Clean Code

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 2023/08/30 

Personal Views / Basic Principles

[IMO] 编程是一件偏主观的事 is very subjective
- 比起数学公式它更像是论文或者有些人可以写成散文
- Engineering rather than Science
- 如果让计算机自己学习，那我们的工作就更像是艺术家 Deep Learning is often considered more of an art than a science because it requires a lot of experimentation and trial-and-error to achieve optimal results.
简单暴力 KISS (Keep it simple, stupid)
- [IMO]Clarity First，Optimization second. 能让人看懂是最优先，性能再好都是其次。
- [IMO] 尽量别用显式的去用Java的多线程，很难不出错。很难debug
  - 为啥要用，异步 & 性能（利用多核）- 用框架去解决。netty akka erlang exilir
  - 如果你一定要用， 1.把你写的并行代码和其他代码隔离开，单独弄个module。 2 使用ExecutorService。
    ExecutorService abstracts away many of the complexities associated with the lower-level abstractions like raw Thread
- 任何人都能写出workable的代码，好程序员写出来的是人类能懂的 “Any fool can write code that a computer can understand. Good programmers write code that humans can understand.” — Martin Fowler**
- 著名的雷神之锤代码 Q_rsqrt https://github.com/id-Software/Quake-III-Arena/blob/master/code/game/q_math.c
- one of our recent candidate’s code

public static int[] transform(int [] array){
    int leftIndex = 0;
    int rightIndex = array.length -1; // array can be null?

    while(leftIndex < rightIndex){

        while(array[leftIndex] % 2 == 0 && leftIndex < rightIndex){
            leftIndex++;
        }
        while(array[rightIndex] % 2 != 0 && leftIndex < rightIndex){
            rightIndex--;
        }

        int temp =array[leftIndex];
        array[leftIndex] = array[rightIndex];
        array[rightIndex] = temp;
    }
}

we can do better than this?


/**
 * Transforms the given array by moving all odd numbers to the left side and even numbers to the right side.
 *
 * @param array The input array to be transformed.
 * @return The transformed array.
 * @throws IllegalArgumentException If the input array is null.
 */
public static int[] transform(int[] array) {
    validateInputArray(array);

    int leftIndex = 0;
    int rightIndex = array.length - 1;

    while (leftIndex < rightIndex) {
        leftIndex = findNextOddIndex(array, leftIndex, rightIndex);
        rightIndex = findNextEvenIndex(array, rightIndex, leftIndex);
        swapElements(array, leftIndex, rightIndex);
    }

    return array;
}

/**
 * Validates the input array to ensure it is not null.
 *
 * @param array The input array to be validated.
 * @throws IllegalArgumentException If the input array is null.
 */
private static void validateInputArray(int[] array) {
    if (array == null) {
        throw new IllegalArgumentException("Input array cannot be null.");
    }
}

/**
 * Finds the next odd index in the array starting from the given startIndex.
 *
 * @param array      The input array to search for the next odd index.
 * @param startIndex The starting index for searching.
 * @param endIndex   The ending index for searching.
 * @return The next odd index.
 */
private static int findNextOddIndex(int[] array, int startIndex, int endIndex) {
    while (startIndex < endIndex && isEven(array[startIndex])) {
        startIndex++;
    }
    return startIndex;
}

/**
 * Finds the next even index in the array starting from the given endIndex.
 *
 * @param array      The input array to search for the next even index.
 * @param startIndex The starting index for searching.
 * @param endIndex   The ending index for searching.
 * @return The next even index.
 */
private static int findNextEvenIndex(int[] array, int startIndex, int endIndex) {
    while (startIndex < endIndex && !isEven(array[startIndex])) {
        endIndex--;
    }
    return endIndex;
}

/**
 * Checks if a number is even.
 *
 * @param number The number to be checked.
 * @return {@code true} if the number is even, {@code false} otherwise.
 */
private static boolean isEven(int number) {
    return number % 2 == 0;
}

