it internally when the argument is not a power of 2. - anyone

I wonder if any of the random number generating methods provided by an Apache Commons Math library would fit the bill.

For example: `RandomDataGenerator.nextInt`

or `RandomDataGenerator.nextLong`

Answered 2023-09-20 20:13:01

I use this:

```
/**
* @param min - The minimum.
* @param max - The maximum.
* @return A random double between these numbers (inclusive the minimum and maximum).
*/
public static double getRandom(double min, double max) {
return (Math.random() * (max + 1 - min)) + min;
}
```

You can cast it to an Integer if you want.

Answered 2023-09-20 20:13:01

- This function produces the same number over and over again. In my case it was: 2147483647 - anyone
- Fail: you have a function that requires a double and then perform + 1? This certainly goes against the principle of least surprise. What happens if you use min = 0.1 and max = 0.2? - anyone
- @sokras the method calls
`new Random`

(check the JavaDoc): "Creates a new random number generator. This constructor sets the seed of the random number generator to a value very likely to be distinct from any other invocation of this constructor." Very likely might just involve using the current time as seed. If that time uses milliseconds then current computers are fast enough to generate the same number. But besides that 2147483647 is`Integer.MAX_VALUE`

; the output obviously depends on the input, which you haven't specified. - anyone

As of Java 7, you should no longer use

Random. For most uses, the random number generator of choice is now ThreadLocalRandom.

For fork join pools and parallel streams, use SplittableRandom.

*Joshua Bloch. Effective Java. Third Edition.*

For fork join pools and parallel streams, use SplittableRandom (it implements SplittableGenerator interface, see Java 17 notes below) that is usually faster, has a better statistical independence and uniformity properties in comparison with *Random*.

To generate a random `int`

in the range `[0, 1_000]:`

```
int n = new SplittableRandom().nextInt(0, 1_001);
```

To generate a random `int[100]`

array of values in the range `[0, 1_000]:`

```
int[] a = new SplittableRandom().ints(100, 0, 1_001).parallel().toArray();
```

To return a Stream of random values:

```
IntStream stream = new SplittableRandom().ints(100, 0, 1_001);
```

It is recommended that multithreaded applications use either ThreadLocalRandom or (preferably) pseudorandom number generators that implement the RandomGenerator.SplittableGenerator or RandomGenerator.JumpableGenerator interface.

*RandomGenerator ^{1} documentation*.

^{1 - RandomGenerator is a modern interface that was introduced in Java 17. It is a common protocol for objects that generate random or pseudorandom sequences of numbers and has more features and methods than Random.}

Answered 2023-09-20 20:13:01

- Is there a reason why the example includes a
`.parallel()`

? It seems to me like generating a 100 random numbers would be too trivial to warrant parallelism. - anyone - @Johnbot Thanks for comment, you are right. But, the main reason was to show an API (of course, the smart path is to measure performance before using
`parallel`

processing). By the way, for array of`1_000_000`

elements, the`parallel`

version was 2 times faster on my machine in comparison with sequential. - anyone

Let us take an example.

Suppose I wish to generate a number between **5-10**:

```
int max = 10;
int min = 5;
int diff = max - min;
Random rn = new Random();
int i = rn.nextInt(diff + 1);
i += min;
System.out.print("The Random Number is " + i);
```

*Let us understand this*...

Initialize max with highest value and min with the lowest value.

Now, we need to determine how many possible values can be obtained. For this example, it would be:

5, 6, 7, 8, 9, 10So, count of this would be max - min + 1.

i.e. 10 - 5 + 1 = 6

The random number will generate a number between

0-5.i.e. 0, 1, 2, 3, 4, 5

Adding the

minvalue to the random number would produce:5, 6, 7, 8, 9, 10

Hence we obtain the desired range.

