2.4.1 HashMap Explained

The HashMap class in Java is a part of the Java Collections Framework and implements the Map interface. It stores data in key-value pairs, allowing efficient retrieval, insertion, and deletion of elements. Understanding HashMap is crucial for managing data in a key-value format efficiently.

Key Concepts

1. Key-Value Pairs

A HashMap stores elements in key-value pairs. Each key is unique within the map, and it maps to a specific value. This structure allows for quick lookups based on the key.

Example

        HashMap<String, Integer> hashMap = new HashMap<>();
        hashMap.put("Apple", 1);
        hashMap.put("Banana", 2);
        System.out.println(hashMap.get("Apple")); // Output: 1
    

2. Hashing Mechanism

The HashMap uses a hashing mechanism to store and retrieve elements. When a key is inserted, its hash code is computed, which determines the bucket where the key-value pair will be stored. This allows for average constant-time performance for basic operations.

Example

        HashMap<Integer, String> hashMap = new HashMap<>();
        hashMap.put(1, "One");
        hashMap.put(2, "Two");
        System.out.println(hashMap.get(2)); // Output: Two
    

3. Collision Handling

Collisions occur when two different keys produce the same hash code. The HashMap handles collisions using a technique called separate chaining, where each bucket contains a linked list of key-value pairs that hash to the same index.

Example

        HashMap<String, String> hashMap = new HashMap<>();
        hashMap.put("Key1", "Value1");
        hashMap.put("Key2", "Value2");
        // Assuming "Key1" and "Key2" have the same hash code
        System.out.println(hashMap.get("Key1")); // Output: Value1
    

4. Performance

The HashMap provides average constant-time performance for operations like put, get, and remove. The actual performance can vary based on the number of elements and the hash function's effectiveness.

Example

        HashMap<String, Integer> hashMap = new HashMap<>();
        hashMap.put("Apple", 1);
        hashMap.put("Banana", 2);
        hashMap.remove("Apple");
        System.out.println(hashMap.get("Apple")); // Output: null
    

Examples and Analogies

Think of a HashMap as a dictionary where each word (key) has a definition (value). When you look up a word, you quickly find its definition. The hashing mechanism is like an index that helps you find the word's definition quickly. Collisions are like two words having the same index, so they are stored in a linked list at that index.

By mastering the HashMap class, you can efficiently manage and manipulate key-value data in your Java SE 11 applications, making your code more performant and scalable.