You can use RandomStringUtils

import org.apache.commons.lang.RandomStringUtils;
public class RandomStringUtilsTrial {
  public static void main(String[] args) {
    System.out.print("8 char string  >>>");
    System.out.println(RandomStringUtils.random(8, true, true));

  }
}

Create a UUID.

String uniqueID = UUID.randomUUID().toString();

• Your code broadly follows the Java conventions.
• Your objective is unclear, why do you need random strings? If they are for hashkeys there are plenty of strong methods for generating these. If you need simple tokens then is there any reason you cannot use random UUIDs?
• Your code is named using the programming domain, not the problem domain. Naming things relevantly improves maintainability and the potential for reuse. So from your response comment below, generateRandomStrings should be called generateCouponCodes.
• Use Unit tests, they can be used to gauge performance for optimisations, but avoid premature optimisation.
• Static methods are an anti-pattern with OOP and should be avoided except in specific circumstances that do not apply here.