📚 Object-Oriented Programming Key Concepts | 面向对象 考点精讲
Object-oriented programming (OOP) is a paradigm that models real-world entities using classes and objects, promoting code reuse, modularity, and maintainability. In the IB and CIE Computer Science syllabi, OOP is a core topic that requires understanding of encapsulation, inheritance, polymorphism, and design principles. This article provides a focused revision of the key concepts and common exam pitfalls.
面向对象编程（OOP）是一种使用类和对象来模拟现实世界实体的编程范式，可提升代码的重用性、模块化和可维护性。在 IB 和 CIE 计算机科学大纲中，OOP 是核心主题，需要掌握封装、继承、多态和设计原则。本文针对关键概念和常见考试陷阱进行精讲。
1. Class and Object | 类与对象
A class is a blueprint that defines the attributes (data) and methods (behaviours) common to a set of objects. An object is a specific instance of a class, created using the 'new' keyword (in Java/C#). Each object maintains its own state stored in instance variables. For example, a 'Student' class may have attributes like name and idNumber, and methods like submitAssignment().
类是定义一组对象共有的属性（数据）和方法（行为）的蓝图。对象是通过 'new' 关键字（在 Java/C# 中）创建的类的具体实例。每个对象维护自己存储在实例变量中的状态。例如，'Student' 类可能拥有 name 和 idNumber 属性，以及 submitAssignment() 方法。
In OOP analysis, you identify objects from problem statements by looking for nouns (potential objects) and verbs (potential methods). An exam question might present a scenario and ask you to propose a class diagram or write pseudocode, so practice mapping real-world entities to classes and objects.
在 OOP 分析中，你通过查找名词（潜在对象）和动词（潜在方法）从问题陈述中识别对象。考试可能给出一个场景，要求你提出类图或编写伪代码，因此要练习将现实世界实体映射为类和对象。
2. Encapsulation & Data Hiding | 封装与数据隐藏
Encapsulation bundles the data and methods that operate on the data within a class and uses access modifiers to hide the internal state from outside interference. Typically, attributes are marked private, and public getters and setters provide controlled access. This enforces data validation and reduces coupling.
封装将数据和操作数据的方法捆绑在类中，并使用访问修饰符隐藏内部状态，防止外部干扰。通常，属性标记为 private，通过 public 的获取器和设置器提供受控访问。这强制执行数据验证并降低耦合。
You should be able to explain why direct public attributes are discouraged: if the representation of a field changes (e.g., storing a date as a string vs. integer), all external code that directly accesses it must be modified. Encapsulation allows you to change the internal implementation without affecting clients.
你应该能解释为什么不鼓励直接使用公共属性：如果某个字段的表示方式发生变化（例如将日期存储为字符串或整数），所有直接访问它的外部代码都必须修改。封装允许你更改内部实现而不影响使用者。
3. Inheritance and 'is-a' Relationship | 继承与“is-a”关系
Inheritance establishes a hierarchical relationship where a subclass inherits all non-private members of a superclass. This models an "is-a" relationship: a Dog is an Animal. Constructors are not inherited, but a subclass can call a superclass constructor using 'super()'. Method overriding allows the subclass to provide specialized behavior.
继承建立了一种层次关系，子类继承父类的所有非私有成员。这模拟了 "is-a" 关系：狗是一种动物。构造函数不被继承，但子类可以使用 'super()' 调用父类构造函数。方法重写允许子类提供专门的行为。
In CIE and IB, you may need to trace code with inheritance, and understand that a subclass reference can be assigned to a superclass variable (upcasting), but not the reverse without explicit casting. This leads to polymorphism.
在 CIE 和 IB 中，你可能需要追踪包含继承的
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