Java Map Interface

Key-value pair storage for efficient data mapping

🗺️ What is Map?

Map is a Java interface that stores data in key-value pairs. Each key is unique and maps to exactly one value, making it perfect for lookups and associations. 


// Simple Map example
Map<String, Integer> ages = new HashMap<>();
ages.put("Alice", 25);
ages.put("Bob", 30);
System.out.println(ages.get("Alice")); // 25

Map Key Features

🔑

Key-Value Pairs

Store data as key-value associations 

Map<String, String> capitals = new HashMap<>();
capitals.put("USA", "Washington");
capitals.put("France", "Paris");
capitals.put("Japan", "Tokyo");
🎯

Unique Keys

Each key can appear only once 

Map<String, Integer> scores = new HashMap<>();
scores.put("Alice", 85);
scores.put("Alice", 90); // Overwrites previous
// Alice now has score 90
⚡

Fast Lookup

Quick access to values by key 

Map<String, String> phonebook = new HashMap<>();
phonebook.put("John", "123-456-7890");
String phone = phonebook.get("John");
// Instant lookup by key
📊

Multiple Types

HashMap, TreeMap, LinkedHashMap 

Map<String, Integer> hashMap = new HashMap<>();
Map<String, Integer> treeMap = new TreeMap<>();
Map<String, Integer> linkedMap = new LinkedHashMap<>();

🔹 Basic Map Operations

Essential operations for working with Maps: 

import java.util.*;

public class MapExample {
    public static void main(String[] args) {
        // Create a Map
        Map<String, Integer> studentGrades = new HashMap<>();
        
        // Add key-value pairs
        studentGrades.put("Alice", 85);
        studentGrades.put("Bob", 92);
        studentGrades.put("Charlie", 78);
        
        // Get value by key
        int aliceGrade = studentGrades.get("Alice");
        System.out.println("Alice's grade: " + aliceGrade);
        
        // Check if key exists
        if (studentGrades.containsKey("Bob")) {
            System.out.println("Bob is in the class");
        }
        
        // Update value
        studentGrades.put("Alice", 88); // Updates Alice's grade
        
        // Remove entry
        studentGrades.remove("Charlie");
        
        // Display all entries
        System.out.println("Final grades: " + studentGrades);
    }
}

Output:

Alice's grade: 85

Bob is in the class

Final grades: {Alice=88, Bob=92}

🔹 Iterating Through Maps

Different ways to loop through Map entries: 

Map<String, Integer> inventory = new HashMap<>();
inventory.put("Apples", 50);
inventory.put("Bananas", 30);
inventory.put("Oranges", 25);

// Method 1: Using entrySet()
System.out.println("Method 1 - Entry Set:");
for (Map.Entry<String, Integer> entry : inventory.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

// Method 2: Using keySet()
System.out.println("\nMethod 2 - Key Set:");
for (String fruit : inventory.keySet()) {
    System.out.println(fruit + ": " + inventory.get(fruit));
}

// Method 3: Using values()
System.out.println("\nMethod 3 - Values only:");
for (Integer quantity : inventory.values()) {
    System.out.println("Quantity: " + quantity);
}

// Method 4: Using forEach (Java 8+)
System.out.println("\nMethod 4 - forEach:");
inventory.forEach((fruit, quantity) -> 
    System.out.println(fruit + " -> " + quantity));

Output:

Method 1 - Entry Set:

Apples: 50

Bananas: 30

Oranges: 25


Method 2 - Key Set:

Apples: 50

Bananas: 30

Oranges: 25

🔹 Common Map Methods

Essential methods for Map manipulation: 

Map<String, String> countries = new HashMap<>();
countries.put("US", "United States");
countries.put("UK", "United Kingdom");
countries.put("FR", "France");

// Size and emptiness
System.out.println("Size: " + countries.size()); // 3
System.out.println("Is empty: " + countries.isEmpty()); // false

// Checking existence
System.out.println("Contains key 'US': " + countries.containsKey("US")); // true
System.out.println("Contains value 'France': " + countries.containsValue("France")); // true

// Safe retrieval with default
String germany = countries.getOrDefault("DE", "Not found");
System.out.println("Germany: " + germany); // Not found

// Replace operations
countries.replace("UK", "Great Britain");
countries.replaceAll((key, value) -> value.toUpperCase());

// Merge operation
countries.merge("DE", "Germany", (oldVal, newVal) -> newVal);

System.out.println("Final map: " + countries);

Output:

Size: 3

Is empty: false

Contains key 'US': true

Contains value 'France': true

Germany: Not found

Final map: {US=UNITED STATES, UK=GREAT BRITAIN, FR=FRANCE, DE=Germany}

🧠 Test Your Knowledge

What happens when you put a key that already exists in a Map?