Map Interface

A map stores key-value pairs.
Each key is unique, but values can be duplicated.

Map<String, Integer> map = new HashMap<>();
map.put("A", 1);
map.put("B", 2);

Common Implementation

Implementation	Key Features
HashMap	Fast, no ordering, allows 1 null key
LinkedHashMap	Maintains insertion order
TreeMap	Sorted by keys (natural or custom order)
Hashtable	Legacy, synchronized, no null keys
ConcurrentHashMap	Thread-safe, no locking entire map

Methods

map.put("X", 100);
map.get("X");            // 100
map.containsKey("X");    // true
map.containsValue(100);  // true
map.remove("X");
map.size();
map.clear();

Iterating over a Map

for (Map.Entry<String, Integer> entry : map.entrySet()) {
    System.out.println(entry.getKey() + " = " + entry.getValue());
}

Comparison

HashMap vs TreeMap vs LinkedHashMap

Feature	HashMap	LinkedHashMap	TreeMap
Order	✗ None	✅ Insertion order	✅ Sorted by key
Null Keys	✅ One null key	✅ One null key	✗
Performance	✅ Fastest	Slightly slower	Slowest
Thread-safe	✗	✗	✗

Key Notes

• Map is not a subtype of Collection.
• Use equals() and hashCode()for key comparison.
• For thread-safe use: prefer ConcurrentHashMap over Hashtable.

Tip

Be ready to:

• explain how keys are compared,
• compare HashMap vs TreeMap vs LinkedHashMap,
• write code using entrySet() loop,
• mention null-handling rules for keys & values.

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Parent: _Collections