boolean anagram(String a, String b) {
    Map<String, Integer> occA = countOccurences(a);
    Map<String, Integer> occB = countOccurences(b);
    Set<String> uniqueCharacters = getUniqueCharacters(a, b);
    for (String ch : uniqueCharacters) {
        if (occA.getOrDefault(ch, 0) != occB.getOrDefault(ch, 0)) {
            return false;
        }
    }
    return true;
}

private Set<String> getUniqueCharacters(String a, String b) {
    Set<String> uniqueCharacters = new HashSet<>();
    for (int i = 0; i < a.length; i++) {
        uniqueCharacters.add(a[i]);
    }
    for (int j = 0; j < b.length; j++) {
        uniqueCharacters.add(b[j]);
    }
    return uniqueCharacters;
}

private Map<String, Integer> countOccurences(String str) {
    Map<String, Integer> occurences = new HashMap<>();
    for (int i = 0; i < str.length; i++) {
        occurences.putIfAbsent(str[i], 0);
        occurences.put(str[i], occurences.get(str[i]) + 1);
    }
    return occurences;
}