Java : Random alphabets, alpha-numeric or numbers only string generator

A simple Java function to generate random string based on given parameter - alpha, alphanum and numeric. Depending on the parameter type, this function will generate random string that is composed of alphabets, mixed of alphabets and numbers or just plain numbers only.

Here you go!

 package socketloop;

 import java.security.SecureRandom;

 public class HumanReadablePasswordGenerator {

 private static SecureRandom random = new SecureRandom();
 private static final String ALPHA_NUMERIC_DICTIONARY = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
 private static final String ALPHA_DICTIONARY = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
 private static final String NUMERIC_DICTIONARY = "0123456789";

 public static void main(String[] args) {

 System.out.format("Random ALPHA NUMERIC string : %s%n", randStr(12,"alphanum"));
 System.out.format("Random ALPHA string : %s%n", randStr(12,"alpha"));
 System.out.format("Random NUMERIC string : %s%n", randStr(12,"numeric"));
 }

 public static String randStr(int strSize, String type) {
 // sanity check
 if (strSize < 1) throw new IllegalArgumentException();

 // initialize  
 StringBuilder data = new StringBuilder(strSize);
 char randomCharacter = 'a';
 int character = 0;

 while (strSize-- != 0) {
 if (type == "alphanum") {
 int characterLength = (int)(Math.random()*ALPHA_NUMERIC_DICTIONARY.length());

 // sometimes using SecureRandom will introduce "bound must be positive" IllegalArgumentExecption here
 if (characterLength > 0) {
 character = random.nextInt(characterLength);
 } else {
 character = characterLength;
 }
 randomCharacter = ALPHA_NUMERIC_DICTIONARY.charAt(character);
 }


 if (type == "alpha") {
 int characterLength = (int)(Math.random()*ALPHA_DICTIONARY.length());

 // sometimes using SecureRandom will introduce "bound must be positive" IllegalArgumentExecption here
 if (characterLength > 0) {
 character = random.nextInt(characterLength);
 } else {
 character = characterLength;
 }
 randomCharacter = ALPHA_DICTIONARY.charAt(character);
 }

 if (type == "numeric") {
 int characterLength = (int)(Math.random()*NUMERIC_DICTIONARY.length());

 // sometimes using SecureRandom will introduce "bound must be positive" IllegalArgumentExecption here
 if (characterLength > 0) {
 character = random.nextInt(characterLength);
 } else {
 character = characterLength;
 }
 randomCharacter = NUMERIC_DICTIONARY.charAt(character);
 }
 data.append(randomCharacter); 
 }  
 return data.toString();

 }
 }

Sample output:

Random ALPHA NUMERIC string : 0M40310F4HR2

Random ALPHA string : ATdXEOYJJMIM

Random NUMERIC string : 103010200000