03-Java常用API与集合框架

03-Java常用API与集合框架

学习目标

• 掌握String类的常用方法和使用技巧
• 理解String、StringBuilder、StringBuffer的区别
• 掌握日期时间API的使用
• 掌握Math和Random类的常用方法
• 深入理解Java集合框架的体系结构
• 掌握List、Set、Map的使用和区别
• 掌握集合的遍历和排序方法
• 理解集合的底层实现原理
• 能够根据场景选择合适的集合类型
• 掌握集合的常见操作和最佳实践

知识结构思维导图

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Java常用API与集合框架

String类

String基础

常用方法

StringBuilder

StringBuffer

字符串池

日期时间API

Date类

Calendar类

SimpleDateFormat

Java 8新API

LocalDate

LocalTime

LocalDateTime

Math与Random

Math常用方法

Random随机数

UUID生成

集合框架

Collection接口

List接口

ArrayList

LinkedList

Vector

Set接口

HashSet

LinkedHashSet

TreeSet

Queue接口

PriorityQueue

Deque

Map接口

HashMap

LinkedHashMap

TreeMap

Hashtable

ConcurrentHashMap

集合操作

遍历方法

排序方法

查找方法

转换方法

一、为什么需要学习常用API？

1.1 API的重要性

什么是API？

• API（Application Programming Interface）是应用程序编程接口
• Java提供了大量的类库和方法供开发者使用
• 掌握常用API可以大大提高开发效率

不使用API的问题：

// 不使用API：手动实现字符串反转
public class WithoutAPI {
public static String reverse(String str) {
char[] chars = new char[str.length()];
for (int i = 0; i < str.length(); i++) {
chars[i] = str.charAt(str.length() – 1 – i);
}
return new String(chars);
}

public static void main(String[] args) {
String str = "Hello";
String reversed = reverse(str);
System.out.println(reversed); // olleH
}
}

使用API的优势：

// 使用API：直接调用StringBuilder的reverse方法
public class WithAPI {
public static void main(String[] args) {
String str = "Hello";
String reversed = new StringBuilder(str).reverse().toString();
System.out.println(reversed); // olleH

// 优势：
// 1. 代码简洁
// 2. 性能优化
// 3. 经过充分测试
// 4. 易于维护
}
}

1.2 集合框架的重要性

为什么需要集合框架？

// 不使用集合：使用数组存储数据
public class WithoutCollection {
public static void main(String[] args) {
// 问题1：数组长度固定
String[] names = new String[3];
names[0] = "张三";
names[1] = "李四";
names[2] = "王五";
// names[3] = "赵六"; // 错误！数组越界

// 问题2：添加元素需要创建新数组
String[] newNames = new String[4];
for (int i = 0; i < names.length; i++) {
newNames[i] = names[i];
}
newNames[3] = "赵六";

// 问题3：删除元素很麻烦
// 需要手动移动元素
}
}

使用集合框架的优势：

import java.util.ArrayList;
import java.util.List;

// 使用集合：动态数组
public class WithCollection {
public static void main(String[] args) {
// 优势1：长度可变
List<String> names = new ArrayList<>();
names.add("张三");
names.add("李四");
names.add("王五");
names.add("赵六"); // 自动扩容

// 优势2：添加元素简单
names.add("孙七");

// 优势3：删除元素简单
names.remove("李四");

// 优势4：提供丰富的方法
System.out.println("元素个数：" + names.size());
System.out.println("是否包含张三：" + names.contains("张三"));
System.out.println("第一个元素：" + names.get(0));
}
}

二、String类详解

2.1 String类基础

String类的特点：

String的创建方式：

public class StringCreation {
public static void main(String[] args) {
// 方式1：字面量创建（推荐）
String str1 = "Hello";

// 方式2：使用new关键字
String str2 = new String("Hello");

// 方式3：使用字符数组
char[] chars = {'H', 'e', 'l', 'l', 'o'};
String str3 = new String(chars);

// 方式4：使用字节数组
byte[] bytes = {72, 101, 108, 108, 111};
String str4 = new String(bytes);

System.out.println(str1); // Hello
System.out.println(str2); // Hello
System.out.println(str3); // Hello
System.out.println(str4); // Hello
}
}

String的内存结构：

字符串池（String Pool） 堆内存（Heap）
┌─────────────────┐ ┌─────────────────┐
│ "Hello" │◄─────────────│ str1 │
└─────────────────┘ └─────────────────┘
▲ ┌─────────────────┐
│ │ str2 │
│ │ new String │
│ │ value="Hello" │
│ └─────────────────┘
│
└────────────────────────字面量直接指向字符串池
new创建的对象在堆内存

String的不可变性：

public class StringImmutable {
public static void main(String[] args) {
String str = "Hello";
System.out.println("原始字符串：" + str);
System.out.println("原始地址：" + System.identityHashCode(str));

// 看起来修改了字符串，实际上创建了新对象
str = str + " World";
System.out.println("修改后字符串：" + str);
System.out.println("修改后地址：" + System.identityHashCode(str));

// 输出结果显示地址不同，说明是新对象

// 为什么String要设计成不可变的？
// 1. 安全性：字符串常用于参数传递，不可变保证安全
// 2. 线程安全：不可变对象天然线程安全
// 3. 字符串池：可以共享字符串，节省内存
// 4. hashCode缓存：不可变对象的hashCode可以缓存
}
}

2.2 String常用方法

字符串长度和字符访问：

public class StringBasicMethods {
public static void main(String[] args) {
String str = "Hello World";

