Java Map Interface

The Map interface of the Java collections framework provides functionality for key/value pair data structures. Keys are unique, and each key is associated with a single value. A map cannot contain duplicate keys.

Classes Implementing Map

• HashMap
• EnumMap
• LinkedHashMap
• WeakHashMap
• TreeMap

Interfaces Extending Map

• SortedMap
• NavigableMap
• ConcurrentMap

Creating a Map

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

// Creating a Map using HashMap
Map<String, Integer> numbers = new HashMap<>();

Common Map Methods

• put(K, V) - Insert key/value pair
• putAll() - Insert all entries from another map
• putIfAbsent(K, V) - Insert only if key is absent
• get(K) - Get value by key
• getOrDefault(K, defaultValue) - Get value or default
• containsKey(K) / containsValue(V)
• replace(K, V) / replace(K, oldV, newV)
• remove(K) / remove(K, V)
• keySet(), values(), entrySet()

Example 1: Using HashMap

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

class Main {
  public static void main(String[] args) {
    Map<String, Integer> numbers = new HashMap<>();
    numbers.put("One", 1);
    numbers.put("Two", 2);
    System.out.println("Map: " + numbers);

    System.out.println("Keys: " + numbers.keySet());
    System.out.println("Values: " + numbers.values());
    System.out.println("Entries: " + numbers.entrySet());

    int removedValue = numbers.remove("Two");
    System.out.println("Removed Value: " + removedValue);
  }
}

Output: {One=1, Two=2}

Example 2: Using TreeMap

import java.util.Map;
import java.util.TreeMap;

class Main {
  public static void main(String[] args) {
    Map<String, Integer> values = new TreeMap<>();
    values.put("Second", 2);
    values.put("First", 1);
    System.out.println("Map using TreeMap: " + values);

    values.replace("First", 11);
    values.replace("Second", 22);
    System.out.println("New Map: " + values);

    int removedValue = values.remove("First");
    System.out.println("Removed Value: " + removedValue);
  }
}

Output: {First=1, Second=2} → After replace: {First=11, Second=22} → Removed Value: 11