Map Interface

The Map interface is part of the Java Collections Framework, but unlike List and Set, it does not extend the Collection interface. A Map stores data in key-value pairs, where each key is unique, and each key maps to exactly one value.

🧩 Key Features of Map:

• Stores key-value pairs

• Keys are unique, but values can be duplicated

• Allows one null key and multiple null values (in some implementations)

• Provides fast lookup using keys

🔧 Common Implementations

✅ Example 1: HashMap

import java.util.HashMap;

public class HashMapExample {
    public static void main(String[] args) {
        HashMap<String, Integer> map = new HashMap<>();

        map.put("Apple", 3);
        map.put("Banana", 5);
        map.put("Mango", 2);
        map.put("Banana", 6);  // Overwrites previous value

        System.out.println(map);
    }
}

{Apple=3, Banana=6, Mango=2}Note: Order is not guaranteed in HashMap.

✅ Example 2: TreeMap

import java.util.TreeMap;

public class TreeMapExample {
    public static void main(String[] args) {
        TreeMap<Integer, String> map = new TreeMap<>();

        map.put(3, "Banana");
        map.put(1, "Apple");
        map.put(2, "Mango");

        System.out.println(map);
    }
}

{1=Apple, 2=Mango, 3=Banana}

Keys are automatically sorted in ascending order.

✅ Example 3: LinkedHashMap

import java.util.LinkedHashMap;

public class LinkedHashMapExample {
    public static void main(String[] args) {
        LinkedHashMap<String, Integer> map = new LinkedHashMap<>();

        map.put("One", 1);
        map.put("Two", 2);
        map.put("Three", 3);

        System.out.println(map);
    }
}

{One=1, Two=2, Three=3}

Maintains insertion order.

🔧 Common Methods in Map

🧠 Use Cases of Map

• Storing student ID → name

• Creating dictionary or glossary

• Mapping URLs → page titles

• Implementing caching mechanisms

⚖️ HashMap vs TreeMap vs LinkedHashMap