// 1. 获取字符串长度
int length = str.length();
System.out.println("字符串长度：" + length); // 11

// 2. 获取指定位置的字符
char ch = str.charAt(0);
System.out.println("第一个字符：" + ch); // H

// 3. 获取字符的索引位置
int index = str.indexOf('o');
System.out.println("字符o的位置：" + index); // 4

// 4. 获取字符最后出现的位置
int lastIndex = str.lastIndexOf('o');
System.out.println("字符o最后出现的位置：" + lastIndex); // 7

// 5. 判断字符串是否为空
boolean isEmpty = str.isEmpty();
System.out.println("字符串是否为空：" + isEmpty); // false

// 6. 判断字符串是否为空或只包含空白字符（Java 11+）
String blank = " ";
// boolean isBlank = blank.isBlank();
// System.out.println("字符串是否为空白：" + isBlank); // true
}
}

字符串比较：

public class StringComparison {
public static void main(String[] args) {
String str1 = "Hello";
String str2 = "Hello";
String str3 = new String("Hello");
String str4 = "hello";

// 1. 使用==比较（比较地址）
System.out.println("str1 == str2: " + (str1 == str2)); // true（字符串池）
System.out.println("str1 == str3: " + (str1 == str3)); // false（不同对象）

// 2. 使用equals比较（比较内容）
System.out.println("str1.equals(str2): " + str1.equals(str2)); // true
System.out.println("str1.equals(str3): " + str1.equals(str3)); // true
System.out.println("str1.equals(str4): " + str1.equals(str4)); // false

// 3. 使用equalsIgnoreCase比较（忽略大小写）
System.out.println("str1.equalsIgnoreCase(str4): " + str1.equalsIgnoreCase(str4)); // true

// 4. 使用compareTo比较（字典顺序）
System.out.println("str1.compareTo(str2): " + str1.compareTo(str2)); // 0（相等）
System.out.println("str1.compareTo(str4): " + str1.compareTo(str4)); // -32（str1小于str4）

// 5. 使用compareToIgnoreCase比较（忽略大小写的字典顺序）
System.out.println("str1.compareToIgnoreCase(str4): " + str1.compareToIgnoreCase(str4)); // 0
}
}

字符串查找和判断：

public class StringSearch {
public static void main(String[] args) {
String str = "Hello World, Welcome to Java";

// 1. 判断是否包含子字符串
boolean contains = str.contains("World");
System.out.println("是否包含World：" + contains); // true

// 2. 判断是否以指定字符串开头
boolean startsWith = str.startsWith("Hello");
System.out.println("是否以Hello开头：" + startsWith); // true

// 3. 判断是否以指定字符串结尾
boolean endsWith = str.endsWith("Java");
System.out.println("是否以Java结尾：" + endsWith); // true

// 4. 查找子字符串的位置
int index = str.indexOf("World");
System.out.println("World的位置：" + index); // 6

// 5. 从指定位置开始查找
int index2 = str.indexOf("o", 5);
System.out.println("从位置5开始查找o：" + index2); // 7

// 6. 查找子字符串最后出现的位置
int lastIndex = str.lastIndexOf("o");
System.out.println("o最后出现的位置：" + lastIndex); // 22
}
}

字符串截取和拆分：

public class StringSubstring {
public static void main(String[] args) {
String str = "Hello World, Welcome to Java";

// 1. 截取子字符串（从指定位置到末尾）
String sub1 = str.substring(6);
System.out.println("从位置6截取：" + sub1); // World, Welcome to Java

// 2. 截取子字符串（指定开始和结束位置）
String sub2 = str.substring(6, 11);
System.out.println("截取6到11：" + sub2); // World

// 3. 拆分字符串
String[] parts = str.split(",");
System.out.println("拆分结果：");
for (String part : parts) {
System.out.println(" " + part.trim());
}

// 4. 按正则表达式拆分
String str2 = "apple,banana;orange:grape";
String[] fruits = str2.split("[,;:]");
System.out.println("水果列表：");
for (String fruit : fruits) {
System.out.println(" " + fruit);
}

// 5. 限制拆分次数
String str3 = "a-b-c-d-e";
String[] parts2 = str3.split("-", 3);
System.out.println("限制拆分次数：");
for (String part : parts2) {
System.out.println(" " + part);
}
}
}

字符串替换和转换：

public class StringReplace {
public static void main(String[] args) {
String str = "Hello World, Hello Java";

// 1. 替换字符
String replaced1 = str.replace('o', 'O');
System.out.println("替换字符o：" + replaced1);

// 2. 替换子字符串
String replaced2 = str.replace("Hello", "Hi");
System.out.println("替换Hello：" + replaced2);

// 3. 替换第一个匹配的子字符串
String replaced3 = str.replaceFirst("Hello", "Hi");
System.out.println("替换第一个Hello：" + replaced3);

// 4. 使用正则表达式替换
String str2 = "abc123def456";
String replaced4 = str2.replaceAll("\\\\d+", "#");
System.out.println("替换所有数字：" + replaced4);

// 5. 转换为大写
String upper = str.toUpperCase();
System.out.println("转换为大写：" + upper);

// 6. 转换为小写
String lower = str.toLowerCase();
System.out.println("转换为小写：" + lower);

// 7. 去除首尾空白字符
String str3 = " Hello World ";
String trimmed = str3.trim();
System.out.println("去除空白：[" + trimmed + "]");

