Java provides, as part of the utils package, a basic pseudo-random number generator, appropriately named Random. This object can be used to generate a pseudo-random value as any of the built-in numerical datatypes (int, float, etc). You can also use it to generate a random Boolean value, or a random array of bytes. An example usage is as follows:

import java.util.Random;  

...
  
Random random = new Random();
int randInt = random.nextInt();
long randLong = random.nextLong();

double randDouble = random.nextDouble(); //This returns a value between 0.0 and 1.0
float randFloat = random.nextFloat(); //Same as nextDouble

byte[] randBytes = new byte[16];
random.nextBytes(randBytes); //nextBytes takes a user-supplied byte array, and fills it with random bytes. It returns nothing.

NOTE: This class only produces fairly low-quality pseudo-random numbers, and should never be used to generate random numbers for cryptographic operations or other situations where higher-quality randomness is critical (For that, you would want to use the SecureRandom class, as noted below). An explanation for the distinction between "secure" and "insecure" randomness is beyond the scope of this example.

The method nextInt(int bound) of Random accepts an upper exclusive boundary, i.e. a number that the returned random value must be less than. However, only the nextInt method accepts a bound; nextLong, nextDouble etc. do not.

Random random = new Random();
random.nextInt(1000); // 0 - 999

int number = 10 + random.nextInt(100); // number is in the range of 10 to 109

Starting in Java 1.7, you may also use ThreadLocalRandom (source (opens new window)). This class provides a thread-safe PRNG (pseudo-random number generator). Note that the nextInt method of this class accepts both an upper and lower bound.

import java.util.concurrent.ThreadLocalRandom;

// nextInt is normally exclusive of the top value,
// so add 1 to make it inclusive
ThreadLocalRandom.current().nextInt(min, max + 1);

Note that the official documentation (opens new window) states that nextInt(int bound) can do weird things when bound is near 2³⁰+1 (emphasis added):

The algorithm is slightly tricky. It rejects values that would result in an uneven distribution (due to the fact that 2^31 is not divisible by n). The probability of a value being rejected depends on n. The worst case is n=2^30+1, for which the probability of a reject is 1/2, and the expected number of iterations before the loop terminates is 2.

In other words, specifying a bound will (slightly) decrease the performance of the nextInt method, and this performance decrease will become more pronounced as the bound approaches half the max int value.

Random and ThreadLocalRandom are good enough for everyday use, but they have a big problem: They are based on a linear congruential generator (opens new window), an algorithm whose output can be predicted rather easily. Thus, these two classes are not suitable for cryptographic uses (such as key generation).

One can use java.security.SecureRandom in situations where a PRNG with an output that is very hard to predict is required. Predicting the random numbers created by instances of this class is hard enough to label the class as cryptographically secure.

import java.security.SecureRandom;
import java.util.Arrays;

public class Foo {
    public static void main(String[] args) {
        SecureRandom rng = new SecureRandom();
        byte[] randomBytes = new byte[64];
        rng.nextBytes(randomBytes); // Fills randomBytes with random bytes (duh)
        System.out.println(Arrays.toString(randomBytes));
    }
}

Besides being cryptographically secure, SecureRandom has a gigantic period of 2¹⁶⁰, compared to Randoms period of 2⁴⁸. It has one drawback of being considerably slower than Random and other linear PRNGs such as Mersenne Twister (opens new window) and Xorshift (opens new window), however.

Note that SecureRandom implementation is both platform and provider dependent. The default SecureRandom (given by SUN provider in sun.security.provider.SecureRandom):

• on Unix-like systems, seeded with data from /dev/random and/or /dev/urandom.
• on Windows, seeded with calls to CryptGenRandom() in CryptoAPI (opens new window).

//Creates a Random instance with a seed of 12345.
Random random = new Random(12345L);

//Gets a ThreadLocalRandom instance
ThreadLocalRandom tlr = ThreadLocalRandom.current();

//Set the instance's seed.
tlr.setSeed(12345L);

Using the same seed to generate random numbers will return the same numbers every time, so setting a different seed for every Random instance is a good idea if you don't want to end up with duplicate numbers.

A good method to get a Long that is different for every call is System.currentTimeMillis() (opens new window):

Random random = new Random(System.currentTimeMillis());
ThreadLocalRandom.current().setSeed(System.currentTimeMillis());

/**
 * returns a array of random numbers with no duplicates
 * @param range the range of possible numbers for ex. if 100 then it can be anywhere from 1-100
 * @param length the length of the array of random numbers
 * @return array of random numbers with no duplicates.
 */
public static int[] getRandomNumbersWithNoDuplicates(int range, int length){
    if (length<range){
        // this is where all the random numbers
        int[] randomNumbers = new int[length];
        
        // loop through all the random numbers to set them
        for (int q = 0; q < randomNumbers.length; q++){
            
            // get the remaining possible numbers
            int remainingNumbers = range-q;
            
            // get a new random number from the remainingNumbers
            int newRandSpot = (int) (Math.random()*remainingNumbers);
            
            newRandSpot++;
            
            // loop through all the possible numbers
            for (int t = 1; t < range+1; t++){
                
                // check to see if this number has already been taken
                boolean taken = false;
                for (int number : randomNumbers){
                    if (t==number){
                        taken = true;
                        break;
                    }
                }
                
                // if it hasnt been taken then remove one from the spots
                if (!taken){
                    newRandSpot--;
                    
                    // if we have gone though all the spots then set the value
                    if (newRandSpot==0){
                        randomNumbers[q] = t;
                    }
                }
            }
        }
        return randomNumbers;
    } else {
        // invalid can't have a length larger then the range of possible numbers
    }
    return null;
}

The method works by looping though an array that has the size of the requested length and finds the remaining length of possible numbers. It sets a random number of those possible numbers newRandSpot and finds that number within the non taken number left. It does this by looping through the range and checking to see if that number has already been taken.

For example if the range is 5 and the length is 3 and we have already chosen the number 2. Then we have 4 remaining numbers so we get a random number between 1 and 4 and we loop through the range(5) skipping over any numbers that we have already used(2).

Now let's say the next number chosen between 1 & 4 is 3. On the first loop we get 1 which has not yet been taken so we can remove 1 from 3 making it 2. Now on the second loop we get 2 which has been taken so we do nothing. We follow this pattern until we get to 4 where once we remove 1 it becomes 0 so we set the new randomNumber to 4.

We can use org.apache.commons.lang3.RandomUtils to generate random numbers using a single line.

int x = RandomUtils.nextInt(1, 1000);

The method nextInt(int startInclusive, int endExclusive) takes a range.

Apart from int, we can generate random long, double, float and bytes using this class.

RandomUtils class contains the following methods-

static byte[] nextBytes(int count) //Creates an array of random bytes.
static double nextDouble() //Returns a random double within 0 - Double.MAX_VALUE
static double nextDouble(double startInclusive, double endInclusive) //Returns a random double within the specified range.
static float nextFloat() //Returns a random float within 0 - Float.MAX_VALUE
static float nextFloat(float startInclusive, float endInclusive) //Returns a random float within the specified range.
static int nextInt() //Returns a random int within 0 - Integer.MAX_VALUE
static int nextInt(int startInclusive, int endExclusive) //Returns a random integer within the specified range.
static long nextLong() //Returns a random long within 0 - Long.MAX_VALUE
static long nextLong(long startInclusive, long endExclusive) //Returns a random long within the specified range.

Remarks

Nothing is really random and thus the javadoc calls those numbers pseudorandom. Those numbers are created with a pseudorandom number generator (opens new window).