Restricting the parameterized type

When implementing a generic type,you might want to limit the types of its parameters.You can do this using extends.

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 subclasses as 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>();