RandomString.java

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package com.fermod.util;

import java.security.SecureRandom;
import java.util.Locale;
import java.util.Objects;
import java.util.Random;

public class RandomString {

    public static final String UPPER = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    public static final String LOWER = UPPER.toLowerCase(Locale.ROOT);
    public static final String DIGITS = "0123456789";
    public static final String ALPHANUM = UPPER + LOWER + DIGITS;

    private final Random random;
    private final char[] symbols;
    private final char[] buffer;

    public RandomString(int length, Random random, String symbols) {
        if (length < 1) {
            throw new IllegalArgumentException();
        }
        if (symbols.length() < 2) {
            throw new IllegalArgumentException();
        }
        this.random = Objects.requireNonNull(random);
        this.symbols = symbols.toCharArray();
        this.buffer = new char[length];
    }


    public RandomString(int length, Random random) {
        this(length, random, ALPHANUM);
    }

    public RandomString(int length, String symbols) {
        this(length, new SecureRandom(), symbols);
    }

    public RandomString(int length) {
        this(length, new SecureRandom());
    }

    public String nextString() {
        for (int i = 0; i < buffer.length; ++i) {
            buffer[i] = symbols[random.nextInt(symbols.length)];
        }
        return new String(buffer);
    }

}