Generics

If you look at the API documentation for the basic array type, List, you'll see that the type is actually List<E>. The <...> notation marks List as a generic (or parameterized) type—a type that has formal type parameters. By convention, most type variables have single-letter names, such as E, T, S, K, and V.

Generics are often required for type safety, but they have more benefits than just allowing your code to run:

• Properly specifying generic types results in better generated code.
• You can use generics to reduce code duplication.

If you intend for a list to contain only strings, you can declare it as List<String> (read that as "list of string"). That way you, your fellow programmers, and your tools can detect that assigning a non-string to the list is probably a mistake. Here's an example:

var names = <String>[];
names.addAll(['Seth', 'Kathy', 'Lars']);
names.add(42); // Error

Another reason for using generics is to reduce code duplication. Generics let you share a single interface and implementation between many types, while still taking advantage of static analysis. For example, say you create an interface for caching an object:

abstract class ObjectCache {
  Object getByKey(String key);
  void setByKey(String key, Object value);
}

You discover that you want a string-specific version of this interface, so you create another interface:

abstract class StringCache {
  String getByKey(String key);
  void setByKey(String key, String value);
}

Later, you decide you want a number-specific version of this interface... You get the idea.

Generic types can save you the trouble of creating all these interfaces. Instead, you can create a single interface that takes a type parameter:

abstract class Cache<T> {
  T getByKey(String key);
  void setByKey(String key, T value);
}

In this code, T is the stand-in type. It's a placeholder that you can think of as a type that a developer will define later.

List, set, and map literals can be parameterized. Parameterized literals are just like the literals you've already seen, except that you add <type> (for lists and sets) or <keyType, valueType> (for maps) before the opening bracket. Here is an example of using typed literals:

var names = <String>['Seth', 'Kathy', 'Lars'];
var uniqueNames = <String>{'Seth', 'Kathy', 'Lars'};
var pages = <String, String>{
  'index.html': 'Homepage',
  'robots.txt': 'Hints for web robots',
  'humans.txt': 'We are people, not machines'
};

To specify one or more types when using a constructor, put the types in angle brackets (<...>) just after the class name. For example:

var nameSet = Set<String>.from(names);

The following code creates a map that has integer keys and values of type View:

var views = Map<int, View>();

Dart generic types are reified, which means that they carry their type information around at runtime. For example, you can test the type of a collection:

var names = <String>[];
names.addAll(['Seth', 'Kathy', 'Lars']);
print(names is List<String>); // true

When implementing a generic type, you might want to limit the types that can be provided as arguments, so that the argument must be a subtype of a particular type. You can do this using extends.

A common use case is ensuring that a type is non-nullable by making it a subtype of Object (instead of the default, Object?).

class Foo<T extends Object> {
  // Any type provided to Foo for T must be non-nullable.
}

You can use extends with other types besides Object. Here's an example of extending SomeBaseClass, so that members of SomeBaseClass can be called on objects of type T:

class Foo<T extends SomeBaseClass> {
  // Implementation goes here...
  String toString() => "Instance of 'Foo<$T>'";
}

class Extender extends SomeBaseClass {...}

It's OK to use SomeBaseClass or any of its subtypes as the generic argument:

var someBaseClassFoo = Foo<SomeBaseClass>();
var extenderFoo = Foo<Extender>();

It's also OK to specify no generic argument:

var foo = Foo();
print(foo); // Instance of 'Foo<SomeBaseClass>'

Specifying any non-SomeBaseClass type results in an error:

var foo = Foo<Object>();

Methods and functions also allow type arguments:

T first<T>(List<T> ts) {
  // Do some initial work or error checking, then...
  T tmp = ts[0];
  // Do some additional checking or processing...
  return tmp;
}

Here the generic type parameter on first (<T>) allows you to use the type argument T in several places:

• In the function's return type (T).
• In the type of an argument (List<T>).
• In the type of a local variable (T tmp).