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Can we generate multiple random strings as per regex /^[0-9A-F]$/

Can we generate multiple random strings as per regex /^[0-9A-F]$/
I have written the below code, is there any better way to generate it, please do not use any lib, use only java API
public static void printRandomStrings(int numOfStrings){
Random random = new Random();
char[] chars = {'A','B','C','D','E','F'}; // 6 characters
for(int i=0;i<numOfStrings;i++){
String randomString = "";
for(int j=0;j<4;j++){
int choice = random.nextInt(2); // 0 -1
if(choice == 1){
randomString= randomString+generateRandomInt(random);
}else{
randomString= randomString+generateRandomChar(random,chars);
}
}
System.out.println(randomString);
}
}
public static int generateRandomInt(Random random){
return random.nextInt(10);
}
public static char generateRandomChar(Random random,char[] chars){
int charIdx = random.nextInt(6);
return chars[charIdx];
}

Alpha Numeric Random Number Sequence [5-digit or 6-digit or any digit] generation

Hi I am trying to generate a 5 digit random number which is alpha numeric in nature. I am doing both with without using Streams.
CODE
public class AlphaRandom {
private static final String ALPHA_NUMERIC_STRING = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
static final Random random = new Random();
public static void main(String args[]) {
int length = 5;
String seq = randomAlphaNumeric(length);
System.out.println("Random Number Normal : " +seq);
IntStream.range(0,1).forEach(i->System.out.println("Random Number Streams : " +generateRandomString(random, length)));
}
// Using Streams
private static String generateRandomString(Random random, int length){
return random.ints(48,122)
.filter(i-> (i<57 || i>65) && (i <90 || i>97))
.mapToObj(i -> (char) i)
.limit(length)
.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append)
.toString();
}
// Normal
public static String randomAlphaNumeric(int count) {
StringBuilder builder = new StringBuilder();
while (count-- != 0) {
int character = (int)(Math.random()*ALPHA_NUMERIC_STRING.length());
builder.append(ALPHA_NUMERIC_STRING.charAt(character));
}
return builder.toString();
}
}
Sample Outputs :
Random Number Normal : VYAXC
Random Number Streams : LdBN6
Random Number Normal : 2ANTT
Random Number Streams : hfegc
Random Number Normal : JWK4Y
Random Number Streams : 8mQXK
But I am unable to generate the sequence always starting with a UpperCase only.
Can someone help me out.

I will use RandomStringUtils from the apache library to do this. The reason to do this is less code and I believe better readability of the code.
This is my code which can do the required thing
import org.apache.commons.lang3.RandomStringUtils;
public class GenerateRandomString {
public static void main(String[] args) {
int keyLength = 5; // Number of character allowed as per requirement
String randomAlphanumeric = RandomStringUtils.randomAlphanumeric(keyLength);
System.out.println("randomAlphanumeric generated is " + randomAlphanumeric);
String upperCaseRandom = randomAlphanumeric.substring(0, 1).toUpperCase() + randomAlphanumeric.substring(1);
System.out.println("upperCaseRandom generated is " + upperCaseRandom);
}
}
It would generate the following output:
randomAlphanumeric generated is m8OiR
upperCaseRandom generated is M8OiR
I am making the first character to uppercase as required by question using the substring method.

