JDK17前后写法对比：现代Java开发的进化之路

// JDK 8及之前
Object obj = "Hello Java";
if (obj instanceof String) {
    String str = (String) obj; // 需要显式强制转换
    System.out.println(str.length());
}

// JDK 16+（JDK 14中作为预览特性引入）
Object obj = "Hello Java";
if (obj instanceof String str) { // 直接绑定变量
    System.out.println(str.length()); // str已经是String类型
}

// JDK 8：传统POJO类
public class User {
    private final String name;
    private final int age;
    
    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }
    
    // 必须手动生成getter、equals、hashCode、toString等方法
    public String getName() { return name; }
    public int getAge() { return age; }
    
    @Override
    public boolean equals(Object o) { /* 冗长的实现 */ }
    @Override
    public int hashCode() { /* 冗长的实现 */ }
    @Override
    public String toString() { /* 冗长的实现 */ }
}

// JDK 16+：记录类（JDK 14预览，JDK 16正式）
public record User(String name, int age) { }
// 自动生成：规范构造器、访问器(name()、age())、equals、hashCode、toString

// JDK 17+：密封类
public sealed class Shape permits Circle, Rectangle, Triangle {
    // 基类定义
}

public final class Circle extends Shape { /* 圆形实现 */ }
public non-sealed class Rectangle extends Shape { /* 矩形实现 */ }
public final class Triangle extends Shape { /* 三角形实现 */ }

// 编译错误：Square不允许继承Shape
// public class Square extends Shape { }

// JDK 8：传统switch语句（容易遗漏break导致穿透）
String dayType;
switch (day) {
    case MONDAY:
    case TUESDAY:
    case WEDNESDAY: 
    case THURSDAY:
    case FRIDAY:
        dayType = "工作日";
        break; // 容易忘记
    case SATURDAY:
    case SUNDAY:
        dayType = "周末";
        break;
    default:
        throw new IllegalArgumentException("无效日期");
}

// JDK 14+：switch表达式（JDK 12预览，JDK 14正式）
String dayType = switch (day) {
    case MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY -> "工作日";
    case SATURDAY, SUNDAY -> "周末";
    default -> throw new IllegalArgumentException("无效日期");
};

// 多行逻辑使用代码块
int daysInMonth = switch (month) {
    case JAN, MAR, MAY, JUL, AUG, OCT, DEC -> 31;
    case APR, JUN, SEP, NOV -> 30;
    case FEB -> {
        if (isLeapYear) 
            yield 29; // JDK 13引入yield关键字
        else 
            yield 28;
    }
};

// JDK 8：多行字符串拼接
String json = "{\n" +
              "  \"name\": \"张三\",\n" +
              "  \"age\": 30,\n" +
              "  \"city\": \"北京\"\n" +
              "}";

// JDK 15+：文本块（JDK 13预览，JDK 15正式）
String json = """
    {
      "name": "张三",
      "age": 30,
      "city": "北京"
    }
    """;

// JDK 10之前
List<String> list = new ArrayList<>();
Map<String, List<Integer>> map = new HashMap<>();

// JDK 10+
var list = new ArrayList<String>(); // 推断为List<String>
var map = new HashMap<String, List<Integer>>(); // 推断为Map<String, List<Integer>>

// 替代Arrays.asList和Collections.unmodifiableXXX
List<String> list = List.of("A", "B", "C");
Set<Integer> set = Set.of(1, 2, 3);
Map<String, Integer> map = Map.of("A", 1, "B", 2);

// JDK 11+
"  Hello  ".strip();          // "Hello" - 去除前后空格
"  Hello  ".stripLeading();   // "Hello  " - 只去前面
"".isBlank();                 // true - 空或仅空白字符
"line1\nline2".lines().count(); // 2 - 行数统计
"Java".repeat(3);             // "JavaJavaJava" - 重复

// JDK 16之前
String s = null;
s.substring(1); 
// 抛出：java.lang.NullPointerException

// JDK 16+
String s = null;
s.substring(1);
// 抛出：java.lang.NullPointerException: 
// Cannot invoke "String.substring(int)" because "s" is null