Hi, and welcome! Today, we'll explore HashMaps, a data structure that organizes data as key-value pairs, much like a treasure box with unique labels for each compartment.

Imagine dozens of toys in a box. If each toy had a unique label (the key), you could directly select a toy (the value) using the label. No rummaging required — that's the power of HashMaps! Today, we'll understand HashMaps and learn how to implement them in Java.

HashMaps are special types of data structures that utilize unique keys instead of indexes. When you know the key (toy's label), you can directly pick up the value (toy). That's how a HashMap works!

Consider an oversized library of books. With HashMaps (which act like the library catalog), you'll quickly locate any book using a unique number (key)!

Java implements HashMaps through the HashMap class in the java.util package. They hold data in key-value pairs.

Let's create a HashMap, functioning as a catalog for a library:

Java
Copy to clipboard
1import java.util.HashMap;
2import java.util.Map;
3
4class Solution {
5    public static void main(String[] args) {
6        // Creating a catalog for the library using HashMap with initialization
7        Map<String, String> libraryCatalog = new HashMap<>(Map.of(
8            "book1", "A Tale of Two Cities",
9            "book2", "To Kill a Mockingbird",
10            "book3", "1984"
11        ));
12    }
13}

In this HashMap, book1, book2, and book3 are keys, while the book titles serve as their respective values.

It's important to remember that the keys should be of a type that supports hashing and equality comparison. Examples include String, Short, Integer, Long, Float, Double, Character, and Boolean. The values can be of any type.

HashMap allows you to access, update, or remove elements:

1. Accessing Elements: You can retrieve a book's title using its key straightforwardly: libraryCatalog.get("book1") would return "A Tale of Two Cities." But what happens if you try to access a key that isn't present in the HashMap? This would return null.

   Java
   Copy to clipboard
   1import java.util.HashMap;
   2import java.util.Map;
   3
   4class Solution {
   5    public static void main(String[] args) {
   6        // Creating a catalog for the library using HashMap with initialization
   7        Map<String, String> libraryCatalog = new HashMap<>(Map.of(
   8            "book1", "A Tale of Two Cities",
   9            "book2", "To Kill a Mockingbird",
   10            "book3", "1984"
   11        ));
   12
   13        // Accessing a book's title
   14        String title1 = libraryCatalog.get("book1");
   15        if (title1 != null)
   16            System.out.println(title1); // Output: "A Tale of Two Cities"
   17        else
   18            System.out.println("Key not found");
   19
   20        // Accessing a nonexistent key
   21        String titleNonexistent = libraryCatalog.get("book100");
   22        if (titleNonexistent != null)
   23            System.out.println(titleNonexistent);
   24        else
   25            System.out.println("Key not found"); // Output: "Key not found"
   26    }
   27}

2. Adding or Updating Elements: Whether you're adding a new book to the catalog or updating an existing book's title, you'll use the put() method.

   If the specified key exists in the HashMap, the assigned value replaces the existing one. For updating a title: libraryCatalog.put("book1", "The Tell-Tale Heart").

   If the key doesn't exist in the HashMap yet, the operation creates a new key-value pair. For adding a new book: libraryCatalog.put("book4", "Pride and Prejudice").

   Java
   Copy to clipboard
   1import java.util.HashMap;
   2import java.util.Map;
   3
   4class Solution {
   5    public static void main(String[] args) {
   6        // Creating a catalog for the library using HashMap with initialization
   7        Map<String, String> libraryCatalog = new HashMap<>(Map.of(
   8            "book1", "A Tale of Two Cities",
   9            "book2", "To Kill a Mockingbird",
   10            "book3", "1984"
   11        ));
   12
   13        // Updating an existing book's title
   14        libraryCatalog.put("book1", "The Tell-Tale Heart");
   15        System.out.println("Updated book1: " + libraryCatalog.get("book1")); // Output: "Updated book1: The Tell-Tale Heart"
   16
   17        // Adding a new book to the catalog
   18        libraryCatalog.put("book4", "Pride and Prejudice");
   19        System.out.println("Added book4: " + libraryCatalog.get("book4")); // Output: "Added book4: Pride and Prejudice"
   20    }
   21}