// 8. 去除所有空白字符（Java 11+）
// String stripped = str3.strip();
// System.out.println("去除空白：[" + stripped + "]");
}
}

字符串格式化和连接：

public class StringFormat {
public static void main(String[] args) {
// 1. 使用+连接字符串（不推荐，性能差）
String name = "张三";
int age = 25;
String info1 = "姓名：" + name + "，年龄：" + age;
System.out.println(info1);

// 2. 使用String.format格式化（推荐）
String info2 = String.format("姓名：%s，年龄：%d", name, age);
System.out.println(info2);

// 3. 常用格式化符号
System.out.println(String.format("整数：%d", 100));
System.out.println(String.format("浮点数：%.2f", 3.14159));
System.out.println(String.format("字符串：%s", "Hello"));
System.out.println(String.format("布尔值：%b", true));
System.out.println(String.format("十六进制：%x", 255));

// 4. 使用String.join连接字符串（Java 8+）
String joined = String.join(", ", "apple", "banana", "orange");
System.out.println("连接结果：" + joined);

// 5. 连接集合中的字符串
List<String> list = Arrays.asList("Java", "Python", "C++");
String languages = String.join(" | ", list);
System.out.println("编程语言：" + languages);

// 6. 重复字符串（Java 11+）
// String repeated = "Hello".repeat(3);
// System.out.println(repeated); // HelloHelloHello
}
}

2.3 StringBuilder和StringBuffer

为什么需要StringBuilder？

public class StringBuilderDemo {
public static void main(String[] args) {
// 问题：使用String拼接字符串，性能差
long start1 = System.currentTimeMillis();
String str = "";
for (int i = 0; i < 10000; i++) {
str += i; // 每次都创建新对象
}
long end1 = System.currentTimeMillis();
System.out.println("String拼接耗时：" + (end1 – start1) + "ms");

// 解决方案：使用StringBuilder
long start2 = System.currentTimeMillis();
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 10000; i++) {
sb.append(i); // 在原对象上修改
}
String result = sb.toString();
long end2 = System.currentTimeMillis();
System.out.println("StringBuilder拼接耗时：" + (end2 – start2) + "ms");

// 性能对比：StringBuilder比String快几十倍甚至上百倍
}
}

StringBuilder常用方法：

public class StringBuilderMethods {
public static void main(String[] args) {
StringBuilder sb = new StringBuilder("Hello");

// 1. 追加内容
sb.append(" World");
System.out.println("追加后：" + sb); // Hello World

// 2. 插入内容
sb.insert(5, ",");
System.out.println("插入后：" + sb); // Hello, World

// 3. 删除内容
sb.delete(5, 6);
System.out.println("删除后：" + sb); // Hello World

// 4. 替换内容
sb.replace(0, 5, "Hi");
System.out.println("替换后：" + sb); // Hi World

// 5. 反转字符串
sb.reverse();
System.out.println("反转后：" + sb); // dlroW iH

// 6. 获取长度
int length = sb.length();
System.out.println("长度：" + length);

// 7. 获取容量
int capacity = sb.capacity();
System.out.println("容量：" + capacity);

// 8. 设置长度
sb.setLength(5);
System.out.println("设置长度后：" + sb);

// 9. 转换为String
String str = sb.toString();
System.out.println("转换为String：" + str);
}
}

StringBuilder vs StringBuffer：

public class StringBuilderVsStringBuffer {
public static void main(String[] args) {
// StringBuilder：线程不安全，性能高（推荐）
StringBuilder sb = new StringBuilder();
sb.append("Hello");
sb.append(" World");

// StringBuffer：线程安全，性能较低
StringBuffer sbf = new StringBuffer();
sbf.append("Hello");
sbf.append(" World");

System.out.println("StringBuilder：" + sb);
System.out.println("StringBuffer：" + sbf);
}
}

对比总结：

2.4 字符串池（String Pool）

字符串池的概念：

public class StringPoolDemo {
public static void main(String[] args) {
// 字符串池：JVM为了优化字符串操作，维护的一个特殊内存区域

// 1. 字面量创建的字符串在字符串池中
String str1 = "Hello";
String str2 = "Hello";
System.out.println("str1 == str2: " + (str1 == str2)); // true

// 2. new创建的字符串在堆内存中
String str3 = new String("Hello");
System.out.println("str1 == str3: " + (str1 == str3)); // false

// 3. intern()方法：将字符串放入字符串池
String str4 = new String("Hello").intern();
System.out.println("str1 == str4: " + (str1 == str4)); // true

// 4. 字符串拼接
String str5 = "Hel" + "lo"; // 编译期优化，直接变成"Hello"
System.out.println("str1 == str5: " + (str1 == str5)); // true

String s1 = "Hel";
String s2 = "lo";
String str6 = s1 + s2; // 运行时拼接，创建新对象
System.out.println("str1 == str6: " + (str1 == str6)); // false

// 5. 使用final修饰的变量拼接
final String s3 = "Hel";
final String s4 = "lo";
String str7 = s3 + s4; // 编译期优化
System.out.println("str1 == str7: " + (str1 == str7)); // true
}
}

字符串池的内存结构：

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指向

指向

指向

字符串池 String Pool

Hello

World

Java

堆内存 Heap

new String Hello

new String World

str1 = Hello

str2 = Hello

str3 = new String Hello

2.5 实战案例：字符串工具类

/**
* 字符串工具类
*/
public class StringUtils {

/**
* 判断字符串是否为空
*/
public static boolean isEmpty(String str) {
return str == null || str.length() == 0;
}

