String str = "Hello World!";
String lowerCaseStr = str.toLowerCase();
System.out.println(lowerCaseStr);  // 輸出: hello world!

String str = "?STANBUL";
String lowerCaseStr = str.toLowerCase(Locale.ENGLISH); // "istanbul"
String lowerCaseTurkish = str.toLowerCase(new Locale("tr", "TR")); // "i?stanbul"

String str = "Hello World!";
String upperCaseStr = str.toUpperCase();
System.out.println(upperCaseStr);  // 輸出: HELLO WORLD!

char c = 'A';
char lowerCaseC = Character.toLowerCase(c);
System.out.println(lowerCaseC);  // 輸出: a

char c = 'a';
char upperCaseC = Character.toUpperCase(c);
System.out.println(upperCaseC);  // 輸出: A

char[] chars = {'H', 'e', 'l', 'l', 'o'};
for (int i = 0; i < chars.length; i++) {
    chars[i] = Character.toLowerCase(chars[i]);
}
System.out.println(chars);  // 輸出: hello

public class Main {
    public static void main(String[] args) {
        String str = "hElLo wOrLd!";
        String[] words = str.split(" ");
        StringBuilder result = new StringBuilder();

        for (String word : words) {
            String capitalized = Character.toUpperCase(word.charAt(0)) + word.substring(1).toLowerCase();
            result.append(capitalized).append(" ");
        }
        System.out.println(result.toString().trim());  // 輸出: Hello World!
    }
}

import java.util.*;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        List<String> words = Arrays.asList("HELLO", "WORLD", "JAVA");
        List<String> lowerCaseWords = words.stream()
                                           .map(String::toLowerCase)
                                           .collect(Collectors.toList());
        System.out.println(lowerCaseWords);  // 輸出: [hello, world, java]
    }
}

String str = "helloWorld";
String part1 = str.substring(0, 5).toUpperCase(); // HELLO
String part2 = str.substring(5).toLowerCase();    // world
System.out.println(part1 + part2);  // 輸出: HELLOworld

char c = 'A';
if (Character.isUpperCase(c)) {
    System.out.println(c + " 是大寫(xiě)字母");
}

char d = 'a';
if (Character.isLowerCase(d)) {
    System.out.println(d + " 是小寫(xiě)字母");
}

public class Main {
    public static void main(String[] args) {
        String str = "Hello World!";
        StringBuilder result = new StringBuilder();

        for (char c : str.toCharArray()) {
            if (Character.isUpperCase(c)) {
                result.append(Character.toLowerCase(c));
            } else if (Character.isLowerCase(c)) {
                result.append(Character.toUpperCase(c));
            } else {
                result.append(c);  // 非字母字符保持不變
            }
        }
        System.out.println(result.toString());  // 輸出: hELLO wORLD!
    }
}