String str = "Hello, World!";
System.out.println("长度: " + str.length()); // 13
System.out.println("第5个字符: " + str.charAt(4)); // o
System.out.println("子字符串: " + str.substring(7, 12)); // World
System.out.println("索引: " + str.indexOf('W')); // 7
System.out.println("是否以Hello开头: " + str.startsWith("Hello")); // true
System.out.println("替换后: " + str.replace("World", "Java")); // Hello, Java!

StringBuilder sb = new StringBuilder("Hello");
sb.append(" World"); // Hello World
sb.insert(5, ","); // Hello, World
sb.delete(5, 6); // Hello World
sb.reverse(); // dlroW olleH
System.out.println(sb.toString());

ArrayList<String> list = new ArrayList<>();
list.add("Apple");
list.add("Banana");
list.add("Orange");
System.out.println("元素个数: " + list.size()); // 3
System.out.println("第二个元素: " + list.get(1)); // Banana
list.set(1, "Grape");
System.out.println("修改后的列表: " + list); // [Apple, Grape, Orange]
list.remove(2);
System.out.println("移除后的列表: " + list); // [Apple, Grape]

HashMap<String, Integer> map = new HashMap<>();
map.put("Apple", 1);
map.put("Banana", 2);
map.put("Orange", 3);
System.out.println("Apple的值: " + map.get("Apple")); // 1
System.out.println("映射大小: " + map.size()); // 3
map.remove("Banana");
System.out.println("移除后映射: " + map); // {Apple=1, Orange=3}
System.out.println("是否包含Orange键: " + map.containsKey("Orange")); // true

System.out.println("绝对值: " + Math.abs(-5.5)); // 5.5
System.out.println("平方根: " + Math.sqrt(25)); // 5.0
System.out.println("3的4次幂: " + Math.pow(3, 4)); // 81.0
System.out.println("最大值: " + Math.max(10, 20)); // 20
System.out.println("随机数: " + Math.random()); // 例如: 0.64523

Date date = new Date();
System.out.println("当前日期时间: " + date.toString()); // Fri Jul 01 10:30:45 CST 2022
long millis = date.getTime();
System.out.println("自1970年1月1日以来的毫秒数: " + millis);

Calendar calendar = Calendar.getInstance();
System.out.println("当前年份: " + calendar.get(Calendar.YEAR)); // 2022
calendar.add(Calendar.DAY_OF_MONTH, 5); // 日期加5天
Date newDate = calendar.getTime();
System.out.println("新日期: " + newDate.toString());

File file = new File("example.txt");
if (!file.exists()) {
    try {
        file.createNewFile();
        System.out.println("文件创建成功");
    } catch (IOException e) {
        e.printStackTrace();
    }
}
System.out.println("文件大小: " + file.length() + "字节");
File dir = new File("testDir");
if (!dir.exists()) {
    dir.mkdirs();
    System.out.println("目录创建成功");
}
File[] files = dir.listFiles();
if (files != null) {
    for (File f : files) {
        System.out.println("目录中的文件: " + f.getName());
    }
}

Scanner scanner = new Scanner(System.in);
System.out.print("请输入您的名字: ");
String name = scanner.nextLine();
System.out.print("请输入您的年龄: ");
int age = scanner.nextInt();
scanner.close();
System.out.println("名字: " + name + ", 年龄: " + age);

PrintStream ps = System.out;
ps.println("Hello, World!");
ps.printf("圆周率: %.2f%n", Math.PI); // 圆周率: 3.14

System.out.println("绝对值: " + Math.abs(-5.5)); // 5.5
System.out.println("平方根: " + Math.sqrt(25)); // 5.0
System.out.println("3的4次幂: " + Math.pow(3, 4)); // 81.0
System.out.println("最大值: " + Math.max(10, 20)); // 20
System.out.println("随机数: " + Math.random()); // 例如: 0.64523

long startTime = System.currentTimeMillis();
// 执行某些操作
long endTime = System.currentTimeMillis();
System.out.println("执行时间: " + (endTime - startTime) + "ms");

String osName = System.getProperty("os.name");
System.out.println("操作系统名称: " + osName);

String numStr = "123";
int num = Integer.parseInt(numStr);
System.out.println("字符串转换为整数: " + num); // 123

int maxNum = Integer.max(10, 20);
System.out.println("最大值: " + maxNum); // 20

int bitCount = Integer.bitCount(7); // 二进制111中有3个1
System.out.println("二进制中1的位数: " + bitCount); // 3

String doubleStr = "3.14";
double value = Double.parseDouble(doubleStr);
System.out.println("字符串转换为浮点数: " + value); // 3.14

boolean isNan = Double.isNaN(0.0 / 0.0);
System.out.println("是否为NaN: " + isNan); // true

char ch = 'A';
System.out.println("是否为字母: " + Character.isLetter(ch)); // true
System.out.println("是否为大写: " + Character.isUpperCase(ch)); // true
System.out.println("转换为小写: " + Character.toLowerCase(ch)); // a

int[] arr = {3, 1, 4, 1, 5, 9};
Arrays.sort(arr);
System.out.println("排序后的数组: " + Arrays.toString(arr)); // [1, 1, 3, 4, 5, 9]

int index = Arrays.binarySearch(arr, 4);
System.out.println("元素4的位置: " + index); // 3

int[] newArr = Arrays.copyOf(arr, 4);
System.out.println("复制后的数组: " + Arrays.toString(newArr)); // [1, 1, 3, 4]

ArrayList<Integer> list = new ArrayList<>();
list.add(3);
list.add(1);
list.add(4);
list.add(1);
list.add(5);
list.add(9);

Collections.sort(list);
System.out.println("排序后的列表: " + list); // [1, 1, 3, 4, 5, 9]