The easiest way is that before to return your string (once you have generated it), you take the first letter in order to apply toUpperCase and on the remaining chars, apply toLowerCase. Also, if in the future you will need to generate longer strings, you can use the same method without changing anything.
Summering what we'll do is:
public static String manipulate (String rand){
String c = rand.substring(0,1); // where rand is the random alphanumeric generated by your methods, we pick the first char
c = c.toUpperCase(); //we make it upperCase
String split = rand.substring(1); //we get the remaining of the string starting from position 1
split = split.toLowerCase(); //let's apply lowercase on the remaining
String result = c+split; //join again
return result;
}
Full Code:
package test;
import java.util.Random;
import java.util.stream.IntStream;
public class AlphaRandom {
private static final String ALPHA_NUMERIC_STRING = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
static final Random random = new Random();
public static void main(String args[]) {
int length = 5;
String seq = randomAlphaNumeric(length);
System.out.println("Random Number Normal : " +seq);
IntStream.range(0,1).forEach(i->System.out.println("Random Number Streams : " +generateRandomString(random, length)));
}
public static String manipulate (String rand){
String c = rand.substring(0,1); // where rand is the random alphanumeric generated by your methods, we pick the first char
c = c.toUpperCase(); //we make it upperCase
String split = rand.substring(1); //we get the remaining of the string starting from position 1
split = split.toLowerCase(); //let's apply lowercase on the remaining
String result = c+split; //join the again
return result;
}
// Using Streams
private static String generateRandomString(Random random, int length){
String rand = random.ints(48,122)
.filter(i-> (i<57 || i>65) && (i <90 || i>97))
.mapToObj(i -> (char) i)
.limit(length)
.collect(StringBuilder::new, StringBuilder::append, StringBuilder::append)
.toString();
return manipulate(rand);
}
// Normal
public static String randomAlphaNumeric(int count) {
StringBuilder builder = new StringBuilder();
while (count-- != 0) {
int character = (int)(Math.random()*ALPHA_NUMERIC_STRING.length());
builder.append(ALPHA_NUMERIC_STRING.charAt(character));
}
return manipulate(builder.toString());
}
}
Output:
Random Number Normal : Mwwsz
Random Number Streams : Q1fqk

Generate Random String in java [duplicate]

This question already has answers here:
How to generate a random alpha-numeric string
(46 answers)
Closed 6 years ago.
I'm trying to generate a string between capital A-Z in java using Secure Random. Currently I'm able to generate an alphanumeric string with special characters but I want a string with only upper case alphabets.
public String createRandomCode(int codeLength, String id){
char[] chars = id.toCharArray();
StringBuilder sb = new StringBuilder();
Random random = new SecureRandom();
for (int i = 0; i < codeLength; i++) {
char c = chars[random.nextInt(chars.length)];
sb.append(c);
}
String output = sb.toString();
System.out.println(output);
return output ;
}
The input parameters are length of the output string & id whhich is alphanumeric string.Can't understand what modifications to make to the above code to generate only upper case alphabet string. Please help..

Your method randomly selects characters out of the id argument. If you want those to only be uppercase letters, then pass a string with those characters:
String randomCode = createRandomCode(length, "ABCDEFGHIJKLMNOPQRSTUVWXYZ");
EDIT If you want to avoid duplicates, you can't just select characters at random. You'll want to shuffle them and pick out the first n characters:
public String createRandomCode(int codeLength, String id) {
List<Character> temp = id.chars()
.mapToObj(i -> (char)i)
.collect(Collectors.toList());
Collections.shuffle(temp, new SecureRandom());
return temp.stream()
.map(Object::toString)
.limit(codeLength)
.collect(Collectors.joining());
}
EDIT 2 Just for fun, here's another way to implement the original random code generator (allowing duplicates):
public static String createRandomCode(int codeLength, String id) {
return new SecureRandom()
.ints(codeLength, 0, id.length())
.mapToObj(id::charAt)
.map(Object::toString)
.collect(Collectors.joining());
}

Here is generator that I wrote and use:
public class RandomGenerator {
private static final String characters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
public static String generateRandom(int length) {
Random random = new SecureRandom();
if (length <= 0) {
throw new IllegalArgumentException("String length must be a positive integer");
}
StringBuilder sb = new StringBuilder(length);
for (int i = 0; i < length; i++) {
sb.append(characters.charAt(random.nextInt(characters.length())));
}
return sb.toString();
}
}
in numChars string you can put any characters you want to be included. int length parameter is the length of generated random string.

