Generate random String of given size in Java

Given a size as n, The task is to generate a random alphanumeric String of this size.

Below are various ways to generate random alphanumeric String of given size:

Prerequisite : Generating random numbers in Java

• Method 1: Using Math.random()

  Here the function getAlphaNumericString(n) generates a random number of length a string. This number is an index of a Character and this Character is appended in temporary local variable sb. In the end sb is returned.

  // Java program generate a random AlphaNumeric String
  // using Math.random() method
  
  public class RandomString {
  
      // function to generate a random string of length n
      static String getAlphaNumericString(int n)
      {
  
          // chose a Character random from this String
          String AlphaNumericString = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
                                      + "0123456789"
                                      + "abcdefghijklmnopqrstuvxyz";
  
          // create StringBuffer size of AlphaNumericString
          StringBuilder sb = new StringBuilder(n);
  
          for (int i = 0; i < n; i++) {
  
              // generate a random number between
              // 0 to AlphaNumericString variable length
              int index
                  = (int)(AlphaNumericString.length()
                          * Math.random());
  
              // add Character one by one in end of sb
              sb.append(AlphaNumericString
                            .charAt(index));
          }
  
          return sb.toString();
      }
  
      public static void main(String[] args)
      {
  
          // Get the size n
          int n = 20;
  
          // Get and display the alphanumeric string
          System.out.println(RandomString
                                 .getAlphaNumericString(n));
      }
  }
  Output:
  
  

  kU9vRVm9T1lFMbi3duO1
  

  Method 2: Using CharSet

  Generate 20 character long alphanumeric string randomly using Charset which is in java.nio.charset package.

  1. First take char between 0 to 256 and traverse.
  2. Check char is alphabetic or numeric.
  3. If yes, then add at the end of our String
  4. Return String

  Below is the implementation of the above approach:

  // Java program generate a random AlphaNumeric String
  // using CharSet method
  
  import java.util.*;
  import java.nio.charset.*;
  
  class RandomString {
  
      static String getAlphaNumericString(int n)
      {
  
          // length is bounded by 256 Character
          byte[] array = new byte[256];
          new Random().nextBytes(array);
  
          String randomString
              = new String(array, Charset.forName("UTF-8"));
  
          // Create a StringBuffer to store the result
          StringBuffer r = new StringBuffer();
  
          // Append first 20 alphanumeric characters
          // from the generated random String into the result
          for (int k = 0; k < randomString.length(); k++) {
  
              char ch = randomString.charAt(k);
  
              if (((ch >= 'a' && ch <= 'z')
                   || (ch >= 'A' && ch <= 'Z')
                   || (ch >= '0' && ch <= '9'))
                  && (n > 0)) {
  
                  r.append(ch);
                  n--;
              }
          }
  
          // return the resultant string
          return r.toString();
      }
  
      public static void main(String[] args)
      {
          // size of random alphanumeric string
          int n = 20;
  
          // Get and display the alphanumeric string
          System.out.println(getAlphaNumericString(n));
      }
  }
  Output:

  jj06CyZKfSBZQHM6KAUd
  

• Method 3: Using Regular Expressions
  1. First take char between 0 to 256.
  2. remove all char except 0-9, a-z and A-Z.
  3. Random select an char
  4. Add at the end our required variable

  Below is the implementation of the above approach:

  // Java program generate a random AlphaNumeric String
  // using Regular Expressions method
  
  import java.util.*;
  import java.nio.charset.*;
  
  class RandomString {
  
      static String getAlphaNumericString(int n)
      {
  
          // length is bounded by 256 Character
          byte[] array = new byte[256];
          new Random().nextBytes(array);
  
          String randomString
              = new String(array, Charset.forName("UTF-8"));
  
          // Create a StringBuffer to store the result
          StringBuffer r = new StringBuffer();
  
          // remove all spacial char
          String  AlphaNumericString
              = randomString
                    .replaceAll("[^A-Za-z0-9]", "");
  
          // Append first 20 alphanumeric characters
          // from the generated random String into the result
          for (int k = 0; k < AlphaNumericString.length(); k++) {
  
              if (Character.isLetter(AlphaNumericString.charAt(k))
                      && (n > 0)
                  || Character.isDigit(AlphaNumericString.charAt(k))
                         && (n > 0)) {
  
                  r.append(AlphaNumericString.charAt(k));
                  n--;
              }
          }
  
          // return the resultant string
          return r.toString();
      }
  
      public static void main(String[] args)
      {
          // size of random alphanumeric string
          int n = 20;
  
          // Get and display the alphanumeric string
          System.out.println(getAlphaNumericString(n));
      }
  }
  Output:

  4J8pirLzX6oIF0IIIaUU
  

• Method 4: Generating random String of UpperCaseLetter/LowerCaseLetter/Numbers

  When some specific characters are needed in the alphanumeric string, like only the UpperCaseLetter or LowerCaseLetter or Numbers, use this method. Below example generates a random String of LowerCase letters of size n.

  Below is the implementation of the above approach:

  // Java program generate a random
  // UpperCase or LowerCase or Number String
  
  import java.util.*;
  
  public class GFG {
  
      static String getAlphaNumericString(int n)
      {
  
          // lower limit for LowerCase Letters
          int lowerLimit = 97;
  
          // lower limit for LowerCase Letters
          int upperLimit = 122;
  
          Random random = new Random();
  
          // Create a StringBuffer to store the result
          StringBuffer r = new StringBuffer(n);
  
          for (int i = 0; i < n; i++) {
  
              // take a random value between 97 and 122
              int nextRandomChar = lowerLimit
                                   + (int)(random.nextFloat()
                                           * (upperLimit - lowerLimit + 1));
  
              // append a character at the end of bs
              r.append((char)nextRandomChar);
          }
  
          // return the resultant string
          return r.toString();
      }
  
      public static void main(String[] args)
      {
          // size of random alphanumeric string
          int n = 20;
  
          // Get and display the alphanumeric string
          System.out.println(getAlphaNumericString(n));
      }
  }
  Output:

  qbhalyuzrenuwgvqidno
  

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