关于代码风格

良好的代码风格能提高代码可读性及可维护性，程序员都应该遵守基本的代码排版规范、代码命名规范。

首先看看以下三张图，在没有代码的情况下能猜猜各是什么开发语言。
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第一张图是CSS，可以看到清晰的选择器和属性键值对。
第二张图是HTML，可以看到清晰的head和body定义。
第三张图是Java，可以看到头部imports，类定义，成员变量，构造函数以及其他方法。
图片来自：https://schneide.wordpress.com/2015/04/26/the-typography-of-source-code/

各个语言都有自己的规范，开发所用的各种IDE也都内置了代码格式化工具，比如Eclipse对Java和JavaScript代码的标准格式化定义：





也可以通过 Checkstyle 检查Java代码的规范性。

这里罗列几个最有争议的。

（1）缩进

使用Tab 还是空格？2个空格还是4个空格？具体关于这个网上的争论太多了，但是需要特别注意的是像Python和CoffeeScript等是通过缩进来控制代码逻辑。



https://softwareengineering.stackexchange.com/questions/57/tabs-versus-spaces-what-is-the-proper-indentation-character-for-everything-in-e
https://medium.com/@hoffa/400-000-github-repositories-1-billion-files-14-terabytes-of-code-spaces-or-tabs-7cfe0b5dd7fd

（2）换行

大括号是否应该换行？

if (you.hasAnswer()) {
    you.postAnswer();
} else {
    you.doSomething();
}

if (you.hasAnswer())
{
    you.postAnswer();
}
else
{
    you.doSomething();
}

if (you.hasAnswer())
    you.postAnswer();
else
    you.doSomething();

https://softwareengineering.stackexchange.com/questions/2715/should-curly-braces-appear-on-their-own-line

大括号是否应该省略？

for (int i = 0; i < size; i++)  {
   a += b;
}

for (int i = 0; i < size; i++)  
   a += b;

for (int i = 0; i < size; i++) a += b;

https://stackoverflow.com/questions/8020228/is-it-ok-if-i-omit-curly-braces-in-java

（3）对齐

赋值等号对齐

int a_variable           = 1;
int another_variable     = 2;
int yet_another_variable = 3;

注释对齐

// whole line comment
int linePointer;                            // inline comment
BufferredReader br = new BufferredReader(); // inline comment
File f = new File();                        // inline comment

参数对齐

int some_call_result = some_object.a_method(
        with, 
        quite_long, 
        list_of,
        parameters
    );

链式方法对齐

protected void configure(HttpSecurity http) throws Exception {
  http
    .authorizeRequests()
      .antMatchers("/resources/**", "/signup", "/about").permitAll()
      .antMatchers("/admin/**").hasRole("ADMIN")
      .antMatchers("/db/**").access("hasRole('ADMIN') and hasRole('DBA')")
      .anyRequest().authenticated()
      .and()
    .formLogin();
}

（4）空行

合理的利用空行来区分代码块。

    /**
     * Initialize this servlet.  Most of the processing has been factored
     * into support methods so that you can override particular functionality
     * at a fairly granular level.

     *
     * @throws ServletException if we cannot configure ourselves correctly
     */
    public void init() throws ServletException {
        final String configPrefix = "config/";
        final int configPrefixLength = configPrefix.length() - 1;

        // Wraps the entire initialization in a try/catch to better handle
        // unexpected exceptions and errors to provide better feedback
        // to the developer
        try {
            initInternal();
            initOther();
            initServlet();
            initChain();

            getServletContext().setAttribute(Globals.ACTION_SERVLET_KEY, this);
            initModuleConfigFactory();

            // Initialize modules as needed
            ModuleConfig moduleConfig = initModuleConfig("", config);

            initModuleMessageResources(moduleConfig);
            initModulePlugIns(moduleConfig);
            initModuleFormBeans(moduleConfig);
            initModuleForwards(moduleConfig);
            initModuleExceptionConfigs(moduleConfig);
            initModuleActions(moduleConfig);
            moduleConfig.freeze();

            Enumeration names = getServletConfig().getInitParameterNames();

            while (names.hasMoreElements()) {
                String name = (String) names.nextElement();

                if (!name.startsWith(configPrefix)) {
                    continue;
                }

                String prefix = name.substring(configPrefixLength);

                moduleConfig =
                    initModuleConfig(prefix,
                        getServletConfig().getInitParameter(name));
                initModuleMessageResources(moduleConfig);
                initModulePlugIns(moduleConfig);
                initModuleFormBeans(moduleConfig);
                initModuleForwards(moduleConfig);
                initModuleExceptionConfigs(moduleConfig);
                initModuleActions(moduleConfig);
                moduleConfig.freeze();
            }

            this.initModulePrefixes(this.getServletContext());

            this.destroyConfigDigester();
        } catch (UnavailableException ex) {
            throw ex;
        } catch (Throwable t) {
            // The follow error message is not retrieved from internal message
            // resources as they may not have been able to have been
            // initialized
            log.error("Unable to initialize Struts ActionServlet due to an "
                + "unexpected exception or error thrown, so marking the "
                + "servlet as unavailable.  Most likely, this is due to an "
                + "incorrect or missing library dependency.", t);
            throw new UnavailableException(t.getMessage());
        }
    }

（5）命名

违反命名规范的例子有很多，这里列2个：

非驼峰命名

package com.test.EC_shop;

public class db_manager {
    private String connection_url;

    public void Init() {
       // ...
    }

}

连续编号

public class UC8010 extends CMN0101 {
    private String FLD0001;

    public int MTD0001() {
       // ...
    }

}
publi class UC8020 extends CMN0101 {
    private String FLD0001;

    public int MTD0001() {
       // ...
    }

}

再列举一些代码书写上的其他Style。

（1）多层嵌套 还是 中途退出？

public String getInsuranceName(Employee employee) {
    if (employee != null) {
        Car car = employee.getCar();
        if (car != null) {
            Insurance insurance = car.getInsurance();
            if (insurance != null) {
                return insurance.getName();
            }
        }
    }
    return "UNKNOWN";
}

public String getInsuranceName(Employee employee) {
    if (employee == null) {
        return "UNKNOWN";
    }
    Car car = employee.getCar();
    if (car == null) {
        return "UNKNOWN";
    }
    Insurance insurance = car.getInsurance();
    if (insurance == null) {
        return "UNKNOWN";
    }
    return insurance.getName();
}

（2）常量放在左侧还是右侧？

if (currentValue == 5) {
    // do work
}
if (5 == currentValue) {
    // do work
}

if (obj == null) {
}
if (null == obj) {
}
if (obj = null) { // 赋值运算、NPE异常
}
if (null = obj) { // 编译错误
}

https://stackoverflow.com/questions/2369226/object-null-or-null-object

（3）定义接口类型还是实现类型？

List<Integer> sampleList = new ArrayList<Integer>();

ArrayList<Integer> sampleList = new ArrayList<Integer>();

（4）标准for循环还是增强for循环？

for (int i = 0; i < peopleList.size(); i++) {
  People p = peopleList.get(i);
  // ..
}

for (People p : peopleList) {
  // ...
}

（5）变量是否需要先定义为null？

Object localVariableObj2 = null;
localVariableObj2 = new Object();

Object localVariableObj2 = new Object();

*** 成员变量有默认值不需要赋值null，本地变量没有默认值需要赋值null。

（6）通过class还是interface定义全局常量？

public final class Constants {
  public static final int PI = 3.14;
}

public interface Constants {
  int PI = 3.14;
}

还有这么定义的：

public interface Constants {
  public static final String TOAST = "toast";
}

这些都是常见的，还有很多。