Chapter 19 Software Trends

As a way of illustrating the principles discussed in this book, this chapter considers several trends and patterns that have become popular in software development over the last few decades. For each trend, I will describe how that trend relates to the principles in this book and use the principles to evaluate whether that trend provides leverage against software complexity.

为了说明本书中讨论的原理，本章考虑了过去几十年来在软件开发中流行的几种趋势和模式。对于每种趋势，我将描述该趋势与本书中的原理之间的关系，并使用这些原理来评估该趋势是否提供了针对软件复杂性的杠杆作用。

19.1 Object-oriented programming and inheritance 面向对象的编程和继承

Object-oriented programming is one of the most important new ideas in software development over the last 30–40 years. It introduced notions such as classes, inheritance, private methods, and instance variables. If used carefully, these mechanisms can help to produce better software designs. For example, private methods and variables can be used to ensure information hiding: no code outside a class can invoke private methods or access private variables, so there can’t be any external dependencies on them.

在过去的 30-40 年中，面向对象编程是软件开发中最重要的新思想之一。它引入了诸如类，继承，私有方法和实例变量之类的概念。如果谨慎使用，这些机制可以帮助产生更好的软件设计。例如，私有方法和变量可用于确保信息隐藏：类外部的任何代码都不能调用私有方法或访问私有变量，所以没有任何外部依赖。

One of the key elements of object-oriented programming is inheritance. Inheritance comes in two forms, which have different implications for software complexity. The first form of inheritance is interface inheritance, in which a parent class defines the signatures for one or more methods, but does not implement the methods. Each subclass must implement the signatures, but different subclasses can implement the same methods in different ways. For example, the interface might define methods for performing I/O; one subclass might implement the I/O operations for disk files, and another subclass might implement the same operations for network sockets.

面向对象编程的关键要素之一是继承。继承有两种形式，它们对软件复杂性有不同的影响。继承的第一种形式是接口继承，其中父类定义一个或多个方法的签名，但不实现这些方法。每个子类都必须实现签名，但是不同的子类可以以不同的方式实现相同的方法。例如，该接口可能定义用于执行 I/O 的方法。一个子类可能对磁盘文件实现 I/O 操作，而另一个子类可能对网络套接字实现相同的操作。

Interface inheritance provides leverage against complexity by reusing the same interface for multiple purposes. It allows knowledge acquired in solving one problem (such as how to use an I/O interface to read and write disk files) to be used to solve other problems (such as communicating over a network socket). Another way of thinking about this is in terms of depth: the more different implementations there are of an interface, the deeper the interface becomes. In order for an interface to have many implementations, it must capture the essential features of all the underlying implementations while steering clear of the details that differ between the implementations; this notion is at the heart of abstraction.

接口继承通过将同一接口用于多种用途，从而提供了对抗复杂性的杠杆作用。它使解决一个问题（例如如何使用 I/O 接口读取和写入磁盘文件）中获得的知识可以用于解决其他问题（例如通过网络套接字进行通信）。关于深度的另一种思考方式是：接口的不同实现越多，接口的深度就越大。为了让一个接口有很多实现，它必须拥有所有底层实现的基本特征，同时避免不同实现之间的差异。这个概念是抽象的核心所在。

The second form of inheritance is implementation inheritance. In this form, a parent class defines not only signatures for one or more methods, but also default implementations. Subclasses can choose to inherit the parent’s implementation of a method or override it by defining a new method with the same signature. Without implementation inheritance, the same method implementation might need to be duplicated in several subclasses, which would create dependencies between those subclasses (modifications would need to be duplicated in all copies of the method). Thus, implementation inheritance reduces the amount of code that needs to be modified as the system evolves; in other words, it reduces the change amplification problem described in Chapter 2.

继承的第二种形式是实现继承。以这种形式，父类不仅定义了一个或多个方法的签名，而且还定义了默认实现。子类可以选择继承方法的父类实现，也可以通过定义具有相同签名的新方法来覆盖它。如果没有实现继承，则可能需要在几个子类中复制相同的方法实现，这将在这些子类之间创建依赖关系（需要在方法的所有副本中复制修改）。因此，实现继承减少了随着系统的发展而需要修改的代码量。换句话说，它减少了第 2 章中描述的变化放大问题。

However, implementation inheritance creates dependencies between the parent class and each of its subclasses. Class instance variables in the parent class are often accessed by both the parent and child classes; this results in information leakage between the classes in the inheritance hierarchy and makes it hard to modify one class in the hierarchy without looking at the others. For example, a developer making changes to the parent class may need to examine all of the subclasses to ensure that the changes don’t break anything. Similarly, if a subclass overrides a method in the parent class, the developer of the subclass may need to examine the implementation in the parent. In the worst case, programmers will need complete knowledge of the entire class hierarchy underneath the parent class in order to make changes to any of the classes. Class hierarchies that use implementation inheritance extensively tend to have high complexity.

