1、泛型類

//泛型類
class Cache<T> {final Map<String, T> _cache = {};void saveData(String key, T value) {_cache[key] = value;}//泛型方法T? getData(String key) {return _cache[key];}
}

void main() {Cache<int> cache1 = Cache();const String name1 = "Leon";cache1.saveData(name1, 18);print("name: $name1   age: ${cache1.getData(name1)}"); //name: leon   age: 18Cache<String> cache2 = Cache();const String name2 = "Alice";cache2.saveData(name2, "woman");print("name: $name2  sex: ${cache2.getData(name2)}"); //name: Alice  sex: woman
}

2、泛型約束

class Person {String? name;int? age;Person(this.name, this.age);void display() {print("name: $name   age: $age");}
}

class Student extends Person {String? _school;Student(name, age, this._school) : super(name, age);@overridevoid display() {print("name: $name   age: $age  school: $_school");}
}

//泛型約束：T 只能是 Person 的子類
class Member<T extends Person> {T? _person;Member(this._person);void show() {if (_person != null) {_person!.display();}}
}

void main() {Member<Student> member = Member(Student("Leon", 18, "hafo"));member.show(); //name: Leon   age: 18  school: hafo
}

3、補充：Flutter 的一些編程技巧

（1）空安全

//安全調用
void safeUse() {List? list;print("list: ${list?.length}"); //list: null
}

（2）默認值

//默認值
void defaultUse() {bool? isOpen;//默認值String result = '';if (isOpen ?? false) {//isOpen == trueresult = '打開';} else {//isOpen == false || isOpen == nullresult = '關閉';}print("result: $result"); //關閉
}

（3）集合判空

//集合判空
void emptyUse() {List list = [];list.add(0);list.add('');list.add(null);list.add(true);for (int i = 0; i < list.length; i++) {if ([0, '', null].contains(list[i])) {print("index: $i   value: 空");} else {print("index: $i   vaule: ${list[i]}");}}// index: 0   value: 空// index: 1   value: 空// index: 2   value: 空// index: 3   vaule: true
}