An interface is a contract that a class must follow. Prefixing a method or variable with _ makes it private, so it won’t be visible when imported from a different file.
interfaces.dart
class Elephant {
// Public interface
final String name;
// In the interface, but visible only in this library. (private)
final int _id = 23;
// Not in the interface, since this is a constructor.
Elephant(this.name);
// Public method.
sayHi() => 'My name is $name.';
// Private method.
_saySecret() => 'My ID is $_id.';
}