但是，实现继承会在父类及其每个子类之间创建依赖关系。父类中的类实例变量经常被父类和子类访问。这导致了继承层次中的类之间的信息泄漏，并且使修改层次中的一个类时很难不考虑其他类。例如，对父类进行修改的开发人员可能需要检查所有子类，以确保所做的修改不会破坏任何内容。同样，如果子类覆盖了父类中的方法，则子类的开发人员可能需要检查父类中的实现。在最坏的情况下，程序员将需要完全了解父类下的整个类层次结构，以便对任何类进行更改。广泛使用实现继承的类层次结构往往具有很高的复杂性。

Thus, implementation inheritance should be used with caution. Before using implementation inheritance, consider whether an approach based on composition can provide the same benefits. For instance, it may be possible to use small helper classes to implement the shared functionality. Rather than inheriting functions from a parent, the original classes can each build upon the features of the helper classes.

因此，应谨慎使用实现继承。在使用实现继承之前，请考虑基于组合的方法是否可以提供相同的好处。例如，可以使用小型辅助类来实现共享功能。与其从父类中继承功能，原始类可以各自建立在辅助类的功能之上。

If there is no viable alternative to implementation inheritance, try to separate the state managed by the parent class from that managed by subclasses. One way to do this is for certain instance variables to be managed entirely by methods in the parent class, with subclasses using them only in a read-only fashion or through other methods in the parent class. This applies the notion of information hiding within the class hierarchy to reduce dependencies.

如果没有实现继承的可行的替代方案，请尝试将父类管理的状态与子类管理的状态分开。一种方法是让某些实例变量完全由父类中的方法管理，子类仅以只读方式或通过父类中的其他方法使用它们。这适用于隐藏在类层次结构中的信息的概念，以减少依赖性。

Although the mechanisms provided by object-oriented programming can assist in implementing clean designs, they do not, by themselves, guarantee good design. For example, if classes are shallow, or have complex interfaces, or permit external access to their internal state, then they will still result in high complexity.

尽管面向对象编程提供的机制可有助于实现干净的设计，但是它们本身不能保证良好的设计。例如，如果类很浅，或者具有复杂的接口，或者允许外部访问其内部状态，那么它们仍将导致很高的复杂性。

19.2 Agile development 敏捷开发

Agile development is an approach to software development that emerged in the late 1990’s from a collection of ideas about how to make software development more lightweight, flexible, and incremental; it was formally defined during a meeting of practitioners in 2001. Agile development is mostly about the process of software development (organizing teams, managing schedules, the role of unit testing, interacting with customers, etc.) as opposed to software design. Nonetheless, it relates to some of the design principles in this book.

敏捷开发是 20 世纪 90 年代末出现的一种软件开发方法，是关于如何使软件开发更加轻量，灵活和渐进的一系列想法。它是在 2001 年的一次从业者会议上正式定义的。敏捷开发主要是关于软件开发的过程（组织团队，管理进度表，单元测试的角色，与客户交互等），而不是软件设计。但是，它与本书中的一些设计原则有关。

One of the most important elements of agile development is the notion that development should be incremental and iterative. In the agile approach, a software system is developed in a series of iterations, each of which adds and evaluates a few new features; each iteration includes design, test, and customer input. In general, this is similar to the incremental approach advocated here. As mentioned in Chapter 1, it isn’t possible to visualize a complex system well enough at the outset of a project to determine the best design. The best way to end up with a good design is to develop a system in increments, where each increment adds a few new abstractions and refactors existing abstractions based on experience. This is similar to the agile development approach.

敏捷开发中最重要的元素之一是开发应该是渐进的和迭代的概念。在敏捷方法中，软件系统是通过一系列迭代开发的，每个迭代都添加并评估了一些新功能。每个迭代都包括设计，测试和客户的意见。通常，这类似于这里提倡的增量方法。如第 1 章所述，在项目开始时就不可能对复杂的系统进行充分的可视化以决定最佳设计。最终获得良好设计的最佳方法是逐步开发一个系统，其中每个增量都会添加一些新的抽象，并根据经验重构现有的抽象。这类似于敏捷开发方法。

One of the risks of agile development is that it can lead to tactical programming. Agile development tends to focus developers on features, not abstractions, and it encourages developers to put off design decisions in order to produce working software as soon as possible. For example, some agile practitioners argue that you shouldn’t implement general-purpose mechanisms right away; implement a minimal special-purpose mechanism to start with, and refactor into something more generic later, once you know that it’s needed. Although these arguments make sense to a degree, they argue against an investment approach, and they encourage a more tactical style of programming. This can result in a rapid accumulation of complexity.

