Kana uppercase ⇔ lowercase conversion in Java, voiced / semi-voiced sound mark deleted

import java.util.HashMap;
import java.util.Map;

public class KanaUtil {
    private static final Map<Character, Character> lower2upperMap = new HashMap<>();
    private static final Map<Character, Character> upper2lowerMap = new HashMap<>();

    private static final String LOWER_CHARS = "Ahhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhhh";
    private static final String UPPER_CHARS = "Aiue Oya Yuyotsu Ai Ue Oya Yuyotsu Ai Ue Oo";
    static {
        char[] lowerChars = LOWER_CHARS.toCharArray();
        char[] upperChars = UPPER_CHARS.toCharArray();
        if (lowerChars.length != upperChars.length) {
            throw new RuntimeException("char count not match. lower=" + lowerChars.length + ", upper=" + upperChars.length);
        }
        for (int i = 0; i < lowerChars.length; i++) {
            lower2upperMap.put(lowerChars[i], upperChars[i]);
            upper2lowerMap.put(upperChars[i], lowerChars[i]);
        }
    }

    public static String toUpperCase(String s) {
        return conv(s, lower2upperMap);
    }

    public static String toLowerCase(String s) {
        return conv(s, upper2lowerMap);
    }

    private static String conv(String s, Map<Character, Character> convMap) {
        StringBuilder sb = new StringBuilder();
        s.chars().mapToObj(c -> (char) c).forEach(c -> {
            Character conv = convMap.get(c);
            if (conv == null) {
                sb.append(c);
            } else {
                sb.append(conv);
            }
        });
        return sb.toString();
    }

    private static final Map<Character, Character> zenDaku2ZenNotDakuMap = new HashMap<>();

    private static final String ZEN_DAKU_CHARS = "ゔ Giguge Gozajizuzezoda Jizude Dobabibubebo Papipupepo Vugagigugegozajizuzezodajizudedobabibubebopapipupepo";
    private static final String ZEN_NOT_DAKU_CHARS = "What is Sashisuseso? Hifuhehohahifuheho Ukakikukekosashisuseso Tachitsutetohahifuhehohahifuheho";
    static {
        char[] zenDakuChars = ZEN_DAKU_CHARS.toCharArray();
        char[] zenNotDakuChars = ZEN_NOT_DAKU_CHARS.toCharArray();
        if (zenDakuChars.length != zenNotDakuChars.length) {
            throw new RuntimeException("char count not match. zenDaku=" + zenDakuChars.length + ", zenNotDaku=" + zenNotDakuChars.length);
        }
        for (int i = 0; i < zenDakuChars.length; i++) {
            zenDaku2ZenNotDakuMap.put(zenDakuChars[i], zenNotDakuChars[i]);
        }
    }

    public static String removeDakuten(String s) {
        StringBuilder sb = new StringBuilder();
        s.chars().mapToObj(c -> (char) c).forEach(c -> {
            if (c == '”' || c == '゜' || c == 'S' || c == '゜') {
                //skip(Delete)
            } else {
                Character conv = zenDaku2ZenNotDakuMap.get(c);
                if (conv == null) {
                    sb.append(c);
                } else {
                    sb.append(conv);
                }
            }
        });
        return sb.toString();
    }
}