Map collections store data in key-value pairs, where each entry consists of a unique key mapped to a corresponding value. The structure follows the pattern {key1=value1, key2=value2, ...}, with keys being unique while values can be duplicated. This design makes Map suitable for storing data with one-to-one relationships.

Map implementations derive their behvaior primarily from key handling, with values being secondary. The main implementations include:

HashMap: Provides unordered storage with no duplicate keys and no indexing.

Map<String, Integer> products = new HashMap<>();
products.put("Television", 1000);
products.put("Computer", 10000);
products.put("Television", 1010); // Overwrites previous entry
products.put("Watch", 10);
products.put(null, null);
System.out.println(products); // {null=null, Watch=10, Computer=10000, Television=1010}

LinkedHashMap: Maintains insertion order while preventing duplicate keys.

Map<String, Integer> products = new LinkedHashMap<>();
products.put("Television", 1000);
products.put("Computer", 10000);
products.put("Television", 1010);
products.put("Watch", 10);
products.put(null, null);
System.out.println(products); // {Television=1010, Computer=10000, Watch=10, null=null}

TreeMap: Orders elements by key in natural ascending order without duplicates.

Core Map Operations

All Map implementations share common functionality:

import java.util.HashMap;
import java.util.Map;

public class MapOperations {
    public static void main(String[] args) {
        Map<String, Integer> data = new HashMap<>();
        
        // Adding elements
        data.put("ItemA", 100);
        data.put("ItemB", 200);
        
        // 1. Retrieve collection size
        System.out.println(data.size());
        
        // 2. Clear all entries
        data.clear();
        
        // 3. Check if empty
        System.out.println(data.isEmpty());
        
        // 4. Get value by key
        data.put("ProductX", 50);
        System.out.println(data.get("ProductX"));
        
        // 5. Remove entry by key
        System.out.println(data.remove("ProductX"));
        
        // 6. Check key existence
        data.put("ProductY", 75);
        System.out.println(data.containsKey("ProductY"));
        
        // 7. Check value existence
        System.out.println(data.containsValue(75));
        
        // 8. Retrieve all keys
        System.out.println(data.keySet());
        
        // 9. Retrieve all values
        System.out.println(data.values());
        
        // 10. Merge another map
        Map<String, Integer> additionalData = new HashMap<>();
        data.putAll(additionalData);
    }
}