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.Â
JAVA
// Java program generate a random AlphaNumeric String // using Math.random() method Â
public class RandomString { Â
 // function to generate a random string of length n  static String getAlphaNumericString(int n)  { Â
  // choose a Character random from this String   String AlphaNumericString = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"         + "0123456789"         + "abcdefghijklmnopqrstuvxyz"; Â
  // create StringBuffer size of AlphaNumericString   StringBuilder sb = new StringBuilder(n); Â
  for (int i = 0; i < n; i++) { Â
   // generate a random number between    // 0 to AlphaNumericString variable length    int index     = (int)(AlphaNumericString.length()       * Math.random()); Â
   // add Character one by one in end of sb    sb.append(AlphaNumericString       .charAt(index));   } Â
  return sb.toString();  } Â
 public static void main(String[] args)  { Â
  // Get the size n   int n = 20; Â
  // 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.
- First take char between 0 to 256 and traverse.
- Check char is alphabetic or numeric.
- If yes, then add at the end of our String
- Return String
- Method 3: Using Regular Expressions
- First take char between 0 to 256.
- remove all char except 0-9, a-z and A-Z.
- Random select an char
- Add at the end our required variable
- 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:Â
JAVA
// Java program generate a random AlphaNumeric String // using CharSet method Â
import java.util.*; import java.nio.charset.*; Â
class RandomString { Â
 static String getAlphaNumericString(int n)  { Â
  // length is bounded by 256 Character   byte[] array = new byte[256];   new Random().nextBytes(array); Â
  String randomString    = new String(array, Charset.forName("UTF-8")); Â
  // Create a StringBuffer to store the result   StringBuffer r = new StringBuffer(); Â
  // Append first 20 alphanumeric characters   // from the generated random String into the result   for (int k = 0; k < randomString.length(); k++) { Â
   char ch = randomString.charAt(k); Â
   if (((ch >= 'a' && ch <= 'z')     || (ch >= 'A' && ch <= 'Z')     || (ch >= '0' && ch <= '9'))     && (n > 0)) { Â
    r.append(ch);     n--;    }   } Â
  // return the resultant string   return r.toString();  } Â
 public static void main(String[] args)  {   // size of random alphanumeric string   int n = 20; Â
  // Get and display the alphanumeric string   System.out.println(getAlphaNumericString(n));  } } |
Output:
qbhalyuzrenuwgvqidno
Time complexity: O(n) where n is size of string to be generated
Auxiliary space: O(n) for StringBuffer
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