Day-91 Map in Java

#webdev #programming #java

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.
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

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)

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