九章ObjectOrientedDesign.md

OOD

• OOD
• Lecture 1 面向对象设计入门 - Elevator System - Strategy Pattern
  • 小结
  • 简介
  • 5C解题法
  • Design Elevator System
    • Clarify
    • Core Object
    • Cases
    • Class
    • Correctness
  • Challenge
    • Design Pattern: Strategy Pattern
• Lecture 2 - 管理类OOD: Parking Lot & Restaurant - Singleton Pattern
  • Clarify
  • Core Object
  • Cases - 站在管理员的角度想
  • Class & Correctness
  • Design Pattern - Singleton
• Lecture 3 - 预定类: Hotel & Airline Ticket
  • 餐馆OOD
    • Clarify
    • Use Case & Classes
  • 预定类OOD
  • Hotel Reservation
• Lecture 4 - 实物类: Vending Machine & Juke Box - Factory & Adaptor Pattern
  • Vending Machine & ATM Machine
    • State Design Pattern
  • Coffee Machine
    • Decorator Design Pattern
  • Kindle
    • Factory Design Pattern
• Lecture 5 - 棋牌类: Black Jack & Chinese Chess
  • 棋牌类解题思路
    • TicTacToe
    • Chinese Chess
    • 牌类 - Blackjack

Lecture 1 面向对象设计入门 - Elevator System - Strategy Pattern

小结

• 管理类 - Parking Lot
  • Use Case: Reserve, Serve, Checkout
  • 技巧: 使用Receipt来储存relationship (Table vs People. Parking vs Car, Book vs Student)
• 预订类 - Restaurant, Hotel
  • Use Case: Reserve
    • Search, make reservation, cancel reservation
  • 技巧: Search criteria -> Search() -> List -> Select() -> Receipt
  • 技巧2: 利用LRU进行优化
• 实物类
  • ATM & Vending Machine - State
  • Coffee Maker - Decorator
  • Kindle - Factory
• 棋牌类总结
  • Clarify : 玩家，规则，胜负，积分
  • Core object: Hand, Board, Deck/Table, Suit, ...
  • Use cases: Initialization / Play / Checkout
  • 对于牌类，需要从Player的角度出发

简介

• OOD特点
  • OOD在面试当中很重要, 尤其对于Intern和Entry level
  • 题目没有标准答案, Everything is an object
  • 题目类型明显，可用固定的模板解题
• OOD面试中的陷阱
  • 起手式的陷阱: 自说自话 & 指鹿为马
  • 解题中的陷阱: 朝出夕改
  • 收手式的陷阱: 虎头蛇尾
• 评判OOD面试
  • S – Single responsibility principle
  • O – Open close principle
  • L – Liskov substitution principle
  • I – Interface segregation principle
  • D – Dependency inversion principle

5C解题法

• Clarify: 通过和面试官交流，去除题目中的歧义，确定答题范围
• Core objects: 确定题目所涉及的类，以及类之间的映射关系
• Cases: 确定题目中所需要实现的场景和功能
• Classes: 通过类图的方式，具体填充题目中涉及的类
• Correctness: 检查自己的设计，是否满足关键点

Design Elevator System

Clarify

• Overview
  • What: 针对题目中的关键字来提问, 通过名词的属性来考虑
  • How: 针对问题主题的规则来提问
  • Who: 以系统为主导
• What - 关键词
  • Elevator:
    • 电梯能够获取当前重量?
    • 客梯和货梯?
  • Building
    • 是否有多处能搭乘的电梯口?
• How - 规则
  • 当按下按钮时，哪一台电梯会相应
  • 当电梯在运行时，哪些按键可以响应
• Who
  • 电梯系统如何获取每位乘客的重量

Core Object

• 为什么要定义Core Object ?
  • 这是和面试官初步的纸面contract
  • 承上启下，来自于Clarify的结果，成为Use case的依据
  • 为画类图打下基础
• 如何定义Core Object ?
  • 以一个Object作为基础，线性思考
    • 只考虑input/output
  • 确定Objects之间的映射关系
• Good Practice
  • Use Access modifier
    • package - 尽量避免
    • public
    • private - 封装
    • protected - 继承
  • package
    • 如果什么都不声明，变量和函数都是package level visible
    • 在同一个package内的其他类都可以访问
    • 在类图中，避免使用default的package level access
    • 常用于写Unit Test

Cases

• 为什么要写Use cases
  • 这是你和面试官白纸黑字达成的第二份共识，把你将要实现的功能列在白板上
  • 帮助你在解题过程中，理清条例，一个一个Case实现
  • 作为检查的标准
• 怎么写Use cases
  • 利用定义的Core Object, 列举出每个Object对应产生的use case.
  • 每个use case只需要先用一句简单的话来􏲼述即可
• ElevatorSystem
  • Handle request
• Request
  • N/A
• Elevator
  • Take external request
  • Take internal request
  • Open gate
  • Close gate
  • Check weight
• ElevatorButton
  • Press button