/**
 * Swaps the elements at the given indices in the array.
 *
 * @param array      The array in which elements are to be swapped.
 * @param leftIndex  The index of the left element to be swapped.
 * @param rightIndex The index of the right element to be swapped.
 */
private static void swapElements(int[] array, int leftIndex, int rightIndex) {
    int temp = array[leftIndex];
    array[leftIndex] = array[rightIndex];
    array[rightIndex] = temp;
}

快速试错 Rapid Iterative Testing and Evaluation (RITE)
- 找到问题的根源，在根源上改。 think before you code, don’t make adhoc change or tweak some if condition without much thinking
- 重复不利于试错不收敛,程序员最懒惰，工作为了不工作 DRY (Don’t Repeat Yourself) principle
- 一切都是代码，ut和ft也是代码需要review.”Talk is cheap. Show me the code.” – Linus Torvalds
没有绝对的正确，核心是我们怎么妥协 “There are no solutions, only tradeoffs - Thomas Sowell”
- 做一个有态度的 reviewer - Having a clear direction helps us converge quickly.
  - code review 包含了主观的偏好 subjective [IMO] objective[FACT]
  - senior 一定是对的吗 I disagree with your disagreement
  - Let’s appeal to rules that all team member respect （We Make Up the Rules as We Go Along）

Replace Conditional with Polymorphis

什么是条件表达
- if elseif else， switch case

为什么条件表达多了不好

无法一眼看清 nested condition 更差
不好修改，修改太分散（shotgun surgery 散弹式修改）有时需要在好几个else if里面加同样的代码

方法容易写太长，这样方法也难复用了

how to make it better

class Employee {
    int payAmount(RoleType type){
        switch(type){
            case ENGINEER:
                return getMonthlySalary();
            case SALESMAN:
                return getMonthlySalary() + getMonthlyCommission();
            case MANAGER:
                return getMonthlySalary() + getBonus();
            default:
                throw new RuntimeException("Invalid employee role type");
        }
    }

}

it works but we can do better:

abstract class Employee{
    
    abstract int payAmount();
}

class Engineer extends Employee{
    int payAmount(){
        return getMonthlySalary();
    }
}
class Salesman extends Employee{
    int payAmount(){
        return getMonthlySalary() + getMonthlyCommission();
    }
}
class Manager extends Employee{
    int payAmount(){
        return getMonthlySalary() + getBonus();
    }
}

Improve Conditional Expression

prefer positive checks
- IsEmpty() NOT IsNotAllEmpty()
- hasContent() NOT HasNotAnyContent()
use gaurd and fail fast
- [NOT] garbage in , garbage out.
- throw error and exception, stop return.
每个方法只做一件事，这么多if是不是因为想做的太多
- 也有可能是level 太多，把低level的abstract出去，形成一个方法。比如和persist相关的。和mq相关的
- 3层最多了别再多了

Be proactive

主动去学，既然像写文章。那肯定是多看好的少看bad code。看多了你也开始这么写了。
那去看什么？
- 大牛写的不适合你他们的代码也很底层你模仿不来
- github上高星的项目都是大公司开源的可以通用的工具或平台 kafka spark flink。你写业务代码和这些优秀的强关注performance的项目目标就不太一致
- 那看点儿啥？你的代码是搭载什么上边的你和平台的风格靠拢。你在别人的项目相关下工作，和别人写的风格靠拢。别突出自己的不同。
  - https://github.com/spring-projects/spring-petclinic spring 自己提供的一个他们觉得要这么写比较的好的事例应用。宠物诊所
  - Spring Music（https://github.com/cloudfoundry-samples/spring-music）：一个使用Spring Boot构建的音乐存储和检索示例应用程序。
  - Spring RESTbucks（https://github.com/restbucks/restbucks-java-spring）：一个使用Spring Boot构建的咖啡订购和配送系统示例。
  - Spring Trader（https://github.com/spring-projects/spring-trade）：一个使用Spring Boot构建的股票交易示例程序，展示了金融交易相关的业务逻辑。
  - Spring PetClinic Microservices（https://github.com/spring-petclinic/spring-petclinic-microservices）： Spring PetClinic的微服务版本，它将业务功能划分为多个微服务。