Java 7+
In Java 1.7 or later, the standard way to do this (generate a basic non-cryptographically secure random integer in the range [min, max]) 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 with ThreadLocalRandom 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.
Java 17+
As of Java 17, the psuedorandom number generating classes in the standard library implement the RandomGenerator interface. See the linked JavaDoc for more information. For example, if a cryptographically strong random number generator is desired, the SecureRandom class can be used.
Earlier Java
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.
Answer from Greg Case on Stack OverflowHow do I use java.util.Random to generate random integers within a given interval?
[Java] Generate random number, and assign to integer array, and then print?
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Java 7+
In Java 1.7 or later, the standard way to do this (generate a basic non-cryptographically secure random integer in the range [min, max]) 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 with ThreadLocalRandom 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.
Java 17+
As of Java 17, the psuedorandom number generating classes in the standard library implement the RandomGenerator interface. See the linked JavaDoc for more information. For example, if a cryptographically strong random number generator is desired, the SecureRandom class can be used.
Earlier Java
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.
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))
I am trying to generate a random number between 25 and 50, however every time I run my code, I keep getting 35. My teacher says it's because I planted a "seed" inside the Random declaration, but how would I change this into an interval?
import java.util.Random;
public class Range{
public static void main(String[] args) {
`// TODO Auto-generated method stub`
Random rand = new Random(25);
int num = rand.nextInt(51);
System.out.println(num);
}
}
I'm working on this challenging assignment outside of my Java course and given this objective:
"In a loop, generate a random party size, assign it to the waitingLine, and print it For generating the random party sizes, use the constants PARTY_SIZE_MIN and PARTY_SIZE_MAX to adjust the result you get when using the nextInt method
I don't understand how to use the min and max values however in tandem with Random variable...
public static int PARTY_SIZE_MIN =1;
public static int PARTY_SIZE_MAX=5;
Random myRandom=new Random();
int x = myRandom.nextInt(6); //max party size is 5
Scanner keyboard = new Scanner(system.in);
System.out.println("Enter the number of parties in the waiting line: ");
int lineSize = keyboard.nextInt();
int[] lineArray = new int[lineSize]; // made a new array to the size of the line input by the user
for (int i = 0; i < lineSize; i++) {
lineArray[i] = rand.nextInt(PARTY_SIZE_MAX-PARTY_SIZE_MIN)+1; //generate random numbers. upperbound-lowerbound, + 1 at end.The numbers have to start a 1, not 0, which further complicates this... Any help is appreciated.
edit: Updated my code