Dart对几种类型的通用约束并设置默认值

class Box<T extends num> {
  T value;

  Box(this.value);
}

abstract class Printable {
  void print();
}

class Document<T extends Printable> {
  T content;

  Document(this.content);

  void display() {
    content.print();
  }
}

class MultiConstraint<T extends num & Comparable<T>> {
  T value;

  MultiConstraint(this.value);

  bool isGreaterThan(T other) {
    return value.compareTo(other) > 0;
  }
}

class DefaultBox<T extends num = int> {
  T value;

  DefaultBox(this.value);
}

abstract class Printable {
  void print();
}

class Text implements Printable {
  final String content;
  Text(this.content);

  @override
  void print() {
    print(content);
  }
}

class Document<T extends Printable = Text> {
  T content;

  Document(this.content);

  void display() {
    content.print();
  }
}

void main() {
  var doc = Document(); // 使用默认类型 Text
  doc.content = Text('Hello, World!');
  doc.display();

  var numBox = Box<int>(); // 指定 int 类型
  numBox.value = 42;
}