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:

varnames=<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:

abstractclassObjectCache{
ObjectgetByKey(Stringkey);
voidsetByKey(Stringkey,Objectvalue);
}

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

abstractclassStringCache{
StringgetByKey(Stringkey);
voidsetByKey(Stringkey,Stringvalue);
}

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:

abstractclassCache<T>{
TgetByKey(Stringkey);
voidsetByKey(Stringkey,Tvalue);
}

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:

varnames=<String>['Seth','Kathy','Lars'];
varuniqueNames=<String>{'Seth','Kathy','Lars'};
varpages=<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:

varnameSet=Set<String>.of(names);

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

varviews=SplayTreeMap<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:

varnames=<String>[];
names.addAll(['Seth','Kathy','Lars']);
print(namesisList<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. This restriction is called a bound. 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?).

classFoo<TextendsObject>{
// 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:

classFoo<TextendsSomeBaseClass>{
// Implementation goes here...
StringtoString()=>"Instance of 'Foo<$T>'";
}
​
classExtenderextendsSomeBaseClass{
...
}

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

varsomeBaseClassFoo=Foo<SomeBaseClass>();
varextenderFoo=Foo<Extender>();

It's also OK to specify no generic argument:

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

Specifying any non-SomeBaseClass type results in an error:

varfoo=Foo<Object>();

When using bounds to restrict parameter types, you can refer the bound back to the type parameter itself. This creates a self-referential constraint, or F-bound. For example:

abstractinterfaceclassComparable<T>{
intcompareTo(To);
}
​
intcompareAndOffset<TextendsComparable<T>>(Tt1,Tt2)=>
t1.compareTo(t2)+1;
​
classAimplementsComparable<A>{
@override
intcompareTo(Aother)=>/*...implementation...*/0;
}
​
intuseIt=compareAndOffset(A(),A());

The F-bound T extends Comparable<T> means T must be comparable to itself. So, A can only be compared to other instances of the same type.

Methods and functions also allow type arguments:

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

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).