# OOD

## 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

```java
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&#x20;

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&#x20;

### 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...&#x20;