## 1. Purpose

This post is to introduce how to generate random number using java.

## 2. Solution

Here is the solution:

``````
/**
* Generates a random boolean value.
*
* @return the random boolean
* @since 3.5
*/
public static boolean nextBoolean() {
return random().nextBoolean();
}

/**
* Generates an array of random bytes.
*
* @param count
*            the size of the returned array
* @return the random byte array
* @throws IllegalArgumentException if {@code count} is negative
*/
public static byte[] nextBytes(final int count) {
Validate.isTrue(count >= 0, "Count cannot be negative.");

final byte[] result = new byte[count];
random().nextBytes(result);
return result;
}

/**
* Generates a random double within 0 - Double.MAX_VALUE.
*
* @return the random double
* @see #nextDouble(double, double)
* @since 3.5
*/
public static double nextDouble() {
return nextDouble(0, Double.MAX_VALUE);
}

/**
* Generates a random double within the specified range.
*
* @param startInclusive
*            the smallest value that can be returned, must be non-negative
* @param endExclusive
*            the upper bound (not included)
* @throws IllegalArgumentException
*             if {@code startInclusive > endExclusive} or if
*             {@code startInclusive} is negative
* @return the random double
*/
public static double nextDouble(final double startInclusive, final double endExclusive) {
Validate.isTrue(endExclusive >= startInclusive,
"Start value must be smaller or equal to end value.");
Validate.isTrue(startInclusive >= 0, "Both range values must be non-negative.");

if (startInclusive == endExclusive) {
return startInclusive;
}

return startInclusive + ((endExclusive - startInclusive) * random().nextDouble());
}

/**
* Generates a random float within 0 - Float.MAX_VALUE.
*
* @return the random float
* @see #nextFloat(float, float)
* @since 3.5
*/
public static float nextFloat() {
return nextFloat(0, Float.MAX_VALUE);
}

/**
* Generates a random float within the specified range.
*
* @param startInclusive
*            the smallest value that can be returned, must be non-negative
* @param endExclusive
*            the upper bound (not included)
* @throws IllegalArgumentException
*             if {@code startInclusive > endExclusive} or if
*             {@code startInclusive} is negative
* @return the random float
*/
public static float nextFloat(final float startInclusive, final float endExclusive) {
Validate.isTrue(endExclusive >= startInclusive,
"Start value must be smaller or equal to end value.");
Validate.isTrue(startInclusive >= 0, "Both range values must be non-negative.");

if (startInclusive == endExclusive) {
return startInclusive;
}

return startInclusive + ((endExclusive - startInclusive) * random().nextFloat());
}

/**
* Generates a random int within 0 - Integer.MAX_VALUE.
*
* @return the random integer
* @see #nextInt(int, int)
* @since 3.5
*/
public static int nextInt() {
return nextInt(0, Integer.MAX_VALUE);
}

/**
* Generates a random integer within the specified range.
*
* @param startInclusive
*            the smallest value that can be returned, must be non-negative
* @param endExclusive
*            the upper bound (not included)
* @throws IllegalArgumentException
*             if {@code startInclusive > endExclusive} or if
*             {@code startInclusive} is negative
* @return the random integer
*/
public static int nextInt(final int startInclusive, final int endExclusive) {
Validate.isTrue(endExclusive >= startInclusive,
"Start value must be smaller or equal to end value.");
Validate.isTrue(startInclusive >= 0, "Both range values must be non-negative.");

if (startInclusive == endExclusive) {
return startInclusive;
}

return startInclusive + random().nextInt(endExclusive - startInclusive);
}

/**
* Generates a random long within 0 - Long.MAX_VALUE.
*
* @return the random long
* @see #nextLong(long, long)
* @since 3.5
*/
public static long nextLong() {
return nextLong(Long.MAX_VALUE);
}

/**
* Generates a {@code long} value between 0 (inclusive) and the specified
* value (exclusive).
*
* @param n Bound on the random number to be returned.  Must be positive.
* @return a random {@code long} value between 0 (inclusive) and {@code n}
* (exclusive).
*/
private static long nextLong(final long n) {
// Extracted from o.a.c.rng.core.BaseProvider.nextLong(long)
long bits;
long val;
do {
bits = random().nextLong() >>> 1;
val  = bits % n;
} while (bits - val + (n - 1) < 0);

return val;
}

/**
* Generates a random long within the specified range.
*
* @param startInclusive
*            the smallest value that can be returned, must be non-negative
* @param endExclusive
*            the upper bound (not included)
* @throws IllegalArgumentException
*             if {@code startInclusive > endExclusive} or if
*             {@code startInclusive} is negative
* @return the random long
*/
public static long nextLong(final long startInclusive, final long endExclusive) {
Validate.isTrue(endExclusive >= startInclusive,
"Start value must be smaller or equal to end value.");
Validate.isTrue(startInclusive >= 0, "Both range values must be non-negative.");

if (startInclusive == endExclusive) {
return startInclusive;
}

return startInclusive + nextLong(endExclusive - startInclusive);
}

}

/**
* {@link RandomUtils} instances should NOT be constructed in standard
* programming. Instead, the class should be used as
* {@code RandomUtils.nextBytes(5);}.
* <p>
* This constructor is public to permit tools that require a JavaBean
* instance to operate.
* </p>
*/
public RandomUtils() {
}
``````

