OOD

Design a Parking Lot

Design a Parking Lot

Step 1: Understand / Analyze the Use Case (明确这个程序/系统是做什么的)

在问题领域 用自然语言描述问题是什么

Use Case: Describe the parking lot building. Vehicle monitoring? What kind of parking lot？

• One level or multiple levels?

• Parking-spot / vehicle sizes?

• Need to track the location?

• Fee?

Step 2: Classes and their relationships

用程序语言 通过建模来描述问题

每出现一个事物的种类，就定义一个class，一个类只做一件事，把这一件事做好。

class relationships 区别 是否可以独立存在:

• Composition: (has a) ownership relationship between two classes 手足

• Aggregation: holds a reference of another, no real ownership 男女朋友

• Association: an activity between 2 classes

inheritance (is a)

对于parking lot的语境，上级hold下级的reference

• level无法脱离parking lot存在，所以是composition

• parking spot无法脱离level存在，也是composition

• vehicle和parking spot是aggregation

• vehicle和car、truck是inheritance的关系

• parking lot和vehicle是association、aggregation

方案比较 有level vs 没有level

1）Parking Lot -- Level -- Parking Spot 2) Parking Lot -- Parking Spot

1的好处是easy to evolve, 未来不但能扩展楼层数，还能specify每一层的功能 2的好处是方便prototyping

Step 3: For complicated designs, first focus on public methods 如何调用

1. Basic functionality: tell whether or not there is available spot in the parking lot

2. Possible Extensions: provide available spot locations, assign spot to the vehicle

public class ParkingLot{
    List<Level> level;
    
    Ticket park(Vehicle v); //开着车拿ticket走
    Vehicle leave(Ticket t); //拿ticket取车
    
    public boolean hasSpot(Vehicle v) {
    //check 
    }
}

class Level {
    boolean hasSpot(Vehicle);
}

class ParkingSpot {
    boolean fit(Vehicle); //check size and availability
}

public abstract class Vehicle {

}

Step 4: Complete Implementation

如果想要enforce一个规定个数的spots, 不能 private final <ParkingSpot> spots因为这只防了一层，并不真的immutable。 Java中可以通过一个”防火墙“似的操作，把list的方法包起来，只通行一些方法。或者用spot=Collections.unmodifiableList(list)

Design a Web Crawler

BFS

• Get web page content by URL

• Parse the page and get hyperlinks

• Deduplicate the links (do not download the same URL twice)

• Put new target links to a Queue

• Get next urls to be downloaded and parsed from the Queue

在领任务时它是consumer，在给出新urls时它是producer。应该用的是blocking queue，因为它是一个multi-threaded process，在queue为空时...