Map Interface

Map is NOT a child of Collection Interface. It stores data in Key-Value pairs. Key Features:

1. Key: Must be Unique. If duplicated, old value is replaced.
2. Value: Can be duplicated.
3. Association: Each key maps to one value.

Implementation Classes

1. HashMap
2. LinkedHashMap
3. TreeMap
4. Hashtable (Legacy)

1. HashMap

• Order: Random. • Synchronization: Not synchronized. • Null: 1 null key allowed, multiple null values. • Working: Uses Hashing (Bucket, HashCode, Equals).

2. LinkedHashMap

• Order: Insertion Order preserved. • Performance: Slightly slower due to linking overhead.

3. TreeMap

• Order: Sorted Order based on Keys. • Null: No null keys allows.

4. Hashtable

• Legacy: Since JDK 1.0. • Synchronization: Thread-safe (All methods are synchronized). • Null: No null key or value allowed. • Performance: Slow.

Iterating a Map

Map is not Iterable directly. We convert it to Set/Collection:

1. keySet(): Returns Set of keys.
2. values(): Returns Collection of values.
3. entrySet(): Returns Set of Entry (Key-Value pairs).