# Dart Extensions Part 2: Extensions for Collections

While Part 1 is an introduction to Dart Extensions. This article focuses on improving the quality of the extensions.

### **Tutorial 4: Extensions for Collections**

In here we are going to cover the following topics:

* Working with collections (List, Map, Set)
    
* Adding utility methods for collections
    
* Examples of useful collection extensions
    

**Example: Working with Collections**

Let's focus on the `Map` collection. We will create an extension to add a method `invert` that swaps the keys and values of the map.

```dart
extension MapExtensions<K, V> on Map<K, V> {
  Map<V, K> invert() {
    return this.map((key, value) => MapEntry(value, key));
  }
}

void main() {
  Map<String, int> original = {'a': 1, 'b': 2, 'c': 3};

  Map<int, String> inverted = original.invert();

  print(inverted);
}
```

We did the following things here:

* The `MapExtensions` extension adds an `invert` method to the `Map` class.
    
* The `invert` method uses the `map` function to create a new map with the keys and values swapped.
    

You can find the source code here: [Tutorial 3 Collection Extensions](https://github.com/khkred/dart-concepts-code/commit/5eef80893c39660e3663235a08fa539b79783ac1)

### Assignment 4:

Write an extension on the `Set` class to add a method `unionAll` that takes a list of sets and returns the union of all sets including the original one.

Solution: [Assignment 4 Set Extensions Solution](https://github.com/khkred/dart-concepts-code/commit/b09e2699fcd04e17f3266a30f18229ab0bfb5b11)

### Tutorial 5: Advanced Extensions and Best Practices

In here we are going to cover following topics:

* Advanced use cases of extensions
    
* Combining extensions with other Dart features
    
* Best practices and performance considerations
    

**Advanced Use Cases of Extensions**

In this tutorial, we'll explore some advanced use cases of extensions and how to combine them with other Dart features for more powerful and flexible code.

**Example: Extending DateTime**

Let's create an extension on the `DateTime` class to add two methods

* `isWeekend` that checks if a date falls on a weekend.
    
* `daysUntil` that returns the number of days until another date.
    

```dart
extension DatetimeExtensions on DateTime {
  bool isWeekend() {
    return this.weekday == DateTime.saturday || this.weekday == DateTime.sunday;
  }

  int daysUntil(DateTime other) {
    return other.difference(this).inDays;
  }
}

void main() {
  DateTime today = DateTime.now();
  DateTime futureDate = today.add(Duration(days: 10));

  print('Is today a Weekend? ${today.isWeekend()}');

  print('Days until $futureDate: ${today.daysUntil(futureDate)}');
}
```

The `DateTimeExtensions` extension adds two methods to the `DateTime` class:

* `isWeekend`: Checks if the `DateTime` instance falls on a Saturday or Sunday.
    
* `daysUntil`: Calculates the number of days between the `DateTime` instance and another date.
    

Source Code: [Tutorial 5 Advanced Extensions](https://github.com/khkred/dart-concepts-code/commit/9dba6b54e53f141c475c3f3132dba2fbdfe2e1c7)

### **Best Practices**

1. **Name Extensions Clearly**: Use clear and descriptive names for your extensions to avoid conflicts and improve readability.
    
2. **Keep Extensions Focused**: Extensions should add related functionality and not become a dumping ground for unrelated methods.
    
3. **Avoid Overusing Extensions**: Use extensions judiciously. Overusing them can lead to confusing and hard-to-maintain code.
    
4. **Document Extensions**: Provide comments and documentation for your extensions, especially if they add complex functionality.
    

### **Assignment 5:**

Create an extension on the `List` class to add a method `average` that calculates the average of a list of numbers. Also, add a method `median` to find the median value of the list.

Solution: [Assignment 5 Solution](https://github.com/khkred/dart-concepts-code/commit/a0f317ba16498caf73a63f3b08c2970b6ebc13f5)

This concludes our series on Dart extensions. Happy coding!