📚 Object-Oriented Programming Essentials | 面向对象编程基础
Object-oriented programming (OOP) is one of the core programming paradigms assessed in Edexcel A-Level Computer Science. You will need to explain its key features, compare it with procedural programming, and apply it in your programming project. This revision guide breaks down the core concepts and exam-style pitfalls.
面向对象编程（OOP）是爱德思 A-Level 计算机科学考查的核心编程范式之一。你需要解释其关键特性、与面向过程编程进行比较，并在编程项目中加以应用。本复习指南将拆解核心概念与考试常见陷阱。
1. Programming Paradigms and Why They Matter | 编程范式及其重要性
A programming paradigm is a fundamental style of writing code. Edexcel expects you to distinguish between procedural, object-oriented, and event-driven paradigms, with a focus on how OOP promotes reuse and maintainability.
编程范式是编写代码的基本风格。爱德思考试要求区分面向过程、面向对象和事件驱动范式，并重点关注 OOP 如何促进代码复用和可维护性。
In procedural programming, the program is organised around functions that operate on separate data. In OOP, data and the functions that work on that data are bundled into objects, which more closely models real-world systems.
在面向过程编程中，程序围绕对独立数据进行操作的函数来组织。而在 OOP 中，数据与操作这些数据的函数被捆绑为对象，这更接近现实系统的建模方式。
Event-driven programming, by contrast, structures code around responses to events such as mouse clicks or key presses. You may see all three paradigms in a single Edexcel question, so focus on the organisation of code and data in each.
与之相比，事件驱动编程围绕对鼠标点击或按键等事件的响应来组织代码。在一道爱德思试题中，你可能会看到三种范式同时出现，因此要重点关注每种范式中代码与数据的组织方式。
2. Classes and Objects: The Blueprint and the Instance | 类与对象：蓝图与实例
A class is a template or blueprint that defines the attributes and methods for a category of objects. An object is a concrete instance created from that class.
类是定义某一类对象的属性和方法的模板或蓝图。对象是由该类创建的具体实例。
For example, a class Car might have attributes such as registration and engineSize, and methods such as accelerate(). Each actual car in the program is an object of the class.
例如，类 Car 可能具有 registration 和 engineSize 等属性，以及 accelerate() 等方法。程序中的每辆真实汽车都是该类的一个对象。
- Class: a static definition; object: a dynamic instance with its own state.
- 类：静态定义；对象：拥有自身状态的动态实例。
- Many objects can be created from one class, each holding different attribute values.
- 一个类可以创建多个对象，每个对象保存不同的属性值。
3. Attributes and Methods: State and Behaviour | 属性与方法：状态与行为
Attributes store the state of an object, while methods define its behaviour. In exam pseudocode, attributes can be private, protected, or public depending on the access modifier.
属性存储对象的状态，而方法定义其行为。在考试伪代码中，根据访问修饰符，属性可以是私有、受保护或公有的。
Methods often include accessor methods (getters) that return an attribute value and mutator methods (setters) that change it. This supports encapsulation by controlling how state is modified.
方法通常包括返回属性值的访问器方法（getter）和修改属性值的修改器方法（setter）。这通过控制状态如何被修改来支持封装。
| Attribute / 属性 | Method / 方法 |
|---|---|
| Stores data, e.g. speed | Performs an action, e.g. brake() |
| Typically private | Public interface |
| Defines current state | Defines available behaviour |
When answering exam questions, link attributes to ‘what an object knows’ and methods to ‘what an object can do’. This small distinction earns marks in definition-style questions.
在回答考试问题时，要将属性与“对象知道什么”联系起来，将方法与“对象能做什么”联系起来。这个细小区别在定义类题目中能得分。
4. Constructors and Instantiation | 构造函数与实例化
A constructor is a special method that runs automatically when an object is created. It often sets initial values for attributes, ensuring the object starts in a valid state.
构造函数是在创建对象时自动运行的特殊方法。它通常为属性设置初始值，确保对象以有效状态开始。
Instantiation is the process of creating an object from a class, using the keyword new in many languages or NEW in Edexcel-style pseudocode. If a class has no constructor, a default empty one is provided.
实例化是从类创建对象的过程，在许多语言中使用 new 关键字，在爱德思风格伪代码中使用 NEW。如果类没有构造函数，则提供一个默认的空构造函数。
myCar = NEW Car(“AB12 CDE”)
This line calls the constructor and returns an object reference stored in myCar. You should be able to trace how constructor parameters become initial attribute values.
这行代码调用构造函数并返回一个存储在 myCar 中的对象引用。你应该能够追踪构造函数参数如何成为初始属性值。
5. Encapsulation and Data Hiding | 封装与数据隐藏
Encapsulation means bundling data and methods together and restricting direct access to an object’s internal state. It is implemented using access modifiers such as private, public, and protected.
