What does "< >" mean in Dart?

Question

Through an online Dart course, I've found some values bracketed with "less than" and "greater than" marks such as"List< E >" .

eg

List<int> fixedLengthList = new List(5);

I couldn't find a direct answer online, probably because that question was too basic. Could someone explain what those marks exactly indicate? Or any links if possible.

Answer 1

This are generic type parameters. It allows specializations of classes.

List is a list that can contain any value (if no type parameter is passed dynamic is used by default). List<int> is a list that only allows integer values and null`.

You can add such Type parameters to your custom classes as well.
Usually single upper-case letters are used for type parameter names like T , U , K but they can be other names like TKey ...

class MyClass<T> {
  T value;
  MyClass(this.value);
}

main() {
  var mcInt = MyClass<int>(5);
  var mcString = MyClass<String>('foo');
  var mcStringError = MyClass<String>(5); // causes error because `5` is an invalid value when `T` is `String`
}

See alsohttps://www.dartlang.org/guides/language/language-tour#generics

Answer 2

例如，如果你打算让一个列表只包含字符串，你可以将它声明为List<String> （读作“字符串列表”）