Java Examples: How to generate a random alpha numeric string?

Question:

How can I generate a random alpha numeric string?

Answer:

Using a random number generator to index valid letters for your string.

Code:

import java.util.Random;
 
public class RandomString {
 
   public static void main(String[] args) {
      System.out.println( RandomString(16) );
   }
 
   static String RandomString(int size) {
 
      String str1 = "abcdefghijklmnopqrstuvwxyz";
      String str2 = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
      String str3 = "0123456789";
 
      // these are the valid charecters 
      String valid = str1 + str2 + str3;
 
      Random random = new Random();
 
      String mystring = "";
      for( int i=0; i<size; i++ ) {
         int randnum = random.nextInt(valid.length()); 
         mystring = mystring + valid.charAt(randnum);
      } 
 
      return mystring;
   }
}

Output:

$ java RandomString 
XGgsYI60JuJHeTr6