Generate Alpha Numeric String for the given size

import java.util.Random;
import java.util.Scanner;

public class GenerateAlphaNumericStringUsingGivenNo {

 public static void main(String[] args) {
  
  Scanner sc=null;
  int size=0; 
  System.out.println("Enter the size");
  sc=new Scanner(System.in);
  //taking the size from the keyboard
  size=sc.nextInt();
  
  //calling the method to generate AlphaNumericString
  System.out.println(generate(size));
  
  
 }
 
 //method for generating AlphaNumericString 
 public static String generate(int size)
 {
  Random rand=null;
  StringBuffer sb=null;
  String numAndChars=null;
  
  //all string which will be choosen to generate
  numAndChars="abcdefghijklmnopqrstuvwxyz"
     +"ABCDEFGHIJKLMNOPQRSTUVWXYZ"
     +"0123456789";
  
  rand=new Random();
  sb=new StringBuffer("");
  
  for(int i=0;i<(size);i++)
   {
    //logic to generate AlphaNumeric String
    int index=(int)(numAndChars.length()*Math.random());
    
    //appending the characters one by one 
    sb.append(numAndChars.charAt(index));
   }
  //returning the AlphaNumeric String
  return sb.toString();
 }
}