Class

• 为什么要画类图?
  • 可交付，Minimal Viable Product
  • 节省时间，不容易在Coding上挣扎
  • 建立在Use case上，和之前的步骤层层递进，条例清晰，便于交流和修改
  • 如果时间允许/面试官要求，便于转化成Code
• 怎么画类图?
  • 遍历你所列出的use cases
  • 对于每一个use case，更加详细的描述这个use case在做什么事情
    • 例如:take external request
      • ElevatorSystem takes an external request, and decide to push this request to an appropriate elevator
  • 针对这个描述，在已有的Core objects里填充进所需要的信息
• Use case: Handle request
  • ElevatorSystem takes an external request, and decide to push this request to an appropriate elevator

Correctness

• 从以下几方面检查:
  • Validate use cases (检查是否支持所有的use case)
  • Follow good practice (面试当中的加分项，展现一个程序员的经验)
  • S.O.L.I.D
  • Design pattern

Challenge

• Q: What if I want to apply different ways to handle external requests during different time of a day?
• A: Use Strategy design pattern

Design Pattern: Strategy Pattern

• 封装了多种 算法/策略
• 使得算法/策略之间能够互相替换

Lecture 2 - 管理类OOD: Parking Lot & Restaurant - Singleton Pattern

• 管理类 - 题目后面都可以接上三个字: 管理员
  • e.g.: Gym, Parking lot, Restaurant, Library, Super market, Hotel
  • 设计一个模拟/代替管理员日常工作的系统
• 解题方法
  • Clarify: 除了题目中问的名词外，还需要从管理的名词来考
  • Core object: 有进有出
  • Cases: 从管理员角度考虑
    • Reserve: 预定
    • Serve: 服务
    • Checkout: 买单
  • Class: 经常可以使用收据的形式，来保管信息
  • Correctness

Clarify

• What
  • 关键词: Parking lot, Vehicle, Parking Spot
  • Parking lot: 考虑多层的Parking lot, 没有错层
  • Vehicle: 考虑三种大小的车
  • 不考虑残疾人停车位/充电车位
• How
  • 规则1: 如何停车
    • 一定要从停车场的角度来考虑,而不是车的角度
      • 停车场: 开进停车场 -> 返回一个能停的地方 ->停进一个位 置
      • 车: 开进停车场 -> 经过每一个位置看看能不能停 -> 停进一个位置
  • 规则2: 付费
    • 免费还是付费
• Who: Optional

Core Object

• Parking Lot
• Parking spot
• Car, Bus, Motorcycle
• 映射关系
  • Parking has a list of Spot
  • 错误方法: spot里面有car,或者parking lot有list of car

Cases - 站在管理员的角度想

• Bus / Car / Motorcycle
  • N/A
• Parking Lot
  • Get available count (reserve)
  • Park vehicle (serve)
  • Clear spot (checkout)
  • Calculate price (checkout)
• Parking Spot
  • N/A

Class & Correctness

• Draw UML per Use Case
• 从以下几方面检查:
  • Validate use cases (检查是否支持所有的use case)
  • Follow good practice (面试当中的加分项，展现一个程序员的经验)
  • S.O.L.I.D
  • Design pattern

Design Pattern - Singleton

public class ParkingLot {
  private static ParkingLot _instance = null;

  private List<Level> levels;

  private ParkingLost() {
    levels = new ArrayList<List>();
  }

  public static synchronized ParkingLot getInstance() {
    if (_instance == null) {
      _instance = new ParkingLot();
    }
    return _instance;
  }
}

• synchronized getInstance()使得Singleton线程安全
• static _instance 使得只有一个instance
• private ParkingLot()使得constructor只被调用一次

Lecture 3 - 预定类: Hotel & Airline Ticket

餐馆OOD

Clarify

• What
  • 管理: Party -> Restaurant -> Table
• How
  • 能否预约
  • 能否外送
• Who - Optional
  • 思考模式1: 过于复杂
    • Party 进入餐馆 -> Host指引到空桌 (find table) -> 一个waiter负责这桌客人 (assign waiter) -> 客人点菜 (take order) -> Chief 拿到order，按顺序做菜 (cook by order) -> Order做好后，waiter上菜 (serve order) -> 客人吃完后付钱 (check out)
  • 思考模式2
    • 客人进入餐馆，餐馆返回一个Table
    • 客人点菜，餐馆返回一桌菜
    • 客人付账，餐馆清空Table

Use Case & Classes

• Restaruant
  • Find Table (serve)
  • Take Order (serve)
  • Checkout (checkout)

预定类OOD

• 题库
  • Restaurant reservation system
  • Hotel reservation system
  • Flight/Bus/Train reservation system
• 解题思路
  • What: 考虑预定的东西
  • Use Case = Reserve
    • Search Criteria -> search() -> List<Result> -> select() -> Receipt
      • Reservation findTableForReservation(Time slot) throws NoTableForReservationException
      • void confirmReservation(Reservation reservation)

