How do I generate a random integer between min and max in Java?

Question

What method returns a random int between a min and max? Or does no such method exist?

What I'm looking for is something like this:

NAMEOFMETHOD (min, max) 

(where min and max are ints), that returns something like this:

8

(randomly)

If such a method does exist could you please link to the relevant documentation with your answer.

Thanks.

---

UPDATE

Attempting to implement the full solution and I get the following error message:

class TestR
{
 public static void main (String[]arg) 
 { 
 Random random = new Random() ;
 int randomNumber = random.nextInt(5) + 2;
 System.out.println (randomNumber) ; 
 } 
} 

I'm still getting the same errors from the compiler:

TestR.java:5: cannot find symbol
symbol : class Random
location: class TestR
 Random random = new Random() ;
 ^
TestR.java:5: cannot find symbol
symbol : class Random
location: class TestR
 Random random = new Random() ;
 ^
TestR.java:6: operator + cannot be applied to Random.nextInt,int
 int randomNumber = random.nextInt(5) + 2;
 ^
TestR.java:6: incompatible types
found : <nulltype>
required: int
 int randomNumber = random.nextInt(5) + 2;
 ^
4 errors

What's going wrong here?

• MAK: Did you forget to declare random and assign a new instance of java.util.Random to it?
• David: could you explain how to do that?
• Batakj: Did you declare import statements for java.util.Random

Answer 1

Construct a Random object at application startup:

Random random = new Random();

Then use Random.nextInt(int):

int randomNumber = random.nextInt(max + 1 - min) + min;

Note that the both lower and upper limits are inclusive.

• David: i updated the question with the error that occurs when i try to do that what is going wrong?
• BalusC: David, you need to instantiate it first. Random random = new Random(); It's still just Java code. There's no means of magic ;)
• Mark Byers: And make sure that you only instantiate the Random object once and reuse it. Don't create a new Random object for each call to your function.
• Mark Byers: You are missing import java.util.Random;.
• Mark Byers: @ataulm: In older versions of Java creating multiple Random objects didn't guarantee randomness. "Two Random objects created within the same millisecond will have the same sequence of random numbers." (source). In newer versions it is probably OK, but I didn't know that when I wrote my comment three years ago.

Answer 2

You can use Random.nextInt(n). This returns a random int in [0,n). Just using max-min+1 in place of n and adding min to the answer will give a value in the desired range.

• David: i updated the question with the error that occurs when i try to do that what is going wrong?

Answer 3

public static int random_int(int Min, int Max)
{
 return (int) (Math.random()*(Max-Min))+Min;
}
random_int(5, 9); // For example

• Ruben: Your algorithm is Random x * positive number + positive number. How can this always result in a number between max and min?
• Pablo Pazos: Random is between 0 and 1. Do the math.
• Synn: Be aware that the lower limit is inclusive but the upper limit is exclusive in this answer. If you wants both to be inclusive use: return (int) (Math.random()*(Max-Min + 1))+Min

Answer 4

With Java 7 or above you could use

ThreadLocalRandom.current().nextInt(int origin, int bound)

Javadoc: ThreadLocalRandom.nextInt

Answer 5

As the solutions above do not consider the possible overflow of doing max-min when min is negative, here another solution (similar to the one of kerouac)

public static int getRandom(int min, int max) {
 if (min > max) {
 throw new IllegalArgumentException("Min " + min + " greater than max " + max);
 } 
 return (int) ( (long) min + Math.random() * ((long)max - min + 1));
}

this works even if you call it with:

getRandom(Integer.MIN_VALUE, Integer.MAX_VALUE) 

Answer 6

Using the Random class is the way to go as suggested in the accepted answer, but here is a less straight-forward correct way of doing it if you didn't want to create a new Random object :

min + (int) (Math.random() * (max - min + 1));

Answer 7

This generates a random integer of size psize

public static Integer getRandom(Integer pSize) {
 if(pSize<=0) {
 return null;
 }
 Double min_d = Math.pow(10, pSize.doubleValue()-1D);
 Double max_d = (Math.pow(10, (pSize).doubleValue()))-1D;
 int min = min_d.intValue();
 int max = max_d.intValue();
 return RAND.nextInt(max-min) + min;
}

• Andrew Thompson: That seems a complex way of achieving something that can be done in a single line of code. Please use code formatting in future.

Answer 8

import java.util.Random;