HashMap處理碰撞主要通過鍊式存儲(chǔ)實(shí)現(xiàn)，當(dāng)多個(gè)鍵映射到相同索引時(shí)，它們會(huì)被存入該位置的鍊錶或樹中。 1. HashMap使用hashCode()方法計(jì)算鍵的哈希值，並通過內(nèi)部邏輯確定數(shù)組中的索引；2. 當(dāng)不同鍵產(chǎn)生相同索引時(shí)，採用鍊錶形式將衝突項(xiàng)鍊接起來；3. Java 8及以上版本，若鍊錶長度超過8，則自動(dòng)轉(zhuǎn)換為紅黑樹以提升性能；4. 當(dāng)元素?cái)?shù)量超過負(fù)載因子與容量乘積時(shí)，HashMap會(huì)擴(kuò)容一倍並重新分配所有條目，降低碰撞概率但帶來一定性能開銷。

HashMap in Java handles collisions using a method called chaining . When two or more keys hash to the same index, their entries are stored in a linked list (or a tree in newer versions of Java) at that index. This way, even if multiple keys collide, they can still be stored and retrieved correctly.

How Hashing Works in HashMap

Before diving into collision resolution, it's important to understand how hashing works in HashMap:

• Each key object has a hashCode() method that returns an integer.
• HashMap uses this hash code and applies its own internal logic (like rehashing) to determine the index in the internal array where the entry should go.
• Since different keys can produce the same index after this calculation, collisions are inevitable.

So, HashMap needs a reliable way to store and retrieve entries when collisions occur.

Chaining with Linked Lists and Trees

Java's HashMap uses chaining as its main strategy for handling collisions. Here's how it works:

• Each bucket in the HashMap's internal array holds a linked list of entries that map to the same index.
• When a new key-value pair hashes to an index that already has an entry, it gets added to the linked list at that index.
• During a get() operation, HashMap uses the key's equals() method to traverse the list and find the correct entry.

Starting from Java 8 , if a bucket becomes too large (default threshold is 8), the linked list is converted into a balanced tree (red-black tree) to improve performance. This helps reduce the time complexity from O(n) for a linked list to O(log n) for a tree in case of many collisions.

What Happens During Resizing?

When the number of entries in the HashMap exceeds the product of the load factor and the current capacity, the HashMap resizes itself:

• It doubles the size of the internal array.
• All existing entries are rehashed and moved to new appropriate buckets based on the updated array size.

This process helps maintain performance by reducing the likelihood of collisions and keeping the linked lists (or trees) short.

A few things to note:

• Resizing is an expensive operation because all entries must be rehashed.
• However, it doesn't happen very often due to the exponential growth pattern.
• Choosing an appropriate initial capacity can help minimize resizing operations.

Key Points to Remember

• HashMap uses chaining with linked lists (and trees for high-collision buckets).
• The equals() and hashCode() methods of keys play a critical role in ensuring correct storage and retrieval.
• Treeification happens when a bucket grows beyond a certain size.
• Resizing improves efficiency but comes with a performance cost.

基本上就這些。

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