敏捷开发的风险之一是它可能导致战术编程。敏捷开发倾向于将开发人员的注意力集中在功能上，而不是抽象上，它鼓励开发人员推迟设计决策，以便尽快生产可以使用的软件。例如，一些敏捷的实践者认为，您不应该马上实现通用机制；应该先实现一个最小的特殊用途机制，然后在知道需要它时重构为更通用的东西。尽管这些论点在一定程度上是合理的，但它们反对投资方法，并鼓励采用更具战术性的编程风格。这会导致复杂性的快速累积。

Developing incrementally is generally a good idea, but the increments of development should be abstractions, not features. It’s fine to put off all thoughts about a particular abstraction until it’s needed by a feature. Once you need the abstraction, invest the time to design it cleanly; follow the advice of Chapter 6 and make it somewhat general-purpose.

渐进式开发通常是一个好主意，但是软件开发的增量应该是抽象而不是功能。可以推迟对特定抽象的所有想法，直到功能需要它为止。一旦需要抽象，就要花一些时间进行简洁的设计。遵循第 6 章的建议并使其具有通用性。

19.3 Unit tests 单元测试

It used to be that developers rarely wrote tests. If tests were written at all, they were written by a separate QA team. However, one of the tenets of agile development is that testing should be tightly integrated with development, and programmers should write tests for their own code. This practice has now become widespread. Tests are typically divided into two kinds: unit tests and system tests. Unit tests are the ones most often written by developers. They are small and focused: each test usually validates a small section of code in a single method. Unit tests can be run in isolation, without setting up a production environment for the system. Unit tests are often run in conjunction with a test coverage tool to ensure that every line of code in the application is tested. Whenever developers write new code or modify existing code, they are responsible for updating the unit tests to maintain proper test coverage.

过去，开发人员很少编写测试。如果测试是由一个独立的 QA 团队编写的，那么它们就是由一个独立的 QA 团队编写的。然而，敏捷开发的原则之一是测试应该与开发紧密集成，程序员应该为他们自己的代码编写测试。这种做法现在已经很普遍了。测试通常分为两类:单元测试和系统测试。单元测试是开发人员最常编写的测试。它们很小，而且重点突出：每个测试通常在单个方法中验证一小段代码。单元测试可以独立运行，而不需要为系统设置生产环境。单元测试通常与测试覆盖工具一起运行，以确保应用程序中的每一行代码都经过了测试。每当开发人员编写新代码或修改现有代码时，他们都要负责更新单元测试以保持适当的测试覆盖率。

The second kind of test consists of system tests (sometimes called integration tests), which ensure that the different parts of an application all work together properly. They typically involve running the entire application in a production environment. System tests are more likely to be written by a separate QA or testing team.

第二种测试包括系统测试（有时称为集成测试），这些测试可确保应用程序的不同部分都能正常协同工作。它们通常涉及在生产环境中运行整个应用程序。系统测试更有可能由独立的质量检查或测试小组编写。

Tests, particularly unit tests, play an important role in software design because they facilitate refactoring. Without a test suite, it’s dangerous to make major structural changes to a system. There’s no easy way to find bugs, so it’s likely that bugs will go undetected until the new code is deployed, where they are much more expensive to find and fix. As a result, developers avoid refactoring in systems without good test suites; they try to minimize the number of code changes for each new feature or bug fix, which means that complexity accumulates and design mistakes don’t get corrected.

测试，尤其是单元测试，在软件设计中起着重要作用，因为它们有助于重构。没有测试套件，对系统进行重大结构更改很危险。没有简单的方法可以找到错误，因此在部署新代码之前，很可能将无法检测到错误，因为在新代码中查找和修复它们的成本要高得多。结果，开发人员避免在没有良好测试套件的系统中进行重构。他们尝试将每个新功能或错误修复的代码更改次数降至最低，这意味着复杂性会累积，而设计错误不会得到纠正。

With a good set of tests, developers can be more confident when refactoring because the test suite will find most bugs that are introduced. This encourages developers to make structural improvements to a system, which results in a better design. Unit tests are particularly valuable: they provide a higher degree of code coverage than system tests, so they are more likely to uncover any bugs.

