Generate random String of given size in Java

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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

// 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) 
 { 
 
  // choose 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
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
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

// 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:

qbhalyuzrenuwgvqidno

Time complexity: O(n) where n is size of string to be generated

Auxiliary space: O(n) for StringBuffer

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