## 3. Other solutions

In Java, there are several ways to generate random numbers using different classes and methods. Here are some common approaches:

### 1. Using `java.util.Random`

The `Random` class is a part of the `java.util` package and is used to generate pseudo-random numbers.

``````import java.util.Random;

public class RandomNumberGenerator {
public static void main(String[] args) {
Random random = new Random();

// Generate a random integer
int randomInt = random.nextInt();

// Generate a random integer within a specific range (from 0 to 99)
int randomIntRange = random.nextInt(100);

// Generate a random double
double randomDouble = random.nextDouble();

// Generate a random boolean
boolean randomBoolean = random.nextBoolean();

// Print the random numbers
System.out.println("Random Integer: " + randomInt);
System.out.println("Random Integer (0-99): " + randomIntRange);
System.out.println("Random Double: " + randomDouble);
System.out.println("Random Boolean: " + randomBoolean);
}
}
``````

### 2. Using `java.util.concurrent.ThreadLocalRandom`

The `ThreadLocalRandom` class is a subclass of `Random` that provides better performance when used in concurrent programs.

``````import java.util.concurrent.ThreadLocalRandom;

public static void main(String[] args) {
// Generate a random integer

// Generate a random double

// Print the random numbers
System.out.println("Random Integer: " + randomInt);
System.out.println("Random Double: " + randomDouble);
}
}
``````

### 3. Using `Math.random()`

The `Math.random()` method returns a double value with a positive sign, greater than or equal to 0.0, and less than 1.0.

``````public class MathRandomExample {
public static void main(String[] args) {
// Generate a random double between 0.0 (inclusive) and 1.0 (exclusive)
double randomDouble = Math.random();

// Print the random number
System.out.println("Random Double: " + randomDouble);
}
}
``````

### 4. Using `SecureRandom` for Cryptographic Purposes

The `SecureRandom` class is used for generating a stream of cryptographically strong pseudo-random numbers.

``````import java.security.SecureRandom;

public class SecureRandomExample {
public static void main(String[] args) {
SecureRandom secureRandom = new SecureRandom();

// Generate a random integer
int randomInt = secureRandom.nextInt();

// Print the random number
System.out.println("Cryptographically Strong Random Integer: " + randomInt);
}
}
``````

### Choosing the Right Method:

• Use `java.util.Random` when you need a simple pseudo-random number generator for general purposes.
• Use `java.util.concurrent.ThreadLocalRandom` when you’re working in a multi-threaded environment and want to avoid contention.
• Use `Math.random()` when you need a simple random double and don’t require integers or other types.
• Use `java.security.SecureRandom` when you need a cryptographically strong random number, such as for security-related operations.

Each method has its use case, and the choice depends on the requirements of your application.

## 4. Summary

In this post, I demonstrated how to generate random numbers using java. That’s it, thanks for your reading.