Java Map Interface

The Java Map Interface

A Map is an object that maps keys to values. A map cannot contain duplicate keys; each key can map to at most one value.

The Map interface is a member of the Java Collections Framework but it does not extend the Collection interface. It represents a different type of collection structure based on key-value pairs.

Key Characteristics of a Map

• Key-Value Pairs: The fundamental unit of a Map is an entry, which consists of a unique key and its corresponding value.
• Unique Keys: A Map cannot have duplicate keys. If you try to put a new value with a key that already exists, the old value associated with that key will be replaced.
• No Index-based Access: You do not access elements by a numerical index. Instead, you access a value by its key.
• Ordering: The Map interface itself does not guarantee any order. Some implementations, like HashMap, are unordered, while others, like TreeMap (sorted by key) and LinkedHashMap (insertion-ordered), do maintain an order.

Common Map Methods

Map Implementations

The three most common implementations are:

1. HashMap: The most widely used Map implementation. It is backed by a hash table. It offers the best performance (O(1) for get and put) but makes no guarantees about order.
2. TreeMap: This implementation stores keys in a sorted order. Its performance is slower (O(log n)) than HashMap.
3. LinkedHashMap: This implementation maintains the insertion order of keys. It provides performance nearly as good as HashMap.

Creating a Map

Here is an example of how to use a HashMap to store student IDs and names.

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

public class Main {
  public static void main(String[] args) {
    // Declare as a Map, instantiate as a HashMap
    Map<Integer, String> studentMap = new HashMap<>();
    // Add key-value pairs
    studentMap.put(101, "Alice");
    studentMap.put(102, "Bob");
    studentMap.put(103, "Charlie");
    System.out.println("Map of students: " + studentMap);
    // Get a value by its key
    String studentName = studentMap.get(102);
    System.out.println("Student with ID 102 is: " + studentName);
    // Remove an entry
    studentMap.remove(103);
    System.out.println("Map after removing student 103: " + studentMap);
    // Loop through the keys
    System.out.println("--- Looping through keys ---");
    for (Integer id : studentMap.keySet()) {
      System.out.println("ID: " + id + ", Name: " + studentMap.get(id));
    }
  }
}