In Java, the Map interface is one of the most powerful data structures that allows us to store data in key-value pairs. Unlike a List or Set, a Map does not allow duplicate keys, but values can be duplicated.
Key Points About Map
- A Map stores data in key-value pairs.
- Keys are unique(no duplicates allowed).
- Values can be duplicated.
- Each key maps to exactly one value.
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Commonly used implementations:
- HashMap
- LinkedHashMap
- TreeMap
Comparison of HashMap, LinkedHashMap, and TreeMap
| Feature | HashMap | LinkedHashMap | TreeMap |
|---|---|---|---|
| Order | No order | Insertion order | Sorted order (by keys) |
| Null Key | Allowed (only one) | Allowed (only one) | Not allowed |
| Null Values | Allowed | Allowed | Allowed |
| Speed | Fast O(1) | Fast O(1) | Slower O(log n) |
Commonly Used Map Methods
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put(key, value)→ Adds or updates a key-value pair -
get(key)→ Returns the value for a given key -
remove(key)→ Removes a key and its value -
containsKey(key)→ Checks if the map contains a specific key -
containsValue(value)→ Checks if a specific value exists -
keySet()→ Returns a set of all keys -
values()→ Returns a collection of all values -
entrySet()→ Returns a set of key-value pairs (Map.Entry objects)
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Example : MapDemo
package collectiondemo; import java.util.HashMap; import java.util.LinkedHashMap; import java.util.TreeMap; public class MapDemo { public static void main(String[] args) { // HashMap: Stores items in key-value pairs (No specific order) HashMap<Integer, String> h1 = new HashMap<>(); h1.put(10, "Tamil"); h1.put(20, "Selvan"); h1.put(30, "kts"); System.out.println("HashMap: " + h1); // LinkedHashMap: Maintains insertion order LinkedHashMap<Integer, String> h2 = new LinkedHashMap<>(); h2.put(2, "Selvan"); h2.put(1, "Tamil"); h2.put(3, "kts"); System.out.println("LinkedHashMap: " + h2); // TreeMap: Stores keys in sorted order TreeMap<Integer, String> h3 = new TreeMap<>(); h3.put(2, "Selvan"); h3.put(3, "kts"); h3.put(1, "Tamil"); System.out.println("TreeMap: " + h3); // Demonstrating methods System.out.println("Get key 1: " + h3.get(1)); h3.remove(1); System.out.println("After remove(1): " + h3); System.out.println("Contains key 1? " + h3.containsKey(1)); System.out.println("Contains value 'kts'? " + h3.containsValue("kts")); System.out.println("Keys: " + h3.keySet()); System.out.println("Values: " + h3.values()); System.out.println("Entries: " + h3.entrySet()); // Iterating through TreeMap for (Integer i : h3.keySet()) { System.out.println("Key: " + i + " Value: " + h3.get(i)); } } } Output
HashMap: {20=Selvan, 10=Tamil, 30=kts} LinkedHashMap: {2=Selvan, 1=Tamil, 3=kts} TreeMap: {1=Tamil, 2=Selvan, 3=kts} Get key 1: Tamil After remove(1): {2=Selvan, 3=kts} Contains key 1? false Contains value 'kts'? true Keys: [2, 3] Values: [Selvan, kts] Entries: [2=Selvan, 3=kts] Key: 2 Value: Selvan Key: 3 Value: kts Conclusion
- HashMap is best when you just need fast access without caring about order.
- LinkedHashMap is useful when you want to preserve the insertion order.
- TreeMap is helpful when you need keys in sorted order.
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