Lecture 1：OOP and UML Class DiagramsOOP with Java

OOP 和 UML 类图 OOP 与 Java 。

Object-Oriented Programming 面向对象编程

Class Hierarchies 类层次结构

Superclass and subclass 超类和子类 。

Pillars of Object-Oriented Programming 面向对象编程的支柱

1. Abstraction
   – Modelling attributes and behaviors of real objects, in specific contexts
   抽象——对真实对象的属性和行为进行建模，具体来说

2. Encapsulation – Hiding parts of an object’s states and behaviors from others, and exposing a limited set of interfaces – public, private, and protected – Interfaces and abstract classes 封装 — 隐藏对象的部分内容和来自他人的状态和行为，并公开一组有限的接口 – 公共、私有和受保护 – 接口和抽象类 。

3. Inheritance – Main benefit: code reuse 继承 — 主要好处：代码重用 。

4. Polymorphism – Performing an action in many forms – A mechanism for detecting the real class of an object and call its implementation 多态性 多种形式执行一个动作 一种检测对象的真实类并调用其实现方法的机制 。

OOP with Java: Declaring Classes and Creating Objects Java 的 OOP：声明类和创建对象

• Class declaration 类声明 。

• Creating objects 创建对象 。

  • Declaration, instantiation, initialization
    声明、实例化、初始化
    
  • The reference returned by the new operator does not have to be assigned to a variable
    new 运算符返回的引用不必分配给变量

OOP with Java: Access Control Java 的 OOP：访问控制

• At the top level
  在顶层
  – public, or package-private (no explicit modifier)
  公共或包私有（无显式修饰符）
• At the member level
  在成员级别
  – public, private, protected, or package-private (no explicit modifier)
  public、private、protected 或 package-private（无显式修饰符）

OOP with Java: Inheritance Java 中的 OOP：继承

• Classes can be derived from other classes, inheriting fields and methods 类可以从其他类派生，继承字段和方法 。

• Definitions 定义 – Subclass (derived class/extended class/child class) – Superclass (base class/parent class) – 子类（派生类/扩展类/子类） – 超类（基类/父类） 。

• Every class has one and only one direct superclass (single inheritance) 每个类都有一个且仅有一个直接超类（单继承） – Excepting Object, which has no superclass 除了Object，它没有超类 。

• A subclass inherits all the members (fields, methods, and nested classes) from its superclass 子类继承其超类的所有成员（字段、方法和嵌套类） 。

OOP with Java: What You Can Do in a Subclass Java 的 OOP：在子类中可以做什么

Use the inherited members as is, replace them, hide them, or supplement them 按原样使用继承的成员、替换它们、隐藏它们或补充它们 。

• Declare a field in the subclass with the same name as the one in the superclass, thus hiding it (NOT recommended)
  – 在子类中声明一个与超类中的字段同名的字段，从而隐藏它（不推荐）
• Write a new instance method in the subclass that has the same signature as the one in the superclass, thus overriding it
  – 在子类中编写一个新的实例方法，其签名与超类中的字段相同，从而覆盖它
• Write a new static method in the subclass that has the same signature as the one in the superclass, thus hiding it
  –在子类中编写一个新的静态方法，该方法与超类中的静态方法具有相同的签名，从而隐藏它
• Write a subclass constructor that invokes the constructor of the superclass
  – 编写一个调用超类构造函数的子类构造函数

How about private members in a superclass？

OOP with Java: Abstract and Final Methods/Classes Java 的 OOP：抽象和最终方法/类

• An abstract class is a class declared abstract: it may or may not include abstract methods
  抽象类是声明为抽象的类：它可能包含也可能不包含抽象方法
• An abstract method is a method declared without an implementation
  抽象方法是声明但没有实现的方法
• Final methods and classes
  最终方法和类
  Methods called from constructors should generally be declared final
  – 从构造函数调用的方法通常应声明为final

OOP with Java: Interfaces Java 的 OOP：接口

• Interfaces are contracts
  接口是契约
• A reference type, containing only constants, method signatures,default methods, static methods, and nested types
  引用类型，仅包含常量、方法签名、默认方法、静态方法和嵌套类型
• Cannot be instantiated
  无法实例化
  – They can only be implemented by classes or extended by other interfaces
  – 它们只能由类实现或由其他接口扩展
• Consisting of modifiers, keyword, interface name, a comma-separated list of parent interfaces (if any), and the interface body
  由修饰符、关键字、接口名称、以逗号分隔的父接口列表（如果有）和接口主体组成
• Interface body can contain abstract methods, default methods,and static methods
  接口体可以包含抽象方法、默认方法和静态方法

OOP with Java: Implementing and Using Interfaces 使用 Java 进行 OOP：实现和使用接口

• Include an implements clause in the class declaration
  在类声明中包含 implements 子句
  – Your class can implement more than one interface
  – 你的类可以实现多个接口
• If you define a reference variable whose type is an interface,any object you assign to it must be an instance of a class that implements the interface
  如果定义类型为接口的引用变量，则分配给它的任何对象都必须是实现该接口的类的实例

OOP with Java: Abstract Classes vs. Interfaces Java 中的 OOP：抽象类与接口

• Consider using abstract classes when 考虑使用抽象类 – You want to share code among several closely related classes – 您希望在几个紧密相关的类之间共享代码 – You expect that classes extending the abstract class have many common methods or fields, or require access modifiers other than public – 您希望扩展抽象类的类具有许多通用方法或字段，或者需要除 public 之外的访问修饰符 – You want to declare non-static or non-final fields — 您想要声明非静态或非最终字段 。

• Consider using interfaces when 考虑使用接口 – You expect that unrelated classes would implement your interface – 您希望不相关的类实现您的接口 – You want to specify the behavior of a particular data type, but not concerned about who implements its behavior – 您想要指定特定数据类型的行为，但不关心谁实现其行为 – You want to take advantage of multiple inheritance – 您想要利用多重继承 。

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