初級開發-編程題

`

public static void main(String[] args) {
    System.out.println(changeStrToUpperCase("user_name_abc"));
    System.out.println(changeStrToLowerCase(changeStrToUpperCase("user_name_abc_tsdfadf_ffa")));
}

//首字母變大寫
public static String changeStrToUpperCase(String str){
    StringBuilder builder = new StringBuilder();
    String[] split = str.split("_");
    for (int i = 0; i < split.length; i++) {
        if (i == 0) {
            builder.append(split[i]);
        } else {
            char[] charArray = split[i].toCharArray();
            charArray[0] -= 32;
            builder.append(String.valueOf(charArray));
        }
    }
    return builder.toString();
}

// 首字母變小寫
public static String changeStrToLowerCase(String str){
    char[] charArray = str.toCharArray();
    List<Integer> firstIndexList = new ArrayList<>();
    for (int i = 0; i < charArray.length; i++) {
        if (charArray[i] >= 'A' && charArray[i] <= 'Z') {
            charArray[i] += 32;
            firstIndexList.add(i);
        }
    }
    
    String newStr = String.valueOf(charArray);
    StringBuilder builder = new StringBuilder(newStr);
    for (int i = 0; i < firstIndexList.size(); i++) {
        if (i == 0) {
            builder.insert(firstIndexList.get(i), "_");
        } else{
            builder.insert(firstIndexList.get(i) + i, "_");
        }
    }
    return builder.toString();
}

`