Java 8至21新特性代码写法对比：Lambda、Record与Switch

List list = Arrays.asList("apple", "banana", "pear");
Collections.sort(list, new Comparator() {
    @Override
    public int compare(String s1, String s2) {
        return Integer.compare(s1.length(), s2.length());
    }
});

List list = Arrays.asList("apple", "banana", "pear");

// 方式 1：标准 Lambda 表达式
list.sort((s1, s2) -> Integer.compare(s1.length(), s2.length()));

// 方式 2：结合 Comparator 静态方法引用
list.sort(Comparator.comparingInt(String::length));

public final class UserOrderDto {
    private final String orderId;
    private final double amount;

    public UserOrderDto(String orderId, double amount) {
        this.orderId = orderId;
        this.amount = amount;
    }

    public String getOrderId() { return orderId; }
    public double getAmount() { return amount; }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof UserOrderDto that)) return false;
        return Double.compare(that.amount, amount) == 0 &&
               Objects.equals(orderId, that.orderId);
    }

    @Override
    public int hashCode() {
        return Objects.hash(orderId, amount);
    }

    @Override
    public String toString() {
        return "UserOrderDto[orderId=" + orderId + ", amount=" + amount + "]";
    }
}

// 一行完成不可变类、属性定义、构造器与三样板方法
public record UserOrderDto(String orderId, double amount) {

    // 进阶：紧凑型构造器（Compact Constructor）
    // 常用于参数边界校验，无需显式写 this.x = x;
    public UserOrderDto {
        if (orderId == null || orderId.isBlank()) {
            throw new IllegalArgumentException("orderId 不能为空");
        }
        if (amount < 0) {
            throw new IllegalArgumentException("金额不能为负数");
        }
    }
}

// 访问方式对比：
UserOrderDto dto = new UserOrderDto("ORD-001", 99.5);
System.out.println(dto.orderId()); // 注意：访问方法没有 get 前缀

// 传统类型匹配必须包含频繁强转
String format(Object obj) {
    String result;
    if (obj instanceof Integer) {
        Integer i = (Integer) obj;
        result = "整数: " + i;
    } else if (obj instanceof Long) {
        Long l = (Long) obj;
        result = "长整数: " + l;
    } else if (obj instanceof String) {
        String s = (String) obj;
        result = "字符串: " + s.trim();
    } else {
        result = "其他类型";
    }
    return result;
}

// Switch 表达式直接返回值，并在分支完成解构
String format(Object obj) {
    return switch (obj) {
        case Integer i -> "整数: " + i;
        case Long l    -> "长整数: " + l;
        // 守卫条件 when: 避免额外嵌套 if
        case String s when !s.isBlank() -> "非空字符串: " + s.trim();
        case String s  -> "空字符串";
        case null      -> "入参为 null";
        default        -> "未知对象: " + obj.toString();
    };
}