Best Ways to Generate Random String in Java

let’s use Random.ints — added in JDK 8 — to generate an alphabetic String,

@Test
public void givenUsingJava8_whenGeneratingRandomAlphabeticString_thenCorrect() {
    int leftLimit = 97; // letter 'a'
    int rightLimit = 122; // letter 'z'
    int targetStringLength = 10;
    Random random = new Random();

    String generatedString = random.ints(leftLimit, rightLimit + 1)
      .limit(targetStringLength)
      .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
      .toString();

    System.out.println(generatedString);
}

We can widen our character set to get an alphanumeric String.

@Test
public void givenUsingJava8_whenGeneratingRandomAlphanumericString_thenCorrect() {
    int leftLimit = 48; // numeral '0'
    int rightLimit = 122; // letter 'z'
    int targetStringLength = 10;
    Random random = new Random();

    String generatedString = random.ints(leftLimit, rightLimit + 1)
      .filter(i -> (i <= 57 || i >= 65) && (i <= 90 || i >= 97))
      .limit(targetStringLength)
      .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
      .toString();

    System.out.println(generatedString);
}

Generate 7 letters random string using unbounded String,

@Test
public void givenUsingPlainJava_whenGeneratingRandomStringUnbounded_thenCorrect() {
    byte[] array = new byte[7]; // length is bounded by 7
    new Random().nextBytes(array);
    String generatedString = new String(array, Charset.forName("UTF-8"));

    System.out.println(generatedString);
}

Now we will create a more constrained random string;

We are going to generate a random String using lowercase alphabetic letters and a set length,

@Test
public void givenUsingPlainJava_whenGeneratingRandomStringBounded_thenCorrect() {
 
    int leftLimit = 97; // letter 'a'
    int rightLimit = 122; // letter 'z'
    int targetStringLength = 10;
    Random random = new Random();
    StringBuilder buffer = new StringBuilder(targetStringLength);
    for (int i = 0; i < targetStringLength; i++) {
        int randomLimitedInt = leftLimit + (int) 
          (random.nextFloat() * (rightLimit - leftLimit + 1));
        buffer.append((char) randomLimitedInt);
    }
    String generatedString = buffer.toString();

    System.out.println(generatedString);
}

The Commons Lang library from Apache helps a lot with random string generation. Let’s take a look at generating a bounded String using only letters,

@Test
public void givenUsingApache_whenGeneratingRandomStringBounded_thenCorrect() {
 
    int length = 10;
    boolean useLetters = true;
    boolean useNumbers = false;
    String generatedString = RandomStringUtils.random(length, useLetters, useNumbers);

    System.out.println(generatedString);
}

Another very simple example — this time a bounded String with only alphabetic characters, but without passing boolean flags into the API,

@Test
public void givenUsingApache_whenGeneratingRandomAlphabeticString_thenCorrect() {
    String generatedString = RandomStringUtils.randomAlphabetic(10);

    System.out.println(generatedString);
}

The same random bounded String but this time — numeric

@Test
public void givenUsingApache_whenGeneratingRandomAlphanumericString_thenCorrect() {
    String generatedString = RandomStringUtils.randomAlphanumeric(10);

    System.out.println(generatedString);
}

And there we have it — creating bounded and unbounded strings with either plain Java, a Java 8 variant, or the Apache Commons Library.

Important Note:

In these Java examples, we’ve used java.util.Random, but one point worth mentioning is that it is not cryptographically secure. Consider using java.security.SecureRandom instead for security-sensitive applications.

Hope you find this article helpful, Please share your thoughts in the comment section!

I obtain an immense amount of satisfaction from helping others attain their goals and reach their potential through technology. Even if you wish to reach out and say "Hello", I sincerely appreciate all of the wonderful correspondence I receive each day from readers. You all enrich my life, and I hope I am able to do the same for you all as well.

If you find joy and value in what I do, please consider supporting my work with a donation — however much you can afford, it means and helps more than you can imagine.

You can give tips too using Buy me a coffee.