Generic Methods

Part of the Object Oriented Programming section of Coddy's Dart journey. Lesson 66 of 110.

Just as classes can be generic, individual methods can also have their own type parameters. This is useful when you need a flexible method without making the entire class generic.

A generic method declares its type parameter before the return type:

T findFirst<T>(List<T> items) {
  return items.first;
}

void main() {
  var firstNumber = findFirst<int>([1, 2, 3]);
  var firstWord = findFirst<String>(['hello', 'world']);
  
  print(firstNumber);  // 1
  print(firstWord);    // hello
}

Dart can often infer the type from the arguments, so you don't always need to specify it explicitly:

var result = findFirst([10, 20, 30]);  // Dart infers int

Generic methods are particularly useful inside classes when only certain operations need type flexibility:

class Printer {
  void printItem<T>(T item) {
    print('Printing: $item');
  }
  
  List<T> duplicate<T>(T item, int count) {
    return List.filled(count, item);
  }
}

void main() {
  var printer = Printer();
  printer.printItem<String>('Hello');
  printer.printItem(42);  // Type inferred as int
  
  var copies = printer.duplicate('Hi', 3);
  print(copies);  // [Hi, Hi, Hi]
}

Generic methods give you fine-grained control over type safety at the method level, keeping your classes simpler while still benefiting from compile-time type checking.

Challenge

Easy

Let's build a data transformation utility that showcases the power of generic methods. You'll create a Transformer class with methods that can work with any type, giving you flexibility without making the entire class generic.

You'll organize your code into two files:

transformer.dart: Create a Transformer class with these generic methods:
- T getFirst<T>(List<T> items) - returns the first element from a list
- T getLast<T>(List<T> items) - returns the last element from a list
- List<T> repeat<T>(T item, int times) - creates a list containing the item repeated the specified number of times
- void printPair<A, B>(A first, B second) - prints two values in the format Pair: [first] and [second]
main.dart: Import your transformer file and demonstrate each generic method with different types:
- Create a Transformer instance
- Use getFirst on [10, 20, 30] and print the result
- Use getFirst on ['apple', 'banana', 'cherry'] and print the result
- Use getLast on [1.5, 2.5, 3.5] and print the result
- Use repeat to create a list of 'Hi' repeated 4 times and print the result
- Use repeat to create a list of 7 repeated 3 times and print the result
- Use printPair with 'Name' and 42
- Use printPair with 100 and true

Notice how each method declares its own type parameter(s), allowing the same Transformer object to work with integers, strings, doubles, and booleans - all with full type safety. The printPair method demonstrates using two type parameters in a single method.

Expected output:

10
apple
3.5
[Hi, Hi, Hi, Hi]
[7, 7, 7]
Pair: Name and 42
Pair: 100 and true

Try it yourself

import 'transformer.dart';

void main() {
  // Create a Transformer instance
  var transformer = Transformer();

  // TODO: Use getFirst on [10, 20, 30] and print the result

  // TODO: Use getFirst on ['apple', 'banana', 'cherry'] and print the result

  // TODO: Use getLast on [1.5, 2.5, 3.5] and print the result

  // TODO: Use repeat to create a list of 'Hi' repeated 4 times and print it

  // TODO: Use repeat to create a list of 7 repeated 3 times and print it

  // TODO: Use printPair with 'Name' and 42

  // TODO: Use printPair with 100 and true
}