JAVA - Generate 5 digits code using Math.random()

Question

I'm writing a short program which be able to generate 5 digits code mixed with 0-9, a-z, A-Z.

This is my code:

import java.util.*;

public class FiveDigitsRandom
{

public static void main(String[] args)
{
  new FiveDigitsRandom().use();
 }

public void use()
{
char choice;
while((choice=readChar()) != 'x')
{
    switch(choice)
    {
        case'n': 
        generateAll();
        break;
    }
  }
}

public char readChar()
{
 System.out.print("Please choose n/x: ");
 Scanner scanner = new Scanner(System.in);
 return scanner.nextLine().charAt(0);
 }

public void generateAll()
{
 String[]code = new String[5];
 for(int i=0; i<code.length; i++)
 {
     int random = generate0_2();
     switch(random)
     {
     case 0: 
         code[i] = generate0_9();
         break;
     case 1:
         code[i] = generate_a_z();
         break;
     case 2:
         code[i] = generate_A_Z();
         break;
     }
   }
 for(int j=0; j<code.length; j++)
 {
     System.out.print(code[j]);
 }
 System.out.println(" ");
 }

public int generate0_2()
{
 return (int)Math.random()*3;
 }

public String generate0_9()
{
 int a = (int)(Math.random() * 10);
 String AA = Integer.toString(a);
 return AA;
 }

public String generate_a_z()
{
 char a = (char)((int)'a'+ Math.random() * ((int)'z' - (int)'a' + 1));
 String AA = Character.toString(a);
 return AA;
 }

public String generate_A_Z()
{
 char a = (char)((int)'A'+ Math.random() * ((int)'Z' - (int)'A' + 1));
 String AA = Character.toString(a);
 return AA;
 }
}

It suppose to generate a random code as 0AzhG, Hg78N. But now I can only have 5 digits code with random number 0-9. Please tell me where is wrong in my code?? Thank you!

Oh, I can use java.util.Random. But I think that doesn't effect the result. The problem I have now is that I can only have a random code like 28375 instead of having a mixed random code like 0AhYb, Uhs76 and so on. — Alex Ma
– Alex Ma, Commented May 10, 2016 at 7:03
For each digit you have to select between 62 symbols. Put all the symbols in an array. For each digit, chose a random int between 0 and 61. Use it as index. — SJuan76
– SJuan76, Commented May 10, 2016 at 7:09
In generate0_2() you're multiplying a random number with 3, unless the random number is 1 its always gonna be greater than 3, you're switching from 0 to 2 — David
– David, Commented May 10, 2016 at 7:16
Why it will be greater than 3? Isn't the value range of Math.random() is 0<= Math.random() <1? If I multiply it with 3, then it should be 0<=Math.random()*3<3, is that right? — Alex Ma
– Alex Ma, Commented May 10, 2016 at 7:23

Burak Uyar · Accepted Answer · 2016-05-10 07:19:19Z

2

Your generate0_2 method is wrong.

public int generate0_2()
{
    return (int)Math.random()*3;
}

When you cast it to int, it works like ((int)Math.random)*3 which means, it provides 0 every time.

change it to

public int generate0_2()
{
    return (int)(Math.random()*3);
}

answered May 10, 2016 at 7:19

Burak Uyar

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Comments

Peter Lawrey · Accepted Answer · 2016-05-10 07:44:02Z

1

A problem you have is that your won't have an even distribution. i.e. individual digits are more likely than individual letters. One way to have an even distribution is you have an even function.

public static char randomChar() {
    // random.nextInt(62) would be faster.
    int n = (int) (Math.random() * 62);
    if (n < 10) return (char) ('0' + n);
    n -= 10;
    if (n < 26) return (char) ('A' + n);
    n -= 26;
    return (char) ('a' + n);
}

public static String randomString(int length) {
    char[] chars = new char[length];
    for (int i = 0; i < length; i++)
        char[i] = randomChar();
    return new String(chars);
}

answered May 10, 2016 at 7:44

Peter Lawrey

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Comments

Romanow Alexey · Accepted Answer · 2016-05-10 08:03:25Z

0

Maybe it's easely to use RandomStringUtils

import org.apache.commons.lang3.RandomStringUtils;

class Main {
    public static void main(String[] args) {
        // Prints only A-Z, a-z, 0-9
        System.out.println(RandomStringUtils.randomAlphabetic(5));
        // Prints only A-Z, a-z
        System.out.println(RandomStringUtils.randomAlphanumeric(5));
    }
}

edited May 10, 2016 at 8:03

answered May 10, 2016 at 7:43

Romanow Alexey

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1 Comment

Peter Lawrey Over a year ago

Perhaps you could illustrate how to use it for this use case.

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