2.4 Maps Explained

Maps in Java are a fundamental part of the Java Collections Framework, providing a way to store and manipulate key-value pairs. Understanding maps is crucial for managing data where each element is associated with a unique key.

Key Concepts

1. Map Interface

The Map interface represents a collection of key-value pairs where each key is unique. It does not extend the Collection interface but is a part of the Java Collections Framework. Common methods include put, get, remove, and containsKey.

Example

        Map map = new HashMap<>();
        map.put("Apple", 1);
        map.put("Banana", 2);
        map.put("Cherry", 3);
        System.out.println(map.get("Banana")); // Output: 2
    

2. HashMap

A HashMap is a collection that uses a hash table for storage. It provides constant-time performance for basic operations like put, get, and remove. The order of elements is not guaranteed.

Example

        Map hashMap = new HashMap<>();
        hashMap.put("Dog", 1);
        hashMap.put("Cat", 2);
        hashMap.put("Bird", 3);
        System.out.println(hashMap); // Output: {Dog=1, Cat=2, Bird=3} (order may vary)
    

3. LinkedHashMap

A LinkedHashMap maintains the insertion order of elements. It provides constant-time performance for basic operations and maintains a doubly-linked list running through all of its entries.

Example

        Map linkedHashMap = new LinkedHashMap<>();
        linkedHashMap.put("Car", 1);
        linkedHashMap.put("Bike", 2);
        linkedHashMap.put("Truck", 3);
        System.out.println(linkedHashMap); // Output: {Car=1, Bike=2, Truck=3}
    

4. TreeMap

A TreeMap stores elements in a sorted order based on the keys. It uses a red-black tree structure and provides logarithmic-time performance for basic operations. The elements are sorted according to their natural ordering or by a specified comparator.

Example

        Map treeMap = new TreeMap<>();
        treeMap.put("Banana", 2);
        treeMap.put("Apple", 1);
        treeMap.put("Cherry", 3);
        System.out.println(treeMap); // Output: {Apple=1, Banana=2, Cherry=3}
    

Examples and Analogies

Think of a Map as a dictionary where each word (key) has a definition (value). A HashMap is like a dictionary where words are stored without any specific order, a LinkedHashMap is like a dictionary where words are stored in the order they were added, and a TreeMap is like a dictionary where words are stored in alphabetical order.

By mastering maps, you can efficiently manage and manipulate key-value pairs in your Java SE 11 applications.