Randomizing with substring

Question

The code I have here only manages to randomize the length, I need to randomize the string within my class createPassword. But a method does not exist that randomizes string, only integers. Anyone have any advice?

The main:

public class Program {
    public static void main(String[] args) {
        PasswordRandomizer randomizer = new PasswordRandomizer(13);
        System.out.println("Password: " + randomizer.createPassword());
        System.out.println("Password: " + randomizer.createPassword());
        System.out.println("Password: " + randomizer.createPassword());
        System.out.println("Password: " + randomizer.createPassword());
}
}

Class:

import java.util.Random;

public class PasswordRandomizer {
    private Random random = new Random();
    private int length;

    public PasswordRandomizer(int length) {
        this.length=length;
        random = new Random();
    }

    public String createPassword() {
        String alphabet = "abcdefghijklmnopqrstuvwxyz";
        int randomCharIndex = random.nextInt(length);
        String randomstr = alphabet.substring(randomCharIndex); 
        return randomstr;
    }
}

Example outputs:

Password: ghijklmnopqrstuvwxyz
Password: lmnopqrstuvwxyz
Password: jklmnopqrstuvwxyz
Password: ijklmnopqrstuvwxyz

And with an integer you can point into a character list and pick one and repeat... — Sami Kuhmonen
– Sami Kuhmonen, Commented Mar 27, 2016 at 5:17
if this is for production use, you should get help of some libraries.. you can look into UUID.getRandomUUID(); — Jos
– Jos, Commented Mar 27, 2016 at 5:18

Maljam · Accepted Answer · 2016-03-27 05:19:20Z

2

This would work fine:

    public String createPassword() {
        StringBuilder pass = new StringBuilder();
        Random r = new Random();

        for(int i = 0 ; i < length ; i++) {
            pass.append( (char)('a' + r.nextInt('z'-'a')) );
        }
        return pass.toString();
    }

The idea is to exploit that char can be used as an int and converted back to a char (and that alphabetical chars are consecutive in the ASCII code). So you use this to get a random char:
minimumValue + random.nextInt(MaxValue-MinValue)

answered Mar 27, 2016 at 5:19

Maljam

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6 Comments

Ken Bekov Over a year ago

you didn't pass length as method's parameter

Maljam Over a year ago

@KenBekov in his sample code, it's a field of his main class

Nebular Over a year ago

So you created a new class and object here? StringBuilder pass = new StringBuilder();. Also can you explain this line pass.append( (char)('a' + r.nextInt('z'-'a')) );

Maljam Over a year ago

@Nebular No StringBuilder is a class in the java.lang package that is preferable to use when creating a string bit by bit, over many steps: docs.oracle.com/javase/7/docs/api/java/lang/StringBuilder.html. The append(char c) method simply adds the char c to the end of the string

Nebular Over a year ago

So it is called to manipulate the characters within the string? not sure what you mean by c.

|

Ken Bekov · Accepted Answer · 2016-03-27 05:32:42Z

public String getPassword(int no) {
        String inputChars= "ABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
        StringBuilder salt = new StringBuilder();
        Random rnd = new Random();
        while (salt.length() < no) {
            int index = (int) (rnd.nextFloat() * inputChars.length());
            salt.append(inputChars.charAt(index));
        }
        String password= salt.toString();
        return password;

    }

But I will recommend to use java.security.SecureRandom; e.g.

private SecureRandom random = new SecureRandom();

public String nextSessionId() {
    return new BigInteger(130, random).toString(32);
}

more secure way to genarate password

Jos · Accepted Answer · 2016-03-27 05:32:55Z

You can use Collections.shuffle() to shuffle a list. You can use the same to shuffle the chars in a string. Have a look into the below code. This code will return a unique randomized string of a given string on each execution.

    List<String> list = new ArrayList<>(Arrays.asList("abcdefghijklmnopqrstuvwxyz".split("")));
    Collections.shuffle(list);
    String randomString = list.stream().collect(Collectors.joining());  // Java 8

If all you need is some random chars, that too for purposes of security, you should use some sort of hashing. May be you can use UUID.randomUUID() for getting a random value, then you can hash the same to get always unique strings.

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