封装意味着将数据和方法捆绑在一起，并限制对对象内部状态的直接访问。它通过 private、public 和 protected 等访问修饰符来实现。
By making attributes private, you force external code to use public methods, which can validate data and prevent invalid changes. This increases reliability and makes maintenance easier.
通过将属性设为私有，你强制外部代码使用公有方法，这些方法可以验证数据并阻止无效更改。这提高了可靠性，并使维护更容易。
- Private: only accessible inside the class.
- 私有：只能在类内部访问。
- Public: accessible from any code.
- 公有：任何代码都可以访问。
- Protected: accessible in the class and its subclasses.
- 受保护：可在类及其子类中访问。
In Edexcel questions, a common mark is awarded for stating that encapsulation prevents invalid data from being assigned directly. Always mention the state of an object is protected.
在爱德思考题中，通常会有一个得分点是说明封装可以防止无效数据被直接赋值。一定要提到对象的状态受到保护。
6. Inheritance and Derived Classes | 继承与派生类
Inheritance allows a new class to take on the attributes and methods of an existing base class. The new class is called a derived class or subclass, and it can add or override members.
继承允许新类获得现有基类的属性和方法。新类称为派生类或子类，它可以增加或覆盖成员。
Inheritance expresses an ‘is-a’ relationship. For example, a SportsCar is a Car, so SportsCar can inherit everything common to all cars while adding specific features such as turboMode.
继承表达“是”（is-a）关系。例如，SportsCar 是一辆 Car，因此它可以继承所有汽车共有的一切，同时添加特定功能，如 turboMode。
In Edexcel pseudocode, you may show inheritance as SportsCar INHERITS Car or with a class diagram arrow from subclass to superclass. Be prepared to identify attributes and methods available in both classes.
在爱德思伪代码中，你可以用 SportsCar INHERITS Car 表示继承，或者用从子类指向超类的类图箭头来表示。要准备好识别两个类中都可用的属性和方法。
7. Polymorphism and Method Overriding | 多态与方法覆盖
Polymorphism means ‘many forms’. In OOP, it allows a derived class to be treated as its base class, while the correct overridden method is called at runtime based on the actual object type.
多态意为“多种形态”。在 OOP 中，它允许派生类被视为其基类，而正确的覆盖方法在运行时根据实际对象类型被调用。
Method overriding occurs when a subclass provides a new version of a method with the same signature. This supports dynamic dispatch, enabling code such as vehicle.move() to behave differently for a bike, car, or train.
方法覆盖发生在子类提供具有相同签名的方法的新版本时。这支持动态分派，使 vehicle.move() 这样的代码对于自行车、汽车或火车表现出不同行为。
Edexcel exam questions often ask for an example of polymorphism. A strong answer uses a base class reference holding a subclass object and explains that the subclass method is executed.
爱德思考试题经常要求举例说明多态。一个高质量答案会使用基类引用持有子类对象，并解释执行的是子类方法。
8. OOP vs Procedural Programming | 面向对象与面向过程编程的对比
A short comparison can help with 4-6 mark exam questions. Procedural programming uses flat functions and shared data, while OOP bundles state and behaviour into objects with encapsulation and inheritance.
一个简短的对比有助于回答 4-6 分的考试题。面向过程编程使用扁平函数和共享数据，而 OOP 将状态和行为捆绑到对象中，并封装和继承。
| Feature / 特性 | Procedural / 面向过程 | OOP / 面向对象 |
|---|---|---|
| Organisation | Functions and data separate | Objects combine data and methods |
| Data security | Low, data often shared | High through encapsulation |
| Reuse | Limited, functions reused | Inheritance and composition |
| Maintenance | Changes can affect whole program | Changes localised in classes |
Use this table to structure comparison answers. Always link a feature to a concrete consequence, such as easier debugging or reduced code duplication.
使用此表来组织比较类答案。始终将一个特性与具体结果联系起来，例如更容易调试或减少代码重复。
9. Common Edexcel Exam Pitfalls | 爱德思考试常见误区
Students often confuse a class with an object, or write ‘encapsulation’ when they mean ‘inheritance’. Use precise vocabulary: encapsulation is data hiding; inheritance is class reuse; polymorphism is runtime behaviour.
学生经常混淆类和对象，或者在表示继承时误写“封装”。请使用精确术语：封装是数据隐藏；继承是类重用；多态是运行时行为。
- Pitfall: saying an object is a blueprint. Correct: a class is a blueprint; an object is an instance.
- 误区：说对象是蓝图。正确：类是蓝图；对象是实例。
- Pitfall: forgetting that private attributes
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