Answered 2023-09-20 20:13:01

Generate a random number for the difference of min and max by using the nextint(n) method and then add min number to the result:

```
Random rn = new Random();
int result = rn.nextInt(max - min + 1) + min;
System.out.println(result);
```

Answered 2023-09-20 20:13:01

- How is this different from previous answers? - anyone

To generate a random number "in between two numbers", use the following code:

```
Random r = new Random();
int lowerBound = 1;
int upperBound = 11;
int result = r.nextInt(upperBound-lowerBound) + lowerBound;
```

This gives you a random number in between 1 (inclusive) and 11 (exclusive), so initialize the upperBound value by adding 1. For example, if you want to generate random number between 1 to 10 then initialize the upperBound number with 11 instead of 10.

Answered 2023-09-20 20:13:01

Just use the Random class:

```
Random ran = new Random();
// Assumes max and min are non-negative.
int randomInt = min + ran.nextInt(max - min + 1);
```

Answered 2023-09-20 20:13:01

- I don't see anything new here that hadn't been posted in countless earlier posts. - anyone
- @MaartenBodewes Those countless posts are hard to find. Some other methods on top - anyone
- Yeah, posting yet another dupe is not going to help against that. Vote up the other answers or add details. Otherwise it is just rep farming. - anyone

These methods might be convenient to use:

This method will return a random number * between* the provided minimum and maximum value:

```
public static int getRandomNumberBetween(int min, int max) {
Random foo = new Random();
int randomNumber = foo.nextInt(max - min) + min;
if (randomNumber == min) {
// Since the random number is between the min and max values, simply add 1
return min + 1;
} else {
return randomNumber;
}
}
```

and this method will return a random number * from* the provided minimum and maximum value (so the generated number could also be the minimum or maximum number):

```
public static int getRandomNumberFrom(int min, int max) {
Random foo = new Random();
int randomNumber = foo.nextInt((max + 1) - min) + min;
return randomNumber;
}
```

Answered 2023-09-20 20:13:01

`// Since the random number is between the min and max values, simply add 1`

. Why? Doesn't min count? Usually the range is [min, max) where min is included and max is excluded. Wrong answer, voted down. - anyone- @MaartenBodewes +1 is added because getRandomNumberBetween generates a random number exclusive of the provided endpoints. - anyone
- The number
`min + 1`

will be twice as likely than the other number to be the result of`getRandomNumberBetween`

! - anyone

In case of rolling a dice it would be random number between 1 to 6 (not 0 to 6), so:

```
face = 1 + randomNumbers.nextInt(6);
```

Answered 2023-09-20 20:13:01

```
int random = minimum + Double.valueOf(Math.random()*(maximum-minimum )).intValue();
```

Or take a look to RandomUtils from Apache Commons.

Answered 2023-09-20 20:13:01

- That's useful, but beware a small flaw: method signatures are like: nextDouble(double startInclusive, double endInclusive), but if you look inside the methods, endInclusive should actually be endExclusive. - anyone
`Double.valueOf(Math.random()*(maximum-minimun)).intValue()`

is quite an obfuscated (and inefficient) way to say`(int)(Math.random()*(maximum-minimun))`

… - anyone- Spelling mismatch for minimum return minimum + Double.valueOf(Math.random() * (maximum - minimum)).intValue(); - anyone

You can achieve that concisely in Java 8:

```
Random random = new Random();
int max = 10;
int min = 5;
int totalNumber = 10;
IntStream stream = random.ints(totalNumber, min, max);
stream.forEach(System.out::println);
```

Answered 2023-09-20 20:13:01

Here's a helpful class to generate random `ints`

in a range with any combination of inclusive/exclusive bounds:

```
import java.util.Random;
public class RandomRange extends Random {
public int nextIncInc(int min, int max) {
return nextInt(max - min + 1) + min;
}
public int nextExcInc(int min, int max) {
return nextInt(max - min) + 1 + min;
}
public int nextExcExc(int min, int max) {
return nextInt(max - min - 1) + 1 + min;
}
public int nextIncExc(int min, int max) {
return nextInt(max - min) + min;
}
}
```