/**
* 判断字符串是否为空或只包含空白字符
*/
public static boolean isBlank(String str) {
if (str == null || str.length() == 0) {
return true;
}
for (int i = 0; i < str.length(); i++) {
if (!Character.isWhitespace(str.charAt(i))) {
return false;
}
}
return true;
}

/**
* 反转字符串
*/
public static String reverse(String str) {
if (isEmpty(str)) {
return str;
}
return new StringBuilder(str).reverse().toString();
}

/**
* 判断是否是回文字符串
*/
public static boolean isPalindrome(String str) {
if (isEmpty(str)) {
return false;
}
int left = 0;
int right = str.length() – 1;
while (left < right) {
if (str.charAt(left) != str.charAt(right)) {
return false;
}
left++;
right—;
}
return true;
}

/**
* 统计字符出现次数
*/
public static int countChar(String str, char ch) {
if (isEmpty(str)) {
return 0;
}
int count = 0;
for (int i = 0; i < str.length(); i++) {
if (str.charAt(i) == ch) {
count++;
}
}
return count;
}

/**
* 首字母大写
*/
public static String capitalize(String str) {
if (isEmpty(str)) {
return str;
}
return str.substring(0, 1).toUpperCase() + str.substring(1);
}

/**
* 驼峰命名转下划线命名
*/
public static String camelToUnderscore(String str) {
if (isEmpty(str)) {
return str;
}
StringBuilder sb = new StringBuilder();
for (int i = 0; i < str.length(); i++) {
char ch = str.charAt(i);
if (Character.isUpperCase(ch)) {
if (i > 0) {
sb.append('_');
}
sb.append(Character.toLowerCase(ch));
} else {
sb.append(ch);
}
}
return sb.toString();
}

/**
* 下划线命名转驼峰命名
*/
public static String underscoreToCamel(String str) {
if (isEmpty(str)) {
return str;
}
StringBuilder sb = new StringBuilder();
boolean nextUpperCase = false;
for (int i = 0; i < str.length(); i++) {
char ch = str.charAt(i);
if (ch == '_') {
nextUpperCase = true;
} else {
if (nextUpperCase) {
sb.append(Character.toUpperCase(ch));
nextUpperCase = false;
} else {
sb.append(ch);
}
}
}
return sb.toString();
}

/**
* 测试方法
*/
public static void main(String[] args) {
// 测试isEmpty
System.out.println("isEmpty(null): " + isEmpty(null));
System.out.println("isEmpty(\\"\\"): " + isEmpty(""));
System.out.println("isEmpty(\\"Hello\\"): " + isEmpty("Hello"));

// 测试isBlank
System.out.println("\\nisBlank(\\" \\"): " + isBlank(" "));
System.out.println("isBlank(\\"Hello\\"): " + isBlank("Hello"));

// 测试reverse
System.out.println("\\nreverse(\\"Hello\\"): " + reverse("Hello"));

// 测试isPalindrome
System.out.println("\\nisPalindrome(\\"level\\"): " + isPalindrome("level"));
System.out.println("isPalindrome(\\"hello\\"): " + isPalindrome("hello"));

// 测试countChar
System.out.println("\\ncountChar(\\"hello\\", 'l'): " + countChar("hello", 'l'));

// 测试capitalize
System.out.println("\\ncapitalize(\\"hello\\"): " + capitalize("hello"));

// 测试camelToUnderscore
System.out.println("\\ncamelToUnderscore(\\"userName\\"): " + camelToUnderscore("userName"));

// 测试underscoreToCamel
System.out.println("\\nunderscoreToCamel(\\"user_name\\"): " + underscoreToCamel("user_name"));
}
}

三、日期时间API

3.1 Date类（旧API）

Date类的基本使用：

import java.util.Date;

public class DateDemo {
public static void main(String[] args) {
// 1. 创建当前时间的Date对象
Date now = new Date();
System.out.println("当前时间：" + now);

// 2. 创建指定时间的Date对象（毫秒值）
Date date = new Date(1000L * 60 * 60 * 24 * 365 * 50); // 1970年后50年
System.out.println("指定时间：" + date);

// 3. 获取时间戳（毫秒值）
long timestamp = now.getTime();
System.out.println("时间戳：" + timestamp);

// 4. 设置时间
now.setTime(timestamp + 1000 * 60 * 60); // 加1小时
System.out.println("1小时后：" + now);

// 5. 比较时间
Date date1 = new Date();
Date date2 = new Date(date1.getTime() + 1000);
System.out.println("date1在date2之前：" + date1.before(date2));
System.out.println("date1在date2之后：" + date1.after(date2));

// 注意：Date类的很多方法已经过时，不推荐使用
}
}

3.2 SimpleDateFormat类

日期格式化和解析：

import java.text.SimpleDateFormat;
import java.text.ParseException;
import java.util.Date;

public class SimpleDateFormatDemo {
public static void main(String[] args) throws ParseException {
// 1. 创建SimpleDateFormat对象
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

// 2. 格式化日期（Date -> String）
Date now = new Date();
String dateStr = sdf.format(now);
System.out.println("格式化后：" + dateStr);

// 3. 解析日期（String -> Date）
String str = "2024-01-06 15:30:00";
Date date = sdf.parse(str);
System.out.println("解析后：" + date);

// 4. 常用日期格式
System.out.println("\\n常用日期格式：");