有了一套很好的测试，开发人员可以在重构时更有信心，因为测试套件将发现大多数引入的 bug。这鼓励开发人员对系统进行结构改进，从而获得更好的设计。单元测试特别有价值：与系统测试相比，它们提供更高的代码覆盖率，因此它们更有可能发现任何 bug。

For example, during the development of the Tcl scripting language, we decided to improve performance by replacing Tcl’s interpreter with a byte-code compiler. This was a huge change that affected almost every part of the core Tcl engine. Fortunately, Tcl had an excellent unit test suite, which we ran on the new byte-code engine. The existing tests were so effective in uncovering bugs in the new engine that only a single bug turned up after the alpha release of the byte-code compiler.

例如，在开发 Tcl 脚本语言期间，我们决定通过用字节码编译器替换 Tcl 的解释器来提高性能。这是一个巨大的变化，几乎影响了核心 Tcl 引擎的每个部分。幸运的是，Tcl 有一个出色的单元测试套件，我们在新的字节码引擎上运行了该套件。现有测试在发现新引擎中的错误方面是如此有效，以至于在字节码编译器的 alpha 版本发布之后仅出现了一个错误。

19.4 Test-driven development 测试驱动的开发

Test-driven development is an approach to software development where programmers write unit tests before they write code. When creating a new class, the developer first writes unit tests for the class, based on its expected behavior. None of the tests pass, since there is no code for the class. Then the developer works through the tests one at a time, writing enough code for that test to pass. When all of the tests pass, the class is finished.

测试驱动开发是一种软件开发方法，程序员可以在编写代码之前先编写单元测试。创建新类时，开发人员首先根据其预期行为为该类编写单元测试。没有一个测试通过，因为该类没有代码。然后，开发人员一次完成一个测试，编写足够的代码以使该测试通过。所有测试通过后，这个类的功能就完成了。

Although I am a strong advocate of unit testing, I am not a fan of test-driven development. The problem with test-driven development is that it focuses attention on getting specific features working, rather than finding the best design. This is tactical programming pure and simple, with all of its disadvantages. Test-driven development is too incremental: at any point in time, it’s tempting to just hack in the next feature to make the next test pass. There’s no obvious time to do design, so it’s easy to end up with a mess.

尽管我是单元测试的坚决拥护者，但我并不热衷测试驱动的开发。测试驱动开发的问题在于，它将注意力集中在使特定功能正常工作上，而不是寻找最佳设计。这是纯粹的战术性编程，有其所有的弊端。测试驱动的开发过于增量：在任何时间点，很容易完成一个功能然后让测试通过。没有明显的时间来做设计，因此很容易搞的一团糟。

As mentioned in Section 19.2, the units of development should be abstractions, not features. Once you discover the need for an abstraction, don’t create the abstraction in pieces over time; design it all at once (or at least enough to provide a reasonably comprehensive set of core functions). This is more likely to produce a clean design whose pieces fit together well.

如第 19.2 节所述，开发单位应该是抽象的，而不是功能。一旦发现了对抽象的需求，就不要在一段时间内零散的创建抽象，而应该一次性的设计（或至少足以提供一组合理全面的核心功能）。这样更有可能产生干净的设计，使各个部分很好地契合在一起。

One place where it makes sense to write the tests first is when fixing bugs. Before fixing a bug, write a unit test that fails because of the bug. Then fix the bug and make sure that the unit test now passes. This is the best way to make sure you really have fixed the bug. If you fix the bug before writing the test, it’s possible that the new unit test doesn’t actually trigger the bug, in which case it won’t tell you whether you really fixed the problem.

有一个地方先编写测试是有意义的，那就是修复 bug 的时候。在修复错误之前，请编写由于该错误而失败的单元测试。然后修复该错误，并确保现在可以通过单元测试。这是确保您已真正修复该错误的最佳方法。如果您在编写测试之前就已修复了该错误，则新的单元测试很可能实际上不会触发该错误，在这种情况下，它不会告诉您是否真的修复了该问题。

19.5 Design patterns 设计模式

A design pattern is a commonly used approach for solving a particular kind of problem, such as an iterator or an observer. The notion of design patterns was popularized by the book Design Patterns: Elements of Reusable Object-Oriented Software by Gamma, Helm, Johnson, and Vlissides, and design patterns are now widely used in object-oriented software development.

