Slow OOP approach

package com.company.dsa;

import java.util.*;
import java.util.function.Function;

public class JustCount {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String input = scanner.nextLine();
        Map<Character, Letter> referenceIndexes = new HashMap<>();
        TreeSet<Letter> lowercase = new TreeSet<>();
        TreeSet<Letter> uppercase = new TreeSet<>();
        TreeSet<Letter> symbol = new TreeSet<>();
        for (char character : input.toCharArray()) {
            if (Character.isLowerCase(character)) {
                addCharacter(character, referenceIndexes, lowercase);
            } else if (Character.isUpperCase(character)) {
                addCharacter(character, referenceIndexes, uppercase);
            } else {
                addCharacter(character, referenceIndexes, symbol);
            }
        }
        System.out.println(getFirst(symbol));
        System.out.println(getFirst(lowercase));
        System.out.println(getFirst(uppercase));

    }

    private static String getFirst(TreeSet<Letter> letters) {
        if (letters.isEmpty()) {
            return "-";
        }
        Letter letter = letters.first();
        return (char) letter.character + " " + letter.count;
    }


    private static void addCharacter(char character, Map<Character, Letter> referenceIndexes, Set<Letter> letters) {
        Letter letter = referenceIndexes.getOrDefault(character, new Letter(character, 0));
        letter.count++;
        if (referenceIndexes.put(character, letter) != null) {
            letters.remove(letter);
            letters.add(letter);
        }
        letters.add(letter);
    }


    private static class Letter implements Comparable<Letter> {
        int character, count;

        Letter(int character, int count) {
            this.character = character;
            this.count = count;
        }

        @Override
        public int compareTo(Letter o) {
            return Comparator
                    .comparing((Function<Letter, Integer>) letter -> -letter.count)
                    .thenComparingInt(l -> l.character)
                    .compare(this, o);
        }
    }
}