深入理解《红宝书》第8章：类及其应用

2023-12-06 12:12:06

1. 类的基本概念

类（Class）是ECMAScript中的一种新语法，它是对原型和构造函数概念的封装。类提供了一种简便的方法来创建对象实例，并对对象进行封装、继承和多态等操作。

2. 类实例化

我们可以通过使用new来实例化类，这将创建一个新的对象，该对象具有类的所有属性和方法。

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }
}

const person = new Person('John', 30);

3. 类属性和方法

类属性和方法可以通过在类定义中使用关键字this来定义。类属性是与类关联的数据，而类方法是与类关联的函数。

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  speak() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
  }
}

const person = new Person('John', 30);
person.speak(); // 输出: "Hello, my name is John and I am 30 years old."

4. 类继承

类继承是允许一个类从另一个类继承属性和方法。子类可以访问父类中的所有属性和方法，并可以重写父类中的方法。

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  speak() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
  }
}

class Student extends Person {
  constructor(name, age, school) {
    super(name, age); // 调用父类构造函数
    this.school = school;
  }

  study() {
    console.log('I am studying.');
  }
}

const student = new Student('John', 30, 'Harvard University');
student.speak(); // 输出: "Hello, my name is John and I am 30 years old."
student.study(); // 输出: "I am studying."

5. 类多态

类多态是允许子类中的方法与父类中的方法具有不同的实现。这使得子类可以根据自己的需要定制方法的行为。

class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  speak() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old.`);
  }
}

class Student extends Person {
  constructor(name, age, school) {
    super(name, age); // 调用父类构造函数
    this.school = school;
  }

  speak() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old. I am a student at ${this.school}.`);
  }
}

const person = new Person('John', 30);
const student = new Student('John', 30, 'Harvard University');

person.speak(); // 输出: "Hello, my name is John and I am 30 years old."
student.speak(); // 输出: "Hello, my name is John and I am 30 years old. I am a student at Harvard University."

6. 类封装

类封装是将数据和操作封装在一个对象中，以实现信息的隐藏和保护。类中的属性和方法可以通过访问权限修饰符来控制访问权限，如public、protected和private。

class Person {
  constructor(name, age) {
    this._name = name; // 私有属性
    this.age = age; // 公有属性
  }

  speak() {
    console.log(`Hello, my name is ${this._name} and I am ${this.age} years old.`);
  }

  get name() {
    return this._name; // 获取私有属性值
  }

  set name(value) {
    this._name = value; // 设置私有属性值
  }
}

const person = new Person('John', 30);
person.speak(); // 输出: "Hello, my name is John and I am 30 years old."
person.name = 'Jane'; // 设置私有属性值
person.speak(); // 输出: "Hello, my name is Jane and I am 30 years old."

7. 类抽象

类抽象是允许类定义一些抽象方法，这些方法没有实现，子类必须实现这些方法。抽象类不能被实例化，只能被继承。

abstract class Person {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  abstract speak(); // 抽象方法

  get name() {
    return this._name; // 获取私有属性值
  }

  set name(value) {
    this._name = value; // 设置私有属性值
  }
}

class Student extends Person {
  constructor(name, age, school) {
    super(name, age); // 调用父类构造函数
    this.school = school;
  }

  speak() {
    console.log(`Hello, my name is ${this.name} and I am ${this.age} years old. I am a student at ${this.school}.`);
  }
}

const person = new Person('John', 30); // 报错: 不能实例化抽象类
const student = new Student('John', 30, 'Harvard University');
student.speak(); // 输出: "Hello, my name is John and I am 30 years old. I am a student at Harvard University."