面向对象和泛型：订单 9527 是怎么被“管住”的

Map order = new HashMap();          // 原始类型
order.put("id", 9527L);
order.put("status", "CREATED");
order.put("lines", lines);

// 任何人、任何地方都能改状态
order.put("status", "CANCELED");   // 拼错了也没人管

// 取出来要自己强转
List ls = (List) order.get("lines");
OrderLine l = (OrderLine) ls.get(0);

// 每加一种优惠，就要找到所有 if 改一遍
if ("MINUS".equals(type))   pay = sum.subtract(off);
else if ("PCT".equals(type)) pay = sum.multiply(rate);
// 新加的 "FULL" 忘了写：不报错，悄悄不打折

class Order {
    private final List<OrderLine> lines;
    private Status status = Status.CREATED;

    Order(long id, long userId, List<OrderLine> lines) {
        …
        this.lines = new ArrayList<>(lines);   // 防御性复制
    }
    List<OrderLine> lines() {
        return Collections.unmodifiableList(lines);
    }
    void pay()    { 只能从 CREATED 到 PAID }
    void cancel() { 付了款就抛异常 }
}

sealed interface Discount
        permits Discount.None, Discount.Minus,
                Discount.Percent, ThresholdDiscount {
    BigDecimal apply(BigDecimal sum);
    default String label() { … }
    record Minus(BigDecimal off) implements Discount {
        public BigDecimal apply(BigDecimal sum) {
            return sum.subtract(off).max(BigDecimal.ZERO);
        }
    }
    …
}

abstract non-sealed class ThresholdDiscount
        implements Discount {
    protected final BigDecimal threshold;
    protected ThresholdDiscount(BigDecimal t) {
        this.threshold = t;
        System.out.println("[构造中] " + describe()); // 坑
    }
    public final BigDecimal apply(BigDecimal sum) {
        return sum.compareTo(threshold) >= 0
             ? reduce(sum) : sum;
    }
    protected abstract BigDecimal reduce(BigDecimal sum);
}

$ cd java && mvn -q -pl javase/oop-generics-erasure compile
$ java -cp javase/oop-generics-erasure/target/classes \
    labs.javase.oop_generics_erasure.Demo
== 1. 封装 ==
  外面清空了原列表，订单里还有 1 行，合计 598.00
  order.lines().add(...) → UnsupportedOperationException
  付款后 cancel() → 9527 已付款，不能直接取消
== 2. 多态 ==
    [构造中] 满 300 减 null
  None          Discount$None      598.00
  Minus         Discount$Minus     578.00
  Percent       Discount$Percent   538.20
  FullReduction FullReduction      568.00
== 3. 擦除 ==
  a.getClass() == b.getClass()：true
  Order.lines 的签名：java.util.List<OrderLine>
  它的运行时类型：java.util.List
== 4. 原始类型绕过检查 ==
  addSomething 之后 size = 2，没有报错
  读到 机械键盘
  ClassCastException 在 main():66 抛出，不在 addSomething 里
  class java.lang.String cannot be cast to class OrderLine
== 5. 桥方法 ==
  你写的  public int Demo$ById.compare(Order,Order)
  bridge  public int Demo$ById.compare(Object,Object)

$ javap -c -p Demo.class | grep -E 'apply|checkcast'
  invokeinterface Discount.apply:(BigDecimal)BigDecimal
  invokeinterface Iterator.next:()Ljava/lang/Object;
  checkcast       OrderLine

输出略去了包名前缀。第 2 段那行“满 300 减 null”和第 4 段的 ClassCastException，⑥ 第一圈会解释。