Here's a Java program that generates random strings of a specified length:

import java.util.Random;

public class RandomStringGenerator {
    public static void main(String[] args) {
        int length = 10; // Length of the random string to generate
        String randomString = generateRandomString(length);
        System.out.println("Random string: " + randomString);
    }

    public static String generateRandomString(int length) {
        String chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890";
        StringBuilder sb = new StringBuilder();

        Random random = new Random();
        for (int i = 0; i < length; i++) {
            int index = random.nextInt(chars.length());
            sb.append(chars.charAt(index));
        }

        return sb.toString();
    }
}

In this program, we define a method generateRandomString that takes an integer length as an argument and returns a random string of that length. We define a string chars that contains all the characters from which we want to generate the random string. We then use a StringBuilder to build the random string character by character.

We use a Random object to generate random numbers between 0 and the length of the chars string. We then use the charAt method to get the character at that index and append it to the StringBuilder. Once the StringBuilder has been filled with length random characters, we return the resulting string.