Challenge and some maths

Found a few bits today, trying to install ubuntu on to a usb stick just so I can try it out and get learning some command line, having to burn a DVD and not having any software to do it, it is now in windows. Click on the iso image file in the explorer ribbon there are options to mount or burn to disc, very handy. 

The maths, frustrating that java does not have a function to use log in a base you choose however it does have natural log where it is then possible to get the needed result by dividing the natural log of some value by natural log of some base.

Math.log(x) / Math.log(2)

The challenge is about calculating shannon entropy. Which returns the correct results to at least five decimal places and got to try some streaming.

Entropy of "122333444455555666666777777788888888" is 2.7942086837942446
Entropy of "563881467447538846567288767728553786" is 2.7942086837942446
Entropy of "https://www.reddit.com/r/dailyprogrammer" is 4.056198332810095
Entropy of "int main(int argc, char *argv[])" is 3.8667292966721747

package com.company;

import java.math.BigDecimal;
import java.math.MathContext;
import java.util.*;
public class Main {

    public static void main(String[] args) {
        String[] inputSequences  = {
                "122333444455555666666777777788888888",
                "563881467447538846567288767728553786",
                "https://www.reddit.com/r/dailyprogrammer",
                "int main(int argc, char *argv[])"
        };

        double result;
        int minimumDecimalPlaces = 5;
        String resultStringDP;
        for (String input : inputSequences) {
            result = calculateEntropy(input);
            resultStringDP = new String(result + "").split(("\\."))[1];
            if( resultStringDP.length() < minimumDecimalPlaces) {
                result = new BigDecimal(result).round(new MathContext(minimumDecimalPlaces)).doubleValue();
            }
            System.out.println("Entropy of \"" + input + "\" is " + result);
        }
    }

    public static double calculateEntropy(String input) {
        List<Double> entropyValuesPerChar = new ArrayList<>(input.length());
        List<Integer> frequencyOfChars = new ArrayList<>();
        // get unique chars
        HashSet<Character> uniqueChars = new HashSet<>(input.length());
        for ( int i = 0; i < input.length(); i++) {
            uniqueChars.add(input.charAt(i));
        }
        // for each char determine frequency and add to counter
        int count;
        for ( char uniqueLetter : uniqueChars) {
            count = 0;
            for ( char letter : input.toCharArray()) {
                if (letter == uniqueLetter) {
                    count++;
                }
            }
            frequencyOfChars.add(count);
        }

        for ( Integer freq : frequencyOfChars) {
            double x = (double)freq / input.length();
            entropyValuesPerChar.add(
                    -(x) * (Math.log(x) / Math.log(2))
            );
        }
        return entropyValuesPerChar.stream()

.mapToDouble(w -> w).sum();
    }
}