How to Easily Generate Random String in Java

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Sometimes we have to generate random string in Java. Usually, a random string is used to create a unique identifier for session, database table primary key, etc.

Algorithm to Generate Random String in Java

  1. Create an alphanumeric string that contains all the ASCII uppercase and lowercase characters and digits.
  2. Use the Random class to generate a random number between 0 and the length of the alphanumeric string.
  3. Add the character at the random index of the alphanumeric string to a StringBuilder object.
  4. Repeat steps 2 and 3 until the StringBuilder size is equal to the required length of the random string.
  5. Return the random string using the StringBuilder toString() method.

Java Program to Generate Random String of Given Length

Here is the utility method implementation to generate a random string based on the above algorithm. We are getting the random string length from the user input.

package net.javastring.strings;

import java.util.Random;
import java.util.Scanner;

public class GenerateRandomString {

	public static void main(String[] args) {
		String asciiUpperCase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
		String asciiLowerCase = asciiUpperCase.toLowerCase();
		String digits = "1234567890";
		String asciiChars = asciiUpperCase + asciiLowerCase + digits;

		Scanner sc = new Scanner(System.in);

		System.out.println("Please enter random string length");
		int length = sc.nextInt();
		sc.close();

		String randomString = generateRandomString(length, asciiChars);

		System.out.println(String.format("The random string is %s", randomString));

	}

	private static String generateRandomString(int length, String seedChars) {
		StringBuilder sb = new StringBuilder();
		int i = 0;
		Random rand = new Random();
		while (i < length) {
			sb.append(seedChars.charAt(rand.nextInt(seedChars.length())));
			i++;
		}
		return sb.toString();
	}
}
Generate Random String In Java
Generate Random String In Java

Important Points

  • If you want more security in generating a random index, use SecureRandom class.
  • We can also add some special characters in the seed string to generate a strong random string. This is required if you are generating a random password string in your program.
  • We can easily modify the generateRandomString() method to add more complexity in generating the random string.

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