// yyyy-MM-dd
SimpleDateFormat sdf1 = new SimpleDateFormat("yyyy-MM-dd");
System.out.println("yyyy-MM-dd: " + sdf1.format(now));

// yyyy年MM月dd日
SimpleDateFormat sdf2 = new SimpleDateFormat("yyyy年MM月dd日");
System.out.println("yyyy年MM月dd日: " + sdf2.format(now));

// HH:mm:ss
SimpleDateFormat sdf3 = new SimpleDateFormat("HH:mm:ss");
System.out.println("HH:mm:ss: " + sdf3.format(now));

// yyyy-MM-dd HH:mm:ss E
SimpleDateFormat sdf4 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss E");
System.out.println("yyyy-MM-dd HH:mm:ss E: " + sdf4.format(now));

// 5. 格式化符号说明
System.out.println("\\n格式化符号说明：");
System.out.println("y – 年");
System.out.println("M – 月");
System.out.println("d – 日");
System.out.println("H – 时（24小时制）");
System.out.println("h – 时（12小时制）");
System.out.println("m – 分");
System.out.println("s – 秒");
System.out.println("S – 毫秒");
System.out.println("E – 星期");
System.out.println("a – 上午/下午");
}
}

SimpleDateFormat线程安全问题：

import java.text.SimpleDateFormat;
import java.util.Date;

public class SimpleDateFormatThreadSafe {
// 问题：SimpleDateFormat不是线程安全的
private static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

public static void main(String[] args) {
// 多线程环境下可能出现问题
for (int i = 0; i < 10; i++) {
new Thread(() -> {
try {
Date date = sdf.parse("2024-01-06 15:30:00");
System.out.println(Thread.currentThread().getName() + ": " + date);
} catch (Exception e) {
e.printStackTrace();
}
}).start();
}

// 解决方案1：每次创建新对象
// SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");

// 解决方案2：使用ThreadLocal
// ThreadLocal<SimpleDateFormat> threadLocal = ThreadLocal.withInitial(
// () -> new SimpleDateFormat("yyyy-MM-dd HH:mm:ss")
// );

// 解决方案3：使用Java 8的DateTimeFormatter（推荐）
}
}

3.3 Calendar类

Calendar类的使用：

import java.util.Calendar;
import java.util.Date;

public class CalendarDemo {
public static void main(String[] args) {
// 1. 获取Calendar实例
Calendar calendar = Calendar.getInstance();
System.out.println("当前时间：" + calendar.getTime());

// 2. 获取日期字段
int year = calendar.get(Calendar.YEAR);
int month = calendar.get(Calendar.MONTH) + 1; // 月份从0开始
int day = calendar.get(Calendar.DAY_OF_MONTH);
int hour = calendar.get(Calendar.HOUR_OF_DAY);
int minute = calendar.get(Calendar.MINUTE);
int second = calendar.get(Calendar.SECOND);

System.out.println(String.format("%d年%d月%d日 %d:%d:%d",
year, month, day, hour, minute, second));

// 3. 设置日期字段
calendar.set(Calendar.YEAR, 2025);
calendar.set(Calendar.MONTH, 11); // 12月
calendar.set(Calendar.DAY_OF_MONTH, 25);
System.out.println("设置后：" + calendar.getTime());

// 4. 增加或减少日期
calendar.add(Calendar.DAY_OF_MONTH, 7); // 加7天
System.out.println("7天后：" + calendar.getTime());

calendar.add(Calendar.MONTH, –1); // 减1个月
System.out.println("1个月前：" + calendar.getTime());

// 5. 获取星期几
int dayOfWeek = calendar.get(Calendar.DAY_OF_WEEK);
String[] weeks = {"", "星期日", "星期一", "星期二", "星期三", "星期四", "星期五", "星期六"};
System.out.println("星期：" + weeks[dayOfWeek]);

// 6. 获取当月最大天数
int maxDay = calendar.getActualMaximum(Calendar.DAY_OF_MONTH);
System.out.println("当月最大天数：" + maxDay);

// 7. Calendar和Date互转
Date date = calendar.getTime(); // Calendar -> Date
calendar.setTime(date); // Date -> Calendar
}
}

3.4 Java 8新日期时间API

LocalDate、LocalTime、LocalDateTime：

import java.time.*;
import java.time.format.DateTimeFormatter;

public class Java8DateTimeDemo {
public static void main(String[] args) {
// 1. LocalDate：只包含日期
LocalDate today = LocalDate.now();
System.out.println("今天：" + today);

LocalDate birthday = LocalDate.of(1990, 1, 1);
System.out.println("生日：" + birthday);

// 2. LocalTime：只包含时间
LocalTime now = LocalTime.now();
System.out.println("现在：" + now);

LocalTime time = LocalTime.of(15, 30, 0);
System.out.println("指定时间：" + time);

// 3. LocalDateTime：包含日期和时间
LocalDateTime dateTime = LocalDateTime.now();
System.out.println("当前日期时间：" + dateTime);

LocalDateTime dt = LocalDateTime.of(2024, 1, 6, 15, 30, 0);
System.out.println("指定日期时间：" + dt);

// 4. 获取日期字段
int year = today.getYear();
int month = today.getMonthValue();
int day = today.getDayOfMonth();
System.out.println(String.format("%d年%d月%d日", year, month, day));

// 5. 日期计算
LocalDate tomorrow = today.plusDays(1);
System.out.println("明天：" + tomorrow);

