My Introduction to Collections - Data Structures

Today I dove into Dart's data structures (Collections). List, HashSet and HashMap. At first I thought "aren't these all the same thing" but each has its own specific use cases.

Live Demo: Dart Collections Examples

Try List, HashSet and HashMap usage interactively:

List - Classic Arrays

Lists are the data structure we know best. Similar to arrays in other languages. Each element has an index (starting from 0) and works in order.

My First List Experiments

void main() {
  List<int> numbers = [3, 4, 5];
  List<String> names = ["Ahmet", "Mehmet", "Selda"];
  List<double> ratios = [3.2, 3.3, 3.5];
  
  print(numbers);    // [3, 4, 5]
  print(names);      // [Ahmet, Mehmet, Selda]
  print(ratios);     // [3.2, 3.3, 3.5]
}

Adding and Removing Data

// Adding
numbers.add(32);
names.add("New name");
ratios.add(3.4);

// Insert at specific position
numbers.insert(0, 100); // Adds to beginning

// Update
numbers[0] = 50; // Change first element

// Remove
numbers.remove(50); // Remove by value
numbers.removeAt(0); // Remove by index

Accessing Elements

print(numbers[0]);        // First element
print(numbers.first);     // First element
print(numbers.last);      // Last element
print(numbers.length);    // Number of elements
print(numbers.isEmpty);   // Is it empty?
print(numbers.isNotEmpty); // Is it not empty?

Useful List Methods

List<int> numbers = [5, 2, 8, 1, 9];

numbers.sort();           // Sort: [1, 2, 5, 8, 9]
numbers.reversed;         // Reverse
numbers.contains(5);      // Does it contain 5? true
numbers.indexOf(8);       // Index of 8? 2
numbers.clear();          // Clear all

HashSet - Unique Values

HashSet is a structure that doesn't allow duplicate values. If you try to add the same value twice, it only keeps one.

Why HashSet?

Imagine you're collecting user emails. You don't want the same email twice. HashSet is perfect for this.

import 'dart:collection';

void main() {
  HashSet<String> emails = HashSet();
  
  emails.add("[email protected]");
  emails.add("[email protected]");
  emails.add("[email protected]"); // Won't be added, already exists
  
  print(emails); // {[email protected], [email protected]}
  print(emails.length); // 2
}

HashSet Features

HashSet<int> numbers = HashSet();

numbers.add(5);
numbers.add(3);
numbers.add(5); // Won't be added

print(numbers.contains(3)); // true
print(numbers.length);      // 2

numbers.remove(3);
print(numbers);             // {5}

List vs HashSet

Feature	List	HashSet
Duplicates	Allowed	Not allowed
Order	Maintains order	No guaranteed order
Access	By index	No index
Speed	Slower for search	Faster for search

HashMap - Key-Value Pairs

HashMap stores data as key-value pairs. Like a dictionary: word (key) and its meaning (value).

First HashMap Example

void main() {
  HashMap<String, int> ages = HashMap();
  
  ages["Ahmet"] = 25;
  ages["Mehmet"] = 30;
  ages["Selda"] = 28;
  
  print(ages["Ahmet"]); // 25
  print(ages);          // {Ahmet: 25, Mehmet: 30, Selda: 28}
}

Practical Example: Product Prices

HashMap<String, double> prices = HashMap();

prices["Bread"] = 5.50;
prices["Milk"] = 12.00;
prices["Cheese"] = 45.00;

print("Bread price: ${prices["Bread"]} TL");

// Update
prices["Bread"] = 6.00;

// Check if exists
if (prices.containsKey("Milk")) {
  print("Milk is available");
}

// Remove
prices.remove("Cheese");

HashMap Methods

HashMap<String, int> scores = HashMap();

scores["Math"] = 85;
scores["Physics"] = 90;
scores["Chemistry"] = 78;

print(scores.keys);        // Keys: (Math, Physics, Chemistry)
print(scores.values);      // Values: (85, 90, 78)
print(scores.length);      // 3
print(scores.isEmpty);     // false

scores.clear();            // Clear all

When to Use Which?

Use List:

When order matters
When you need to access by index
When duplicates are allowed
Shopping list, todo list, etc.

Use HashSet:

When you need unique values
When order doesn't matter
When you need fast search
Email list, unique IDs, tags, etc.

Use HashMap:

When you need key-value pairs
When you need to access by key
Settings, translations, user info, etc.

Real Flutter Example

Product List with List

List<String> products = ["Apple", "Banana", "Orange"];

ListView.builder(
  itemCount: products.length,
  itemBuilder: (context, index) {
    return ListTile(
      title: Text(products[index]),
    );
  },
)

User Settings with HashMap

HashMap<String, bool> settings = HashMap();
settings["notifications"] = true;
settings["darkMode"] = false;
settings["autoSave"] = true;

SwitchListTile(
  title: Text("Notifications"),
  value: settings["notifications"] ?? false,
  onChanged: (value) {
    settings["notifications"] = value;
  },
)

What Did I Learn Today?

List: Ordered, allows duplicates, access by index
HashSet: Unique values, no guaranteed order, fast search
HashMap: Key-value pairs, access by key

Tomorrow I'll look at try-catch and async-await topics. Error handling and asynchronous operations are very important in Flutter!

Learning Resources

Dart's official documentation
My own experimental code
StackOverflow examples

I hope this article was useful. Feel free to comment if you have questions!

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Dart Collections: Data Management with List, HashSet and HashMap