[Lecture Notes] CMU 17-514 Software Construction

Lec 2. Object-Oriented Basics

Tradeoffs?

Version 1

void sort(int[] list, String order) {
	// ...
	boolean mustswap;
	if (order.equals("up")) {
		mustswap = list[i] < list[j];
	} else if (order.equals("down")) {
		mustswap = list[i] > list[j];
	}
}

Version 2

void sort(int[] list, Comparator cmp) {
	// ...
  boolean mustswap;
	mustswap = cmp.compare(list[i], list[j]);
}

interface Comparator {
	boolean compare(int i, int j);
}
class UpComparator implements Comparator {
	boolean compare(int i, int j) { return i < j; }
}
class DownComparator implements Comparator {
	boolean compare(int i, int j) { return i > j; }
}

Version 1: Shorter

Version 2: Expandable.

TypeScript is a superset of JavaScript because it allows for gradual migration from JS to TS

Design for Change (flexibility, extensibility, modifiability)
Design for Division of Labor
Design for Understandability

Java Primitives: int, long, byte, short, char, float, double, boolean. 注意没有string！

JavaScript Primitives: null, undefined, boolean, number, string, symbol, bigint

Primitives are immutable, and passed by value

Object is a non-primitive type.

Lec 3. Object-oriented Analysis

如何把用户需求转变成代码实现？

OO Analysis: understanding the problem

domain model
find key concepts in the problem domain
using UML (Unified Modeling Language) class diagrams as informal notation
glossary
system sequence diagram: a model that shows, for one usage scenario, sequence of events that occur on the system’s boundary.
system behavioral contracts

OO Design: Defining a solution

object interaction diagrams
object model

Domain Model长这样

空心菱形：aggregation 聚合关系，例如：班级 ◇--- 学生（学生可以存在于班级之外）
实心菱形：composition 组合关系，例如：人 ♦--- 心脏（心脏不能离开人体独立存在）
三角形：inheritance/generalization 继承/泛化关系，例如：狗 ---△ 动物（狗继承自动物）

UML Sequence Diagram 长这样

Behavioural Contract 长这样

完成了Domain Model, System Sequence Diagram 和 Behavioural Contract 之后，就是从Problem Space到Solution Space的转换。希望能low representational gap

Lec 4. Responsibility Assignment

Problem Space (Domain model) —> Solution Space (Object model)

Representational gap

Design Principle

Low representational gap
为什么要low representational gap? 你可以建立一个class LibrarySystem然后把所有东西放在单个巨大的class里，但是这样不好做后续修改（我们需要 design for change）
Benefits
- facilitates understanding of design and implementation
- facilitates traceability from problem to solution
- facilitates evolution (design for change)
Low coupling
一个module要和尽量少的module之间有依赖关系
Benefits
- Enhances understanability
- Reduce the cost of change (如果每个module最多只和另外两个module有关系，那么改变module时最多只需要修改两个module，不需要牵一发而动全身)
A related design heuristic: law of demeter: each module should have only limited knowledge about other units
a.getB().getC().foo is a bad practice !!! This means you are not distributing knowledge correctly !!!
Prefer coupling to interfaces over coupling to implementations (interface相比implementation改变得更少)
High cohesion (or single-responsibility principle)
每个component（每个class/object）只负责较少的职责