LocalDate nextWeek = today.plusWeeks(1);
System.out.println("下周：" + nextWeek);

LocalDate lastMonth = today.minusMonths(1);
System.out.println("上个月：" + lastMonth);

// 6. 日期比较
boolean isBefore = today.isBefore(tomorrow);
System.out.println("今天在明天之前：" + isBefore);

boolean isAfter = today.isAfter(lastMonth);
System.out.println("今天在上个月之后：" + isAfter);

// 7. 格式化和解析
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
String formatted = dateTime.format(formatter);
System.out.println("格式化后：" + formatted);

LocalDateTime parsed = LocalDateTime.parse("2024-01-06 15:30:00", formatter);
System.out.println("解析后：" + parsed);

// 8. 时间间隔
Period period = Period.between(birthday, today);
System.out.println("年龄：" + period.getYears() + "岁");

Duration duration = Duration.between(time, now);
System.out.println("时间差：" + duration.toHours() + "小时");
}
}

Java 8日期时间API的优势：

public class Java8DateTimeAdvantage {
public static void main(String[] args) {
// 优势1：不可变性（线程安全）
LocalDate date = LocalDate.now();
LocalDate newDate = date.plusDays(1); // 返回新对象，原对象不变
System.out.println("原日期：" + date);
System.out.println("新日期：" + newDate);

// 优势2：清晰的API
LocalDateTime dt = LocalDateTime.now();
System.out.println("年：" + dt.getYear());
System.out.println("月：" + dt.getMonthValue());
System.out.println("日：" + dt.getDayOfMonth());

// 优势3：链式调用
LocalDate result = LocalDate.now()
.plusDays(1)
.plusMonths(1)
.plusYears(1);
System.out.println("1年1月1天后：" + result);

// 优势4：时区支持
ZonedDateTime zonedDateTime = ZonedDateTime.now(ZoneId.of("Asia/Shanghai"));
System.out.println("上海时间：" + zonedDateTime);

// 优势5：DateTimeFormatter线程安全
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
String formatted = date.format(formatter);
System.out.println("格式化：" + formatted);
}
}

四、Math和Random类

4.1 Math类常用方法

Math类的基本使用：

public class MathDemo {
public static void main(String[] args) {
// 1. 常量
System.out.println("圆周率π：" + Math.PI);
System.out.println("自然对数e：" + Math.E);

// 2. 绝对值
System.out.println("\\n绝对值：");
System.out.println("abs(-10): " + Math.abs(–10));
System.out.println("abs(-3.14): " + Math.abs(–3.14));

// 3. 最大值和最小值
System.out.println("\\n最大值和最小值：");
System.out.println("max(10, 20): " + Math.max(10, 20));
System.out.println("min(10, 20): " + Math.min(10, 20));

// 4. 幂运算
System.out.println("\\n幂运算：");
System.out.println("pow(2, 3): " + Math.pow(2, 3)); // 2的3次方
System.out.println("pow(5, 2): " + Math.pow(5, 2)); // 5的2次方

// 5. 平方根
System.out.println("\\n平方根：");
System.out.println("sqrt(16): " + Math.sqrt(16));
System.out.println("sqrt(2): " + Math.sqrt(2));

// 6. 立方根
System.out.println("\\n立方根：");
System.out.println("cbrt(27): " + Math.cbrt(27));

// 7. 向上取整
System.out.println("\\n向上取整：");
System.out.println("ceil(3.1): " + Math.ceil(3.1)); // 4.0
System.out.println("ceil(3.9): " + Math.ceil(3.9)); // 4.0
System.out.println("ceil(-3.1): " + Math.ceil(–3.1)); // -3.0

// 8. 向下取整
System.out.println("\\n向下取整：");
System.out.println("floor(3.1): " + Math.floor(3.1)); // 3.0
System.out.println("floor(3.9): " + Math.floor(3.9)); // 3.0
System.out.println("floor(-3.1): " + Math.floor(–3.1)); // -4.0

// 9. 四舍五入
System.out.println("\\n四舍五入：");
System.out.println("round(3.4): " + Math.round(3.4)); // 3
System.out.println("round(3.5): " + Math.round(3.5)); // 4
System.out.println("round(3.6): " + Math.round(3.6)); // 4

// 10. 随机数
System.out.println("\\n随机数：");
double random = Math.random(); // [0.0, 1.0)
System.out.println("random(): " + random);

// 生成[0, 100)的随机整数
int randomInt = (int)(Math.random() * 100);
System.out.println("0-99的随机整数：" + randomInt);

// 11. 三角函数
System.out.println("\\n三角函数：");
System.out.println("sin(π/2): " + Math.sin(Math.PI / 2));
System.out.println("cos(π): " + Math.cos(Math.PI));
System.out.println("tan(π/4): " + Math.tan(Math.PI / 4));
}
}

Math类实战案例：

public class MathPractice {
/**
* 计算两点之间的距离
*/
public static double distance(double x1, double y1, double x2, double y2) {
double dx = x2 – x1;
double dy = y2 – y1;
return Math.sqrt(dx * dx + dy * dy);
}

/**
* 计算圆的面积
*/
public static double circleArea(double radius) {
return Math.PI * radius * radius;
}

/**
* 计算圆的周长
*/
public static double circlePerimeter(double radius) {
return 2 * Math.PI * radius;
}

/**
* 判断是否是质数
*/
public static boolean isPrime(int n) {
if (n <= 1) return false;
if (n == 2) return true;
if (n % 2 == 0) return false;

int sqrt = (int)Math.sqrt(n);
for (int i = 3; i <= sqrt; i += 2) {
if (n % i == 0) {
return false;
}
}
return true;
}

