Java Collections Framework Examples

Understanding the Java Collections Framework

The Java Collections Framework provides a set of classes and interfaces for storing and manipulating groups of data as a single unit. It offers a variety of data structures, including lists, sets, and maps, allowing developers to choose the most appropriate one for their specific needs. Below are three practical examples that illustrate the versatility and utility of the Java Collections Framework.

Example 1: Using ArrayList to Store a List of Students

Context

In this example, we’ll use an ArrayList to manage a dynamic list of students in a school application. The ArrayList allows for easy addition, removal, and access of student records.

import java.util.ArrayList;

public class StudentList {
    public static void main(String[] args) {
        ArrayList<String> students = new ArrayList<>();
        students.add("Alice");
        students.add("Bob");
        students.add("Charlie");

        System.out.println("Students list: " + students);

        // Remove a student
        students.remove("Bob");
        System.out.println("Updated students list: " + students);

        // Access a student
        String firstStudent = students.get(0);
        System.out.println("First student: " + firstStudent);
    }
}

Notes

• ArrayList is ideal for scenarios where frequent access and modification of elements are required.
• The initial size of the ArrayList can be specified for optimization.

Example 2: Using HashSet to Track Unique Items

Context

This example demonstrates how to use a HashSet to store unique items, such as user IDs, ensuring that there are no duplicates. This is particularly useful in applications where unique identification is critical.

import java.util.HashSet;

public class UniqueIDs {
    public static void main(String[] args) {
        HashSet<String> userIDs = new HashSet<>();
        userIDs.add("user1");
        userIDs.add("user2");
        userIDs.add("user3");
        userIDs.add("user1"); // Duplicate entry

        System.out.println("Unique user IDs: " + userIDs);

        // Check if a specific ID exists
        if (userIDs.contains("user2")) {
            System.out.println("User2 exists!");
        }
    }
}

Notes

• HashSet does not allow duplicate values and has a constant time complexity for basic operations such as add, remove, and contains.
• Consider using LinkedHashSet if you require the order of insertion.

Example 3: Using HashMap for Key-Value Pairs

Context

In this example, we utilize a HashMap to store a mapping of product names to their prices in an inventory system. This allows for quick lookups of product prices based on their names.

import java.util.HashMap;

public class ProductInventory {
    public static void main(String[] args) {
        HashMap<String, Double> inventory = new HashMap<>();
        inventory.put("Laptop", 999.99);
        inventory.put("Smartphone", 699.99);
        inventory.put("Tablet", 299.99);

        System.out.println("Product Inventory: " + inventory);

        // Retrieve the price of a specific product
        double laptopPrice = inventory.get("Laptop");
        System.out.println("Price of Laptop: $" + laptopPrice);

        // Remove a product
        inventory.remove("Tablet");
        System.out.println("Updated Inventory: " + inventory);
    }
}

Notes

• HashMap allows for efficient retrieval of values based on their keys, making it suitable for associative arrays.
• For ordered maps, consider using TreeMap or LinkedHashMap depending on the requirements.

These examples of Java Collections Framework examples demonstrate how different collection types can be effectively used to manage data in various applications.