Java Program to Create random strings

In this example, we will learn to generate a random string and an alphanumeric random string in Java.

Example 1: Java program to generate a random string

import java.util.Random;

class Main {
  public static void main(String[] args) {

    // create a string of all characters
    String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

    // create random string builder
    StringBuilder sb = new StringBuilder();

    // create an object of Random class
    Random random = new Random();

    // specify length of random string
    int length = 7;

    for(int i = 0; i < length; i++) {

      // generate random index number
      int index = random.nextInt(alphabet.length());

      // get character specified by index
      // from the string
      char randomChar = alphabet.charAt(index);

      // append the character to string builder
      sb.append(randomChar);
    }

    String randomString = sb.toString();
    System.out.println("Random String is: " + randomString);

  }
}

Output

Random String is: IIYOBRK

In the above example, we have first created a string containing all the alphabets. Next, we have generated a random index number using the nextInt() method of the Random class.

Using the random index number, we have generated the random character from the string alphabet. We then used the StringBuilder class to append all the characters together.

If we want to change the random string into lower case, we can use the toLowerCase() method of the String.

randomString.toLowerCase()

Note: The output will be different every time you run the program.

Example 2: Java Program to generate a random alphanumeric string

import java.util.Random;

class Main {
  public static void main(String[] args) {

    // create a string of uppercase and lowercase characters and numbers
    String upperAlphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    String lowerAlphabet = "abcdefghijklmnopqrstuvwxyz";
    String numbers = "0123456789";

    // combine all strings
    String alphaNumeric = upperAlphabet + lowerAlphabet + numbers;

    // create random string builder
    StringBuilder sb = new StringBuilder();

    // create an object of Random class
    Random random = new Random();

    // specify length of random string
    int length = 10;

    for(int i = 0; i < length; i++) {

      // generate random index number
      int index = random.nextInt(alphaNumeric.length());

      // get character specified by index
      // from the string
      char randomChar = alphaNumeric.charAt(index);

      // append the character to string builder
      sb.append(randomChar);
    }

    String randomString = sb.toString();
    System.out.println("Random String is: " + randomString);

  }
}

Output

Random Alphanumeric String is: pxg1Uzz9Ju

Here, we have created a string that contains numbers from 0 to 9 and the alphabets in uppercase and lowercase.

From the string, we have randomly generated an alphanumeric string of length 10.

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