/**
* 生成指定范围的随机整数
*/
public static int randomInt(int min, int max) {
return (int)(Math.random() * (max – min + 1)) + min;
}

public static void main(String[] args) {
// 测试距离计算
double dist = distance(0, 0, 3, 4);
System.out.println("两点距离：" + dist);

// 测试圆的面积和周长
double radius = 5.0;
System.out.println("圆的面积：" + circleArea(radius));
System.out.println("圆的周长：" + circlePerimeter(radius));

// 测试质数判断
System.out.println("17是质数：" + isPrime(17));
System.out.println("18是质数：" + isPrime(18));

// 测试随机数生成
System.out.println("1-100的随机数：" + randomInt(1, 100));
}
}

4.2 Random类

Random类的使用：

import java.util.Random;

public class RandomDemo {
public static void main(String[] args) {
// 1. 创建Random对象
Random random = new Random();

// 2. 生成随机整数
int randomInt = random.nextInt();
System.out.println("随机整数：" + randomInt);

// 3. 生成指定范围的随机整数
int randomInt10 = random.nextInt(10); // [0, 10)
System.out.println("0-9的随机整数：" + randomInt10);

int randomInt100 = random.nextInt(100) + 1; // [1, 100]
System.out.println("1-100的随机整数：" + randomInt100);

// 4. 生成随机浮点数
double randomDouble = random.nextDouble(); // [0.0, 1.0)
System.out.println("随机浮点数：" + randomDouble);

// 5. 生成随机布尔值
boolean randomBoolean = random.nextBoolean();
System.out.println("随机布尔值：" + randomBoolean);

// 6. 生成随机长整数
long randomLong = random.nextLong();
System.out.println("随机长整数：" + randomLong);

// 7. 使用种子创建Random对象（相同种子产生相同序列）
Random random1 = new Random(100);
Random random2 = new Random(100);
System.out.println("random1: " + random1.nextInt(10));
System.out.println("random2: " + random2.nextInt(10)); // 相同
}
}

Random实战案例：

import java.util.*;

public class RandomPractice {
private static Random random = new Random();

/**
* 生成随机密码
*/
public static String generatePassword(int length) {
String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*";
StringBuilder password = new StringBuilder();
for (int i = 0; i < length; i++) {
int index = random.nextInt(chars.length());
password.append(chars.charAt(index));
}
return password.toString();
}

/**
* 随机打乱数组
*/
public static void shuffle(int[] arr) {
for (int i = arr.length – 1; i > 0; i—) {
int j = random.nextInt(i + 1);
int temp = arr[i];
arr[i] = arr[j];
arr[j] = temp;
}
}

/**
* 从数组中随机选择n个元素
*/
public static <T> List<T> randomSelect(T[] arr, int n) {
if (n > arr.length) {
n = arr.length;
}

List<T> result = new ArrayList<>();
List<Integer> indices = new ArrayList<>();
for (int i = 0; i < arr.length; i++) {
indices.add(i);
}

for (int i = 0; i < n; i++) {
int index = random.nextInt(indices.size());
result.add(arr[indices.get(index)]);
indices.remove(index);
}

return result;
}

/**
* 生成随机颜色（RGB）
*/
public static String randomColor() {
int r = random.nextInt(256);
int g = random.nextInt(256);
int b = random.nextInt(256);
return String.format("RGB(%d, %d, %d)", r, g, b);
}

public static void main(String[] args) {
// 测试生成随机密码
System.out.println("随机密码：" + generatePassword(12));

// 测试随机打乱数组
int[] arr = {1, 2, 3, 4, 5};
shuffle(arr);
System.out.println("打乱后的数组：" + Arrays.toString(arr));

// 测试随机选择
String[] fruits = {"apple", "banana", "orange", "grape", "watermelon"};
List<String> selected = randomSelect(fruits, 3);
System.out.println("随机选择3个水果：" + selected);

// 测试生成随机颜色
System.out.println("随机颜色：" + randomColor());
}
}

4.3 UUID生成

UUID的使用：

import java.util.UUID;

public class UUIDDemo {
public static void main(String[] args) {
// 1. 生成随机UUID
UUID uuid = UUID.randomUUID();
System.out.println("UUID：" + uuid);
System.out.println("UUID字符串：" + uuid.toString());

// 2. 生成多个UUID
System.out.println("\\n生成5个UUID：");
for (int i = 0; i < 5; i++) {
System.out.println(UUID.randomUUID());
}

// 3. 去掉UUID中的横线
String uuidStr = UUID.randomUUID().toString().replace("-", "");
System.out.println("\\n无横线UUID：" + uuidStr);

// 4. UUID的应用场景
System.out.println("\\nUUID应用场景：");
System.out.println("1. 生成唯一的文件名");
System.out.println("2. 生成唯一的订单号");
System.out.println("3. 生成唯一的会话ID");
System.out.println("4. 分布式系统中的唯一标识");
}
}

五、集合框架概述

5.1 集合框架体系结构

集合框架的层次结构：

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Collection接口

List接口

Set接口

Queue接口

ArrayList

LinkedList

Vector

HashSet

LinkedHashSet

TreeSet

PriorityQueue

Deque接口

ArrayDeque

Map接口

HashMap

LinkedHashMap

TreeMap

Hashtable

Collection接口和Map接口的区别：

5.2 集合框架的选择

如何选择合适的集合？

public class CollectionChoice {
public static void main(String[] args) {
// 1. 需要存储单个元素，且需要保证顺序 -> List
// – 需要频繁随机访问 -> ArrayList
// – 需要频繁插入删除 -> LinkedList