设计模式是解决特定类型问题（例如迭代器或观察器）的常用方法。设计模式的概念在 Gamma，Helm，Johnson 和 Vlissides 的《设计模式：可复用的面向对象软件的基础》一书中而普及，现在设计模式已广泛用于面向对象的软件开发中。

Design patterns represent an alternative to design: rather than designing a new mechanism from scratch, just apply a well-known design pattern. For the most part, this is good: design patterns arose because they solve common problems, and because they are generally agreed to provide clean solutions. If a design pattern works well in a particular situation, it will probably be hard for you to come up with a different approach that is better.

设计模式代表了设计的另一种选择：与其从头设计新机制，不如应用一种众所周知的设计模式。在大多数情况下，这是很好的：设计模式的出现是因为它们解决了常见的问题，并且因为它们被普遍认为提供干净的解决方案。如果设计模式在特定情况下运作良好，那么您可能很难想出另一种更好的方法。

The greatest risk with design patterns is over-application. Not every problem can be solved cleanly with an existing design pattern; don’t try to force a problem into a design pattern when a custom approach will be cleaner. Using design patterns doesn’t automatically improve a software system; it only does so if the design patterns fit. As with many ideas in software design, the notion that design patterns are good doesn’t necessarily mean that more design patterns are better.

设计模式的最大风险是过度使用。不是每个问题都可以用现有的设计模式来解决。当自定义方法更加简洁时，请勿尝试将问题强加到设计模式中。使用设计模式并不能自动改善软件系统。只有在设计模式合适的情况下才会如此。与软件设计中的许多想法一样，设计模式良好的概念并不一定意味着更多的设计模式会更好。

19.6 Getters and setters Getter 和 Setters

In the Java programming community, getter and setter methods are a popular design pattern. A getter and a setter are associated with an instance variable for a class. They have names like getFoo and setFoo, where Foo is the name of the variable. The getter method returns the current value of the variable, and the setter method modifies the value.

在 Java 编程社区中，getter 和 setter 方法是一种流行的设计模式。一个 getter 和一个 setter 与一个类的实例变量相关联。它们具有类似 getFoo 和 setFoo 的名称，其中 Foo 是变量的名称。getter 方法返回变量的当前值，setter 方法修改该值。

Getters and setters aren’t strictly necessary, since instance variables can be made public. The argument for getters and setters is that they allow additional functions to be performed while getting and setting, such as updating related values when a variable changes, notifying listeners of changes, or enforcing constraints on values. Even if these features aren’t needed initially, they can be added later without changing the interface.

由于实例变量可以公开，因此不一定必须使用 getter 和 setter 方法。getter 和 setter 的论点是，它们允许在获取和设置时执行额外功能，例如在变量更改时更新相关值，通知监听器的变化或对值实施约束。即使最初不需要这些功能，以后也可以在不更改界面的情况下添加它们。

Although it may make sense to use getters and setters if you must expose instance variables, it’s better not to expose instance variables in the first place. Exposed instance variables mean that part of the class’s implementation is visible externally, which violates the idea of information hiding and increases the complexity of the class’s interface. Getters and setters are shallow methods (typically only a single line), so they add clutter to the class’s interface without providing much functionality. It’s better to avoid getters and setters (or any exposure of implementation data) as much as possible.

如果必须公开实例变量，则可以使用 getter 和 setter 方法，但最好不要首先公开实例变量。暴露的实例变量意味着类的实现的一部分在外部是可见的，这违反了信息隐藏的思想，并增加了类接口的复杂性。Getter 和 Setter 是浅层方法（通常只有一行），因此它们在不提供太多功能的情况下使类的接口变得混乱。最好避免使用 getter 和 setter（或任何公开的实现数据）。

One of the risks of establishing a design pattern is that developers assume the pattern is good and try to use it as much as possible. This has led to overusage of getters and setters in Java.

建立设计模式的风险之一是开发人员认为该模式是好的，并试图尽可能多地使用它。这导致 Java 中的 getter 和 setter 的过度使用。

19.7 Conclusion 结论

Whenever you encounter a proposal for a new software development paradigm, challenge it from the standpoint of complexity: does the proposal really help to minimize complexity in large software systems? Many proposals sound good on the surface, but if you look more deeply you will see that some of them make complexity worse, not better.

每当您遇到有关新软件开发范例的提案时，就必须从复杂性的角度对其进行挑战：该提案确实有助于最大程度地降低大型软件系统的复杂性吗？许多建议表面上听起来不错，但是如果您深入研究，您会发现其中一些会使复杂性恶化，而不是更好。