Sun Mar 20 18:40:11 IST 2016::Starting
Sun Mar 20 18:43:03 IST 2016::Generated test data, count 40000000
Doing 20000000 validations.
Sun Mar 20 18:43:30 IST 2016 Done in 26860 msecs.
0.001343 milliseconds per validation

import java.util.Date;
import java.util.HashSet;
import java.util.Random;
import java.util.Set;

public class UserValidator {

    Set<String> validCredentials=new HashSet<>();
    long numCredentials = 1000*1000*60;
    Random rnd = new Random();
    char[] validChars=null;

    public UserValidator(){
        String chars="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz!@#$%^&*~*()";
        validChars = chars.toCharArray();
    }

    String generateRandomString(int len){
           StringBuilder sb = new StringBuilder( len );
           for( int i = 0; i < len; i++ )
              sb.append( validChars[ rnd.nextInt(validChars.length) ] );
           return sb.toString();
    }

    String getRansomString(){
        int len = 4;
        len = len+rnd.nextInt(20);
        return generateRandomString(len);
    }


    void generateTestData(){
        for(long i=0;i<numCredentials;i++){
            String username=getRansomString();
            String password=getRansomString();
            String credentials=username+":::"+new String(username+":"+password).hashCode();
            validCredentials.add(credentials);
        }
    }

    boolean isValidUser(String user, String password){
        String credentials=user+":::"+new String(user+":"+password).hashCode();
        return validCredentials.contains(credentials);
    }

    public static void main(String[] args) {
        UserValidator u = new UserValidator();
        System.out.println(new Date().toString()+"::Starting");
        u.generateTestData();
        System.out.println(new Date().toString()+"::Generated test data, count "+u.validCredentials.size());
        int lookups=20*1000*1000;
        System.out.println("Doing "+lookups+" validations.");
        long timeStart = new Date().getTime();
        for(int i=0;i<lookups;i++){
            u.isValidUser(u.getRansomString(), u.getRansomString());
        }
        long timeEnd = new Date().getTime();
        long timeSpent = (timeEnd-timeStart);
        System.out.println(new Date().toString()+" Done in "+timeSpent+" msecs.");
        System.out.println((double)timeSpent/(double)lookups +" milliseconds per validation");
    }

}