Generate random String of given size in Java

0 1 2 minutes read

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

Recommended

Solve DSA problems on GfG Practice.

Solve Problems

Feeling lost in the world of random DSA topics, wasting time without progress? It’s time for a change! Join our DSA course, where we’ll guide you on an exciting journey to master DSA efficiently and on schedule.
Ready to dive in? Explore our Free Demo Content and join our DSA course, trusted by over 100,000 zambiatek!