Java HashMap Tutorial

Welcome to the Java HashMap tutorial! 🎉 In this lesson, we'll dive into the world of collections and learn about the HashMap data structure. We'll cover the basics, real-world applications, and advanced examples to help you master this essential Java tool.

Let's kick things off! 🎯

What is a HashMap?

A HashMap is a data structure that stores key-value pairs. It's a popular choice for implementing a dictionary or associative array in Java. The HashMap allows you to store data in a more organized and efficient way compared to traditional arrays.

💡 Pro Tip: The HashMap is implemented using a hash table under the hood, which makes it faster to access values using their associated keys.

Creating a HashMap

To create a new HashMap, you can use the HashMap constructor:

java

import java.util.HashMap;

HashMap<String, Integer> myHashMap = new HashMap<>();

In this example, we've created a HashMap called myHashMap that stores String keys and Integer values.

Adding Key-Value Pairs

To add key-value pairs to a HashMap, you can use the put() method:

java

myHashMap.put("Apple", 100);
myHashMap.put("Banana", 50);

You can also add key-value pairs while creating the HashMap:

java

HashMap<String, Integer> myHashMap = new HashMap() {{
    put("Apple", 100);
    put("Banana", 50);
}};

Accessing Values

To access the value associated with a key, you can use the get() method:

java

int bananaQuantity = myHashMap.get("Banana");

Removing Key-Value Pairs

To remove a key-value pair, you can use the remove() method:

java

myHashMap.remove("Apple");

HashMap Methods

The HashMap class provides several useful methods for managing key-value pairs, such as:

containsKey(Object key): Check if the HashMap contains a specific key.
containsValue(Object value): Check if the HashMap contains a specific value.
isEmpty(): Check if the HashMap is empty.
size(): Get the number of key-value pairs in the HashMap.

HashMap Quiz

Let's move on to some practical examples! 📝

Example 1: Student Grades

Let's create a HashMap to store student grades:

java

HashMap<String, Integer> studentGrades = new HashMap<>();
studentGrades.put("John", 95);
studentGrades.put("Mike", 85);
studentGrades.put("Emma", 80);

Now, we can calculate the average grade by looping through the HashMap:

java

int totalGrades = 0;
int count = 0;

for (Integer grade : studentGrades.values()) {
    totalGrades += grade;
    count++;
}

double averageGrade = (double) totalGrades / count;

Example 2: Shopping Cart

Next, let's create a HashMap to simulate a shopping cart:

java

HashMap<String, Integer> shoppingCart = new HashMap<>();
shoppingCart.put("Apples", 5);
shoppingCart.put("Bananas", 3);
shoppingCart.put("Oranges", 2);

Now, let's calculate the total cost of the items:

java

int appleCost = 1.50;
int bananaCost = 0.50;
int orangeCost = 1.00;

int totalCost = 0;

for (Integer quantity : shoppingCart.values()) {
    String item = shoppingCart.keySet().stream().findFirst().get();

    if (item.equals("Apples")) {
        totalCost += appleCost * quantity;
    } else if (item.equals("Bananas")) {
        totalCost += bananaCost * quantity;
    } else if (item.equals("Oranges")) {
        totalCost += orangeCost * quantity;
    }
}

That's it for our comprehensive Java HashMap tutorial! You now have the knowledge to work with key-value pairs using the HashMap data structure. Practice using HashMap in various scenarios to strengthen your understanding and become a proficient Java developer.

Happy coding! 💻🥳