Java Random Number Examples

Use the Random class and the Math.random method. Generate random numbers.
Random. The weather is hard to predict. One day rain pours down. The next day the sun is out. This is not random—many factors affect it.
As with the weather, most random number types are hard to predict. They are pseudo-random. In Java we use the Random class (from java.util.Random) and Math.random.
Math.random. Consider this program. It calls the Math.random method and assigns a double variable to the result. That double is anywhere between 0 and 1.

Tip: For simple random needs, Math.random is sufficient. But using Random directly is better for more complex features.

Java program that uses Math.random import java.lang.Math; public class Program { public static void main(String[] args) { // Random number between 0 and 1. double value = Math.random(); System.out.println(value); } } Output 0.24645081324805196
Random Object. Here we instantiate directly the Random class. With a Random instance, we call methods (like nextInt) to get the "next" random number.

Next: The Random object internally stores a state, based (by default) on a time seed. The stream returns the "next" number.

Argument: When we pass an argument to nextInt, the max integer returned is less than that value (not equal to it). It is greater than 0.

Java program that creates Random object import java.util.Random; public class Program { public static void main(String[] args) { // Create new Random and call nextInt on it twice. Random random = new Random(); int random1 = random.nextInt(); int random2 = random.nextInt(10); // Display results. System.out.println(random1); System.out.println(random2); } } Output -1838898462 6
Static Random. This is a common design pattern for the Random class. We create it as a field (often a static field) in the enclosing class. Then we use that field to generate numbers.Static

Tip: With a static Random field, we reduce object creation, and avoid any time-based seed issues from code that executes near in time.

Note: Random uses a seed based on the time. So if we call it many times near in time, it may lead to poor results. A field helps.

Java program that uses static Random field import java.util.Random; public class Program { static Random _random = new Random(); static void test() { // Use static Random instance. // ... This reduces object creation. int number = _random.nextInt(5); System.out.println(number); } public static void main(String[] args) { // Call test method three times. test(); test(); test(); } } Output 3 0 2
Random byte array. With the nextBytes() method on the Random class, we can fill an entire byte array with random byte values. The size is not important—the entire array is filled.

Here: We use a single array of size 10. We then fill it with random bytes 5 times and print them to the terminal.

Java program that gets random byte array import java.util.Random; public class Program { public static void main(String[] args) { Random random = new Random(); byte[] array = new byte[10]; // ... Generate random bytes 5 times. for (int i = 0; i < 5; i++) { // Fill existing array with random bytes. random.nextBytes(array); // Display. for (int x = 0; x < array.length; x++) { System.out.print(array[x]); System.out.print(" "); } System.out.println(); } } } Output -95 49 92 -106 117 67 -97 116 109 100 -8 -50 -18 63 82 -10 122 6 -51 74 -25 -36 54 37 -115 -8 -61 21 43 63 -44 105 107 -109 -100 -4 41 -19 127 -91 -116 12 49 100 -110 -58 -83 -84 44 26
Ints. We can get a stream of random ints from the Random class. We call ints() on Random. And we can use the stream like any other—here we use iterator() can call nextInt 5 times.

Stream: An IntStream is a specialized stream. For stream-based designs, the Random.ints() method can help simplify code.

Java program that uses Random.ints import java.util.PrimitiveIterator; import java.util.Random; import; public class Program { public static void main(String[] args) { // Use ints to get a random stream of ints. Random r = new Random(); IntStream stream = r.ints(); PrimitiveIterator.OfInt iterator = stream.iterator(); // Call nextInt on the iterator to get random numbers. for (int i = 0; i < 5; i++) { int n = iterator.nextInt(); System.out.println(n); } } } Output -1775913741 1599836970 515709788 1578549166 7961094
Random text. We can use Random to generate random phrases (like a spam email). We use nextInt to generate random indexes into an array or ArrayList.

Then: We append the words at those random indexes into a StringBuilder. We use the array's length as the exclusive upper bound.


Finally: We capitalize the resulting phrase and remove its trailing punctuation. More advanced techniques are possible.

Java program that generates random phrases import java.util.Random; import java.lang.StringBuilder; public class Program { public static void main(String[] args) { // Create String array of words. String[] words = { "hello,", "cat,", "food", "buy", "free", "click", "here" }; Random random = new Random(); StringBuilder builder = new StringBuilder(); // Append five random words to the StringBuilder. for (int i = 0; i < 5; i++) { // Get random index (use length as nextInt is exclusive). int index = (int) random.nextInt(words.length); builder.append(words[index]); builder.append(" "); } // Remove final space and trailing punctuation. builder.setLength(builder.length() - 1); if (builder.charAt(builder.length() - 1) == ',') { builder.setLength(builder.length() - 1); } builder.append("!"); // Uppercase the StringBuilder. builder.setCharAt(0, Character.toUpperCase(builder.charAt(0))); // Print result. System.out.println(builder); } } Output Cat, free hello, food cat!
Markov chains. These are an algorithm used in the field of artificial intelligence. Markov chains can generate good random text. For random text, some hidden models are helpful.

Quote: Markov processes can also be used to generate superficially real-looking text given a sample document.

Markov chain: Wikipedia
Shuffle array. With the Fisher-Yates shuffling algorithm, we make just one pass through an array, and rearrange all elements. This is fast. But the code seems prone to errors.Shuffle
Random lowercase letter. With nextInt we can get random integers in a range. And we can convert those ints into chars to get a stream of random lowercase letters.Random Lowercase Letter
Modulo division. With nextInt() we can specify a range of numbers. But a modulo division can help us test random numbers in additional ways.Modulo
A performance note. For the fastest program, it is usually best to avoid generating more random numbers than needed. A random sequence can be cached (as in an array) in memory.
With the Random class, we get a stream of pseudo-random numbers. For scientific work, or money-related programs, a better random stream can employed. But often Random is enough.
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