generate random string in java

import java.util.Random;

public class Generator {
	public static String generateRandomPassword(int len) {
		String chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijk"
          +"lmnopqrstuvwxyz!@#$%&";
		Random rnd = new Random();
		StringBuilder sb = new StringBuilder(len);
		for (int i = 0; i < len; i++)
			sb.append(chars.charAt(rnd.nextInt(chars.length())));
		return sb.toString();
	}
}

public class RandomString { 
  
    // function to generate a random string of length n 
    static String getAlphaNumericString(int n) 
    { 
  
        // chose 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)); 
    } 
} 

@Test
public void givenUsingJava8_whenGeneratingRandomAlphanumericString_thenCorrect() {
    int leftLimit = 48; // numeral '0'
    int rightLimit = 122; // letter 'z'
    int targetStringLength = 10;
    Random random = new Random();
 
    String generatedString = random.ints(leftLimit, rightLimit + 1)
      .filter(i -> (i <= 57 || i >= 65) && (i <= 90 || i >= 97))
      .limit(targetStringLength)
      .collect(StringBuilder::new, StringBuilder::appendCodePoint, StringBuilder::append)
      .toString();
 
    System.out.println(generatedString);
}