Here is an example method that uses the int range for characters A to Z (also this method avoids duplicate characters in the String) :
public String createRandomCode(final int codeLength) {
int min = 65;// A
int max = 90;// Z
StringBuilder sb = new StringBuilder();
Random random = new SecureRandom();
for (int i = 0; i < codeLength; i++) {
Character c;
do {
c = (char) (random.nextInt((max - min) + 1) + min);
} while (sb.indexOf(c.toString()) > -1);
sb.append(c);
}
String output = sb.toString();
System.out.println(output);
return output;
}
The range part comes from this topic : Generating random integers in a specific range

Code verifcation and optimization - Two String common substring

I was solving Two String problem. I have written below code.
It passed 4 test cases but for two test cases it showed timeout. Kindly let me know how can I optimize it to avoid timeouts? Also any links which explains and shows examples of such optimization is welcome.
public class TwoStrings
{
private static final String YES = "YES";
private static final String NO = "NO";
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
int testCases = Integer.parseInt(in.nextLine());
String input1[] = new String[testCases];
String input2[] = new String[testCases];
for (int i = 0; i < testCases; i++)
{
input1[i] = in.nextLine();
input2[i] = in.nextLine();
}
in.close();
for (int i = 0; i < testCases; i++)
{
displayResult(input1[i], input2[i]);
}
}
private static void displayResult(String string1, String string2)
{
// choosing smaller String for iterating through it.
String smallerString = string1.length() <= string2.length() ? string1
: string2;
String biggerString = string1 == smallerString ? string2 : string1;
boolean constains = false;
// Concept - Even if single letter is common, substring exists.
// So checking just one string.
for (int i = 0; i < smallerString.length(); i++)
{
if (biggerString.contains(String.valueOf(smallerString.charAt(i))))
{
constains = true;
break;
}
}
if (constains)
System.out.println(YES);
else
System.out.println(NO);
}
}

What you are currently doing is O(n^2) because you loop through the small string and the search for that character in the longer string is a linear search because it is not sorted (all letters in alphabetical order).
Below is a O(n) solution. The concept is to have a size 26 boolean array (one for each letter), and make an index true if a letter is in the small (could actually be small or long string, doesn't matter) string. Creating the array from the small string is O(n), and checking the letters in the long string is O(n), yielding a grand total of O(n + n), which reduces to O(n).
private static void displayResult(String string1, String string2)
{
boolean[] contains = new boolean[26];
boolean noSubstring = true;
// populate the contains array
for (char c : string1.toCharArray())
{
int value = (int)c - (int)'a'; // make the char 0-25
contains[value] = true;
}
for (char c : string2.toCharArray())
{
int value = (int)c - (int)'a'; // make the char 0-25
if (contains[value])
{
noSubstring = false;
break;
}
}
if (noSubstring) System.out.println("NO");
else System.out.println("YES");
}

4 digits numbers in an array as String

I need a String array with the following attributes:
4 digits numbers
No repeating digits ("1214" is invalid)
No 0's
Is there an easier way to do this than manually type it? Like:
String[] s = {"1234","1235",1236",1237",1238",1239","1243","1245"};
Sorry for my English!

The following code will generate an array with your specifications.
public class Test {
public static void main(String[] args) {
List<String> result = new ArrayList<>();
Set<Character> set = new HashSet<>();
for (int i = 1234; i <= 9876; i++) {
set.clear();
String iAsString = Integer.toString(i);
char[] chars = iAsString.toCharArray();
boolean valid = true;
for (char c : chars) {
if (c == '0' || !set.add(c)) {
valid = false;
break;
}
}
if (valid) {
result.add(iAsString);
}
}
String[] yourStringArray = result.toArray(new String[result.size()]);
System.out.println(Arrays.toString(yourStringArray));
}
}

****edit****
Just saw that it is in Java. So use this function: String.valueOf(number) to convert integer to string if none of the digits are repeats in the loop.
Not sure what language you are doing but I am assuming no repeats not replies.
So what you can do is have a loop from 0 to 9999 and then run through all the numbers while checking if each digit has repeats if so discard number (do not store it into array).
You can convert integers to strings in many languages in their function so you can do that then store it into array.
Good luck.
Hope this helped (fastest solution from my head...there could be more efficient ones)

Try this method for creating Random number with your structure :
ArrayList<Integer> rawNumbers = new ArrayList<Integer>(Arrays.asList(1,2,3,4,5,6,7,8,9));
public String createRandomNumberSring()
{
String result = "";
ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.addAll(rawNumbers);
for(int i = 0; i < 4; i++)
{
int index = (int)(Math.random() * (numbers.size() + 1));
result += numbers.get(index).toString();
numbers.remove(index);
}
return result;
}