>Digital-flame

Yes, a HashMap can degenerate into a linked list even if the keys have different hashCode() values, under certain conditions. Let me explain how and why.

🔍 How HashMap organizes entries:

1. Each key’s hashCode() is used to determine its bucket index via this internal hash function:

Even if two keys have different hashCode() values, their hashed indices may still collide if:

• They map to the same bucket index due to modulo with the table size.
• The internal hash function doesn’t spread them out well enough.

int tableSize = 16;
int hash1 = 0b00010000; // 16
int hash2 = 0b00110000; // 48
// Both (hash & (tableSize - 1)) = 0

➡️ Both keys, despite different hashCode()s, can end up in the same bucket.

✅ Estimated Time Complexity of HashMap operations

🧠 Why the difference?

• In a perfect hash scenario, keys are evenly distributed across buckets → each bucket has few elements → O(1) time.
• In the worst case, all keys hash to the same bucket:
  • Before Java 8, collisions were handled via linked lists → O(n) traversal in a bucket.
  • Since Java 8, if a bucket becomes too dense (≥ 8 entries), it is converted into a balanced tree (a red-black tree), so lookup time becomes O(log n).

In Java, the initial number of buckets in a HashMap depends on the constructor used when creating the map.

By default:

Map<K, V> map = new HashMap<>();

👉 The initial capacity is 16 buckets.

Key details:

• Initial Capacity: 16 (this means 16 buckets internally).
• Load Factor: 0.75 (this means the map resizes when 75% of the capacity is filled, i.e., 12 entries in this case).
• Buckets: These are actually positions in an internal array (Node[] table) where hash collisions are resolved using linked lists or balanced trees (since Java 8).

You can also specify a custom capacity:

Map<K, V> map = new HashMap<>(32); // 32 buckets initially

🎯 TL;DR:

✅ HashMap can store multiple keys with the same hashCode() as long as equals() returns false — because it uses both hashCode() and equals() to handle collisions properly.

🧠 Hash Table Implementations (per Knuth & CLRS)

1. Chaining

Each bucket contains a list (or tree) of entries. Collisions are handled by appending to that list.

2. Open Addressing

All entries live in the array itself. On collision, we probe (linear, quadratic, or double hashing) to find the next free slot.

🔧 How is a HashMap constructed?

🔹 At a high level, a HashMap is:

A hash table implementation that maps keys to values using a combination of array + linked list + (sometimes) red-black trees.

✅ Yes — SortedMap is an interface in Java.

It is part of the Java Collections Framework, and it defines the contract for a map that maintains sorted order of keys.

public interface SortedMap<K, V> extends Map<K, V> {
    Comparator<? super K> comparator();
    SortedMap<K, V> subMap(K fromKey, K toKey);
    SortedMap<K, V> headMap(K toKey);
    SortedMap<K, V> tailMap(K fromKey);
    K firstKey();
    K lastKey();
}

🔍 What is LinkedHashMap?

LinkedHashMap is a hash table + doubly linked list implementation of the Map interface.
It maintains the insertion order (or access order) of entries, unlike regular HashMap.

🧩 What is a WeakHashMap?

A WeakHashMap is a hash table-based map where the keys are stored as weak references.

If a key is no longer referenced elsewhere in your application, then the entry will be automatically removed by the garbage collector — poof 💨

It’s like a self-cleaning map that helps you avoid memory leaks when you’re associating data with objects you don’t “own”.

🔍 Quick Refresher