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  "contentMarkdown": "# Java `HashMap` 的结构与复杂度\n\n## 基本性质\n\n`java.util.HashMap` 基于哈希表实现 `Map` 接口。它允许一个 `null` 键和多个 `null` 值，不保证遍历顺序，也不保证顺序长期不变。若多个线程并发修改，必须在外部同步或改用合适的并发容器。\n\n在哈希分布合理时，`get` 与 `put` 的预期成本为常数时间。最坏情况取决于碰撞、容量以及具体 JDK 实现，不能无条件写成始终 `O(1)`。\n\n![](assets/Java-HashMap-bucket-structure.png)\n\n> [!warning] 实现细节\n> Java 8 及后续 OpenJDK 实现会在满足容量和碰撞阈值等条件时，把过长桶结构树化以改善严重碰撞下的性能；阈值和节点布局属于实现细节，不是使用 `Map` 接口时应依赖的行为契约。\n\n## 容量与负载因子\n\n- **capacity**：桶数组容量。\n- **load factor**：触发扩容的装载比例，默认值通常在时间与空间之间折中。\n- **rehash/resize**：扩容会重新组织桶，成本较高。\n\n能预估元素数量时，可设置足够的初始容量以减少扩容，但过大的容量会浪费空间，并使遍历成本增加，因为遍历与容量和元素数都有关。\n\n## 正确使用自定义键\n\n作为键的对象必须保持 `equals` 与 `hashCode` 契约一致：相等对象必须有相同哈希值。键参与比较的字段在放入映射后不应改变，否则对象可能仍在表中却无法按新状态查到。\n\n## 参考资料\n\n- [Oracle Java API：HashMap](https://docs.oracle.com/en/java/javase/25/docs/api/java.base/java/util/HashMap.html)\n"
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