Answered 2023-09-20 20:13:01

Another option is just using Apache Commons:

```
import org.apache.commons.math.random.RandomData;
import org.apache.commons.math.random.RandomDataImpl;
public void method() {
RandomData randomData = new RandomDataImpl();
int number = randomData.nextInt(5, 10);
// ...
}
```

Answered 2023-09-20 20:13:01

I found this example Generate random numbers :

This example generates random integers in a specific range.

```
import java.util.Random;
/** Generate random integers in a certain range. */
public final class RandomRange {
public static final void main(String... aArgs){
log("Generating random integers in the range 1..10.");
int START = 1;
int END = 10;
Random random = new Random();
for (int idx = 1; idx <= 10; ++idx){
showRandomInteger(START, END, random);
}
log("Done.");
}
private static void showRandomInteger(int aStart, int aEnd, Random aRandom){
if ( aStart > aEnd ) {
throw new IllegalArgumentException("Start cannot exceed End.");
}
//get the range, casting to long to avoid overflow problems
long range = (long)aEnd - (long)aStart + 1;
// compute a fraction of the range, 0 <= frac < range
long fraction = (long)(range * aRandom.nextDouble());
int randomNumber = (int)(fraction + aStart);
log("Generated : " + randomNumber);
}
private static void log(String aMessage){
System.out.println(aMessage);
}
}
```

An example run of this class :

```
Generating random integers in the range 1..10.
Generated : 9
Generated : 3
Generated : 3
Generated : 9
Generated : 4
Generated : 1
Generated : 3
Generated : 9
Generated : 10
Generated : 10
Done.
```

Answered 2023-09-20 20:13:01

```
public static Random RANDOM = new Random(System.nanoTime());
public static final float random(final float pMin, final float pMax) {
return pMin + RANDOM.nextFloat() * (pMax - pMin);
}
```

Answered 2023-09-20 20:13:01

Here is a simple sample that shows how to generate random number from closed `[min, max]`

range, while `min <= max is true`

You can reuse it as field in hole class, also having all `Random.class`

methods in one place

**Results example:**

```
RandomUtils random = new RandomUtils();
random.nextInt(0, 0); // returns 0
random.nextInt(10, 10); // returns 10
random.nextInt(-10, 10); // returns numbers from -10 to 10 (-10, -9....9, 10)
random.nextInt(10, -10); // throws assert
```

**Sources:**

```
import junit.framework.Assert;
import java.util.Random;
public class RandomUtils extends Random {
/**
* @param min generated value. Can't be > then max
* @param max generated value
* @return values in closed range [min, max].
*/
public int nextInt(int min, int max) {
Assert.assertFalse("min can't be > then max; values:[" + min + ", " + max + "]", min > max);
if (min == max) {
return max;
}
return nextInt(max - min + 1) + min;
}
}
```

Answered 2023-09-20 20:13:01

It's better to use SecureRandom rather than just Random.

```
public static int generateRandomInteger(int min, int max) {
SecureRandom rand = new SecureRandom();
rand.setSeed(new Date().getTime());
int randomNum = rand.nextInt((max - min) + 1) + min;
return randomNum;
}
```

Answered 2023-09-20 20:13:01

- This is not that good, because if it is executed in the same mili second then you will get the same number, you need to put the rand initialization and the setSeet outside of the method. - anyone
- You need seed, yes, but using SecureRandom. - anyone
- I'm sorry, but the one who rejected the change request has no clue of Java programming
**It is a good suggestion, but as is it is wrong**because if executed in the same mili second it will give the same number, not random. - anyone - The correct solution isn't anywhere to be found, not many people know static initializer blocks... that is what you should use to set the seed: 1:
`private static int SecureRandom rand = new SecureRandom();`

2:`static {`

3:`rand.setSeed(...);`

4:`}`

- anyone - There is absolutely no need to seed
`SecureRandom`

, it will be seeded by the system. Directly calling`setSeed`

is very dangerous, it may replace the (really random) seed with the date. And that will certainly*not*result in a`SecureRandom`

, as anybody can guess the time and try and seed their own`SecureRandom`

instance with that information. - anyone