How do I generate random string?

package org.kodejava.example.security;

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

public class RandomString {
    public static final String SOURCES =
            "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";

    public static void main(String[] args) {
        RandomString rs = new RandomString();
        System.out.println(rs.generateString(new Random(), SOURCES, 10));
        System.out.println(rs.generateString(new Random(), SOURCES, 10));
        System.out.println(rs.generateString(new SecureRandom(), SOURCES, 15));
        System.out.println(rs.generateString(new SecureRandom(), SOURCES, 15));
    }

    /**
     * Generate a random string.
     *
     * @param random the random number generator.
     * @param characters the characters for generating string.
     * @param length the length of the generated string.
     * @return
     */
    public String generateString(Random random, String characters, int length) {
        char[] text = new char[length];
        for (int i = 0; i < length; i++) {
            text[i] = characters.charAt(random.nextInt(characters.length()));
        }
        return new String(text);
    }
}

Example string produced by the code snippets are:

uxEUFqTqS0
vr89vdF4gh
ysYF9XEHhO5FtDf
aBANhrLObZ1XLJi

How to get random key-value pair from Hashtable?

package org.kodejava.example.util;

import java.util.Hashtable;
import java.util.Random;

public class HashtableGetRandom {
    public static void main(String[] args) {
        // Create a hashtable and put some key-value pair.
        Hashtable<String, String> colors = new Hashtable<>();
        colors.put("black", "#000");
        colors.put("red", "#f00");
        colors.put("green", "#0f0");
        colors.put("blue", "#00f");
        colors.put("white", "#fff");

        // Get a random entry from the hashtable.
        String[] keys = colors.keySet().toArray(new String[colors.size()]);
        String key = keys[new Random().nextInt(keys.length)];
        System.out.println(key + " = " + colors.get(key));
    }
}