Hotel Reservation

• Can you design a hotel reservation system
• What
  • 是为一间酒店设计预定房间系统，还是先选择酒店的系统?
  • 搜索条件区别: 人数+时间 VS 人数+时间+地址
  • Search criteria -> Search() -> List -> Select() -> Receipt

Lecture 4 - 实物类: Vending Machine & Juke Box - Factory & Adaptor Pattern

• 实物类OOD题型
  • Vending machine
  • Jukebox
  • CD Player
  • Coffee maker
  • ATM
• 实物类
  • 考虑对于实物的输入输出
• 技巧
  • State pattern
  • Decorate pattern
  • Factory pattern

Vending Machine & ATM Machine

State Design Pattern

• States
  • HAS_SELECTION - NO_SELECTION
  • COINS_INSERTED - NO_COIN
  • SOLD
  • SOLD_OUT
• State related actions:
  • select item
  • insert coin
  • execute transaction
  • cancel transaction

public interface State {
    public oid selectItem(String selection);
    public void insertMoney(int value);
    public void executeTransaction();
    pulic int cancelTransaction();
}

public class NoSelectionState implements State {
  VendingMachine vendingMachine;

  public NoSectionsState(VendingMachine vendingMachine) {
    this.vendingMachine = vendingMachine;
  }

  //Implement, and change state
  @Override
  public void selectItem(String selection) {
    vendingMachine.setSelectedItem(selection);
    vendingMachine.changeToHasSelectionState();
  }
}

public class VendingMachine {
  private State state;
  private NoSelectionState noSelectionState;
  privte HasSelectionState hasSelectionState

  //Constructor: init all state, and set inital state
  public VendingMachine() {
    noSelectionState = new NoSelectionState(this);
    hasSelectionState = new hasSelectionState(this);
    state = noSelectonState;
  }

  //Expose to other State class for changing 
  public void changeToHasSelectionState() {
    state = hasSelectionState();
  }

  //API, use different state to implement
  public void selectItem(String selection) {
    state.selectItem(selection);
  }
}

Coffee Machine

Decorator Design Pattern

public interface Coffee {
  public double getCost();
  public String getDescripton();
}

public class Expresso implements Coffee {
  public Expresso() {
    description = "Expresso";
  }

  public float getCost() {
    return 1.99;
  }

  public String getDecription() {
    return description;
  }
}

public abstract class CoffeeDecorator implements Coffee {
  protected final Coffe decoratedCoffee;

  public CoffeeDecorator(Coffee c) {
    this.decoratedCoffee = c;
  }

  public double getCost() {
    return decoratedCoffee.getCost();
  }

  public double getDescription() {
    return decoratedCoffee.getDescription();
  }
}

public class WithMocha extends CoffeeDecorator {
  public WithMocha(Coffee coffee) {
    super(c);
  }

  public String getDescription() {
    return super.getDescription() + ", Mocha";
  }

  public float getCost() {
    return super.getCost() + 0.5;
  }
}

public void test() {
  Coffee c = new Expresso();
  c = new WithMocha(c);
  c = new WithMocha(c);
  // Expresso, Mocha, Mocha $2.99
  System.out.println(c.getDescription() + "$ " + c.getCost());
}

Kindle

Factory Design Pattern

• Simple Factory: 将if-else放入一个单独的factory里面
• Factory method
• Abstract Factory

Lecture 5 - 棋牌类: Black Jack & Chinese Chess

棋牌类解题思路

• 特点 - Clarify
  • 玩家
  • 规则
  • 胜负
  • 积分
• 术语 - Core Object
  • Board
  • Suit
  • Hand
• State - Use Case
  • Initialization (摆盘，洗牌...)
  • Play (下棋，出牌...)
  • Win/Lose check (胜负结算) + Tie (流局)

TicTacToe

//Simulator.java
makeMove(1,1)

//TicTacToe.java
public void makeMove(int row, int col) {
  board.makeMove(row, col, currentMove);
  if (board.checkWin()) {
    print (currentMove + " win!");
  } else if (board.isBoardFull()) {
    print("It's a tie");
  }
  changePlayer();
}

Chinese Chess

• 特点 - Clarify
  • 玩家
  • 规则
  • 胜负
  • 积分
• 术语 - Core Object
  • Board
  • Suit
  • Hand
• State - Use Case
  • Initialization (摆盘，洗牌...)
  • Play (下棋，出牌...)
  • Win/Lose check (胜负结算) + Tie (流局)

牌类 - Blackjack

• Core Objects - 牌类较为固定的framework
  • Hand, Player, Dealer, Card, Deck
• Use Case
  • Initialization (摆盘，洗牌...)
    • Join table
    • Place bet
    • Get initial cards
  • Play (下棋，出牌...)
    • Deal
    • Increase bet
    • Stop dealing
  • Win/Lose check (胜负结算) + Tie / Draw (平局)
    • Compare score
    • Take/Lose bets