Collections.reverse(list);
System.out.println("反转后的列表: " + list); // [9, 5, 4, 3, 1, 1]

int max = Collections.max(list);
System.out.println("最大值: " + max); // 9

int count = Collections.frequency(list, 1);
System.out.println("元素1的出现次数: " + count); // 2

Random random = new Random();
int randInt = random.nextInt(100); // 0到99之间的随机整数
System.out.println("随机整数: " + randInt);

double randDouble = random.nextDouble();
System.out.println("随机浮点数: " + randDouble);

boolean randBool = random.nextBoolean();
System.out.println("随机布尔值: " + randBool);

Formatter formatter = new Formatter();
formatter.format("姓名: %s, 年龄: %d, 身高: %.2f", "张三", 25, 175.6);
System.out.println(formatter.toString());
formatter.close();

Pattern pattern = Pattern.compile("\\d+");
Matcher matcher = pattern.matcher("年龄: 25, 身高: 175.6");
while (matcher.find()) {
    System.out.println("匹配到的数字: " + matcher.group());
    System.out.println("起始索引: " + matcher.start());
    System.out.println("结束索引: " + matcher.end());
}

String replaced = matcher.replaceAll("XX");
System.out.println("替换后的字符串: " + replaced); // 年龄: XX, 身高: XX.6

// 写入文件
FileWriter writer = new FileWriter("example.txt");
writer.write("Hello, World!");
writer.close();

// 读取文件
FileReader reader = new FileReader("example.txt");
char[] buffer = new char[100];
int length = reader.read(buffer);
System.out.println("读取的内容: " + new String(buffer, 0, length));
reader.close();

// 写入文件
BufferedWriter writer = new BufferedWriter(new FileWriter("example.txt"));
writer.write("第一行文本");
writer.newLine();
writer.write("第二行文本");
writer.close();

// 读取文件
BufferedReader reader = new BufferedReader(new FileReader("example.txt"));
String line;
while ((line = reader.readLine()) != null) {
    System.out.println("读取的行: " + line);
}
reader.close();

// 写入字节到文件
OutputStream os = new FileOutputStream("example.bin");
os.write(65); // 写入字符'A'的ASCII码
os.close();

// 从文件读取字节
InputStream is = new FileInputStream("example.bin");
int data = is.read();
System.out.println("读取的字节: " + data); // 65
is.close();

// 写入字节到文件
BufferedOutputStream bos = new BufferedOutputStream(new FileOutputStream("example.bin"));
bos.write(65);
bos.flush();
bos.close();

// 从文件读取字节
BufferedInputStream bis = new BufferedInputStream(new FileInputStream("example.bin"));
int data = bis.read();
System.out.println("读取的字节: " + data);
bis.close();

Properties props = new Properties();
props.setProperty("name", "张三");
props.setProperty("age", "25");

// 将属性写入文件
try (OutputStream output = new FileOutputStream("config.properties")) {
    props.store(output, "配置文件");
} catch (IOException e) {
    e.printStackTrace();
}

// 从文件读取属性
Properties props = new Properties();
try (InputStream input = new FileInputStream("config.properties")) {
    props.load(input);
    System.out.println("name: " + props.getProperty("name"));
    System.out.println("age: " + props.getProperty("age"));
} catch (IOException e) {
    e.printStackTrace();
}

GregorianCalendar calendar = new GregorianCalendar();
System.out.println("当前年份: " + calendar.get(Calendar.YEAR));
calendar.add(Calendar.MONTH, 1); // 月份加1
Date date = calendar.getTime();
System.out.println("新日期: " + date.toString());

TimeZone defaultZone = TimeZone.getDefault();
System.out.println("默认时区ID: " + defaultZone.getID());
System.out.println("默认时区名称: " + defaultZone.getDisplayName());
TimeZone newYorkZone = TimeZone.getTimeZone("America/New_York");
System.out.println("纽约时区是否使用夏令时: " + newYorkZone.useDaylightTime());

SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date currentDate = new Date();
String formattedDate = sdf.format(currentDate);
System.out.println("格式化后的日期: " + formattedDate);

try {
    Date parsedDate = sdf.parse("2022-07-01 10:30:45");
    System.out.println("解析后的日期: " + parsedDate.toString());
} catch (ParseException e) {
    e.printStackTrace();
}

BigInteger a = new BigInteger("12345678901234567890");
BigInteger b = new BigInteger("98765432109876543210");

BigInteger sum = a.add(b);
System.out.println("加法结果: " + sum);

BigInteger product = a.multiply(b);
System.out.println("乘法结果: " + product);

BigInteger quotient = a.divide(b);
System.out.println("除法结果: " + quotient);

BigInteger remainder = a.mod(b);
System.out.println("取模结果: " + remainder);

BigDecimal a = new BigDecimal("1234567890.1234567890");
BigDecimal b = new BigDecimal("9876543210.9876543210");

BigDecimal sum = a.add(b);
System.out.println("加法结果: " + sum);

BigDecimal product = a.multiply(b);
System.out.println("乘法结果: " + product);

BigDecimal quotient = a.divide(b, 2, RoundingMode.HALF_UP);
System.out.println("除法结果（保留两位小数）: " + quotient);

BigDecimal scaled = a.setScale(4, RoundingMode.HALF_UP);
System.out.println("设置小数位数为4: " + scaled);

System.out.println("绝对值: " + Math.abs(-5.5)); // 5.5
System.out.println("平方根: " + Math.sqrt(25)); // 5.0
System.out.println("3的4次幂: " + Math.pow(3, 4)); // 81.0
System.out.println("最大值: " + Math.max(10, 20)); // 20
System.out.println("随机数: " + Math.random()); // 例如: 0.64523