3. Removing Elements: If book1 no longer exists, you can remove it using libraryCatalog.remove("book1").

   Java
   Copy to clipboard
   1import java.util.HashMap;
   2import java.util.Map;
   3
   4class Solution {
   5    public static void main(String[] args) {
   6        // Creating a catalog for the library using HashMap with initialization
   7        Map<String, String> libraryCatalog = new HashMap<>(Map.of(
   8            "book1", "A Tale of Two Cities",
   9            "book2", "To Kill a Mockingbird",
   10            "book3", "1984"
   11        ));
   12
   13        // Removing an existing book from the catalog
   14        libraryCatalog.remove("book1");
   15        System.out.println("Removed book1: " + libraryCatalog.get("book1")); // Output: "Removed book1: null"
   16    }
   17}

Java HashMap offers several useful methods to interact with and manage your data:

Iterating over the HashMap: Loop through your HashMap to access each key-value pair in turn. One common way is to use the entrySet() method, which returns a set of key-value pairs.

Java
Copy to clipboard
1import java.util.HashMap;
2import java.util.Map;
3
4class Solution {
5    public static void main(String[] args) {
6        HashMap<String, String> libraryCatalog = new HashMap<>();
7        libraryCatalog.put("book1", "A Tale of Two Cities");
8        libraryCatalog.put("book2", "To Kill a Mockingbird");
9        libraryCatalog.put("book3", "1984");
10
11        // Looping over the HashMap
12        for (Map.Entry<String, String> entry : libraryCatalog.entrySet()) {
13            System.out.println(entry.getKey() + " : " + entry.getValue());
14        }
15    }
16}

When run, this code may output:

Plain text
Copy to clipboard
1book1 : A Tale of Two Cities
2book2 : To Kill a Mockingbird
3book3 : 1984

Please note that the order of the output might differ because HashMap does not maintain any order of keys. It could be in any sequence, such as:

Plain text
Copy to clipboard
1book2 : To Kill a Mockingbird
2book3 : 1984
3book1 : A Tale of Two Cities

This unordered nature is a characteristic of the HashMap class in Java.

HashMaps are popular because they save time! Operations like adding, updating, and locating elements take average constant time, O(1), which means they require nearly the same amount of time regardless of the library size.

However, to fully understand this, let's delve into the concept of hash functions and the difference between average-case and worst-case scenarios.

Hash Functions: A hash function takes an input (or "key") and returns a fixed-size string of bytes. The output is typically a numerical value (hashcode) that represents the original string. The idea is to distribute keys evenly across the array to minimize hash collisions, where two different keys produce the same hashcode.

For example, hash("book1") might return 123 and hash("book2") might return 456.

Average-Case Scenario: In the average-case scenario, the hash function distributes keys uniformly across the HashMap. Thanks to this uniform distribution, the operations of adding, updating, or retrieving keys have a time complexity of O(1) because they involve a simple arithmetic operation and direct addressing.

Worst-Case Scenario: In the worst-case scenario, a poor hash function could cause too many collisions, making the HashMap resemble a linked list. When this happens, operations degrade to O(n) time complexity because they involve traversing a list of n elements. Java's HashMap handles collisions internally using a mechanism called separate chaining or, in some cases, tree-based structure to mitigate performance degradation.

Understanding the balance between these scenarios helps in selecting or designing an effective hash function to maintain the desired performance of O(1) in average cases.

By leveraging well-designed hash functions and recognizing potential pitfalls, you can maximize the efficiency and apply HashMaps effectively in your Java programs.

Well done! You've mastered HashMaps, understood Java's implementation of HashMaps through HashMap, learned their operations, and grasped the concept of time complexity. Now, gear up for some practice exercises to reinforce your learning. Happy coding!