// 2. 需要存储单个元素，且不允许重复 -> Set
// – 不需要排序 -> HashSet
// – 需要保持插入顺序 -> LinkedHashSet
// – 需要自动排序 -> TreeSet

// 3. 需要存储键值对 -> Map
// – 不需要排序 -> HashMap
// – 需要保持插入顺序 -> LinkedHashMap
// – 需要自动排序 -> TreeMap

// 4. 需要队列功能 -> Queue
// – 需要优先级队列 -> PriorityQueue
// – 需要双端队列 -> ArrayDeque

System.out.println("集合选择指南：");
System.out.println("1. 需要快速随机访问 -> ArrayList");
System.out.println("2. 需要频繁插入删除 -> LinkedList");
System.out.println("3. 需要去重 -> HashSet");
System.out.println("4. 需要排序 -> TreeSet/TreeMap");
System.out.println("5. 需要键值对 -> HashMap");
System.out.println("6. 需要线程安全 -> ConcurrentHashMap");
}
}

六、List接口详解

6.1 ArrayList详解

ArrayList的特点：

ArrayList的基本使用：

import java.util.ArrayList;
import java.util.List;

public class ArrayListDemo {
public static void main(String[] args) {
// 1. 创建ArrayList
List<String> list = new ArrayList<>();

// 2. 添加元素
list.add("Java");
list.add("Python");
list.add("C++");
System.out.println("添加后：" + list);

// 3. 在指定位置插入元素
list.add(1, "JavaScript");
System.out.println("插入后：" + list);

// 4. 获取元素
String first = list.get(0);
System.out.println("第一个元素：" + first);

// 5. 修改元素
list.set(0, "Java SE");
System.out.println("修改后：" + list);

// 6. 删除元素
list.remove(1); // 按索引删除
System.out.println("删除索引1后：" + list);

list.remove("C++"); // 按对象删除
System.out.println("删除C++后：" + list);

// 7. 判断是否包含元素
boolean contains = list.contains("Python");
System.out.println("是否包含Python：" + contains);

// 8. 获取元素索引
int index = list.indexOf("Python");
System.out.println("Python的索引：" + index);

// 9. 获取集合大小
int size = list.size();
System.out.println("集合大小：" + size);

// 10. 判断集合是否为空
boolean isEmpty = list.isEmpty();
System.out.println("集合是否为空：" + isEmpty);

// 11. 清空集合
list.clear();
System.out.println("清空后：" + list);
}
}

ArrayList的遍历方式：

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class ArrayListTraversal {
public static void main(String[] args) {
List<String> list = new ArrayList<>();
list.add("Java");
list.add("Python");
list.add("C++");
list.add("JavaScript");

// 方式1：普通for循环
System.out.println("方式1：普通for循环");
for (int i = 0; i < list.size(); i++) {
System.out.println(list.get(i));
}

// 方式2：增强for循环（推荐）
System.out.println("\\n方式2：增强for循环");
for (String item : list) {
System.out.println(item);
}

// 方式3：Iterator迭代器
System.out.println("\\n方式3：Iterator迭代器");
Iterator<String> iterator = list.iterator();
while (iterator.hasNext()) {
String item = iterator.next();
System.out.println(item);
}

// 方式4：forEach + Lambda表达式（Java 8+）
System.out.println("\\n方式4：forEach + Lambda");
list.forEach(item -> System.out.println(item));

// 方式5：Stream API（Java 8+）
System.out.println("\\n方式5：Stream API");
list.stream().forEach(System.out::println);
}
}

ArrayList的底层原理：

public class ArrayListPrinciple {
public static void main(String[] args) {
// ArrayList底层原理：
// 1. 底层使用Object[]数组存储元素
// 2. 默认初始容量为10
// 3. 当容量不足时，扩容为原来的1.5倍
// 4. 扩容过程：创建新数组，复制旧数组元素到新数组

List<Integer> list = new ArrayList<>();

// 添加元素时的扩容过程
System.out.println("添加元素观察扩容：");
for (int i = 0; i < 15; i++) {
list.add(i);
// 第11个元素时会触发扩容
if (i == 10) {
System.out.println("添加第11个元素，触发扩容");
}
}

// ArrayList的优缺点
System.out.println("\\nArrayList的优点：");
System.out.println("1. 查询快：O(1)时间复杂度");
System.out.println("2. 支持随机访问");
System.out.println("3. 内存连续，缓存友好");

System.out.println("\\nArrayList的缺点：");
System.out.println("1. 插入删除慢：需要移动元素");
System.out.println("2. 扩容有性能开销");
System.out.println("3. 线程不安全");
}
}

6.2 LinkedList详解

LinkedList的特点：

LinkedList的基本使用：

import java.util.LinkedList;

public class LinkedListDemo {
public static void main(String[] args) {
LinkedList<String> list = new LinkedList<>();

// 1. 添加元素
list.add("Java");
list.add("Python");
list.add("C++");
System.out.println("添加后：" + list);

// 2. 在头部添加元素
list.addFirst("JavaScript");
System.out.println("头部添加后：" + list);

// 3. 在尾部添加元素
list.addLast("Go");
System.out.println("尾部添加后：" + list);