Object-Oriented Design Principles PMD (1) ● A static source code analyzer (written in: Java; license: BSD-style) https://pmd.github.io/ Péter Jeszenszky https://github.com/pmd/pmd Faculty of Informatics, University of Debrecen – Supported programming languages: ECMAScript [email protected] (JavaScript), Java, Scala, … Last modified: April 16, 2021 3 Static Code Analysis PMD (2) ● Static code analysis is the process of analyzing program ● Tool integration: code without executing it. – Apache Maven: ● Apache Maven PMD Plugin (license: Apache License 2.0) – Analysis can be aimed at: detecting errors and bugs, checking https://maven.apache.org/plugins/maven-pmd-plugin/ whether the code conforms to a coding standard, … – Gradle: ● The PMD Plugin (license: Apache License 2.0) ● Static code analyzer, static code analysis tool: an automated https://docs.gradle.org/current/userguide/pmd_plugin.html tool for performing static code analysis. – Eclipse IDE: ● pmd-eclipse-plugin (license: BSD-style) https://marketplace.eclipse.org/content/pmd-eclipse-plugin – Examples: https://github.com/pmd/pmd-eclipse-plugin ● PMD Plug-in (license: Eclipse Public License 2.0) https://acanda.github.io/eclipse-pmd/ ● Checkstyle https://checkstyle.org/ https://github.com/checkstyle/checkstyle https://github.com/acanda/eclipse-pmd/ ● PMD https://pmd.github.io/ https://github.com/pmd/pmd – IntelliJ IDEA: ● SpotBugs https://spotbugs.github.io/ https://github.com/spotbugs/spotbugs ● PMDPlugin (license: MIT License) https://plugins.jetbrains.com/plugin/1137-pmdplugin https://github.com/amitdev/PMD-Intellij ● QAPlug (license: proprietary) https://qaplug.com/ 2 4 DRY (1) DRY (3) ● Don't Repeat Yourself ● Categories of duplication: – – Imposed duplication: Developers feel they have no choice – “Every piece of knowledge must have a single, the environment seems to require duplication. unambiguous, authoritative representation within a – Inadvertent duplication: Developers don’t realize that they are system.” duplicating information. ● The opposite of DRY is WET: – Impatient duplication: Developers get lazy and duplicate because it seems easier. – “We enjoy typing”, “write everything twice”, “waste – Interdeveloper duplication: Multiple people on a team (or on everyone's time”, … different teams) duplicate a piece of information. ● Related concept: code duplication, duplicate code, copy- and-paste programming 5 7 DRY (2) DRY (4) ● Source: ● Duplicate code is identical (or very similar) – Andrew Hunt, David Thomas. The Pragmatic piece of source code that occurs more than Programmer: From Journeyman to Master. once in a program. Addison-Wesley, 1999. ● Not all code duplication is knowledge – David Thomas, Andrew Hunt. The Pragmatic duplication! Programmer: Your Journey to Mastery, 20th Anniversary Edition. Addison Wesley, 2019. https://pragprog.com/book/tpp20/the-pragmatic-pro grammer-20th-anniversary-edition ● Free chapter: DRY – The Evils of Duplication https://media.pragprog.com/titles/tpp20/dry.pdf 6 8 DRY (5) DRY (7) ● PMD support: Copy/Paste Detector (CPD) ● Example (continued): – Finding duplicated code with CPD – The violation can be eliminated by replacing the https://pmd.github.io/latest/pmd_userdocs_cpd.html length field with a method: – Supported programming languages: C/C++, C#, class Line { ECMAScript (JavaScript), Java, Kotlin, Python, Point start; Scala, … Point end; ● See: double length() { https://pmd.github.io/latest/pmd_userdocs_cpd.html#sup ported-languages return start.distanceTo(end); } } 9 11 DRY (6) DRY (8) ● DRY violations do not always take the form of ● Example (continued): duplicate code. – For performance reasons, one may choose to – DRY is about the duplication of knowledge. The violate the DRY principle. same piece of knowledge can be expressed in two ● In this case, the violation should be hidden from the totally different ways in two different places. outside world. – Example (Thomas & Hunt, 2019): class Line { Point start; Point end; double length; // DRY violation } 10 12 class Line { private Point start; private Point end; DRY (11) private double length; public Line(Point start, Point end) { this.start = start; ● Representational duplication (Thomas & Hunt, this.end = end; calculateLength(); 2019): } – public void setStart(Point p) { This duplication is inevitable. this.start = p; calculateLength(); – Tools that help to cope with this kind of duplication: } ● Tools that generate code from schemas (e.g., JAXB, JPA) public void setEnd(Point p) { ● this.end = p; OpenAPI calculateLength(); ● } … public Point getStart() { return start; } public Point getEnd() { return end; } public double getLength() { return length; } private void calculateLength() { this.length = start.distanceTo(end); }13 15 } DRY (10) KISS ● Representational duplication (Thomas & Hunt, ● “Keep it simple, stupid” 2019): – 1960s, U.S. Navy – Code often depends on the outside world: e.g., – The phrase is attributed to aeronautical engineer Kelly other libraries via APIs, data in external data Johnson (1910–1990). sources, that always introduces some kind of DRY ● The pursuit of simplicity: violation: code has to have knowledge that is also – Leonardo da Vinci (1452–1519): “Simplicity is the ultimate present in the external thing. sophistication.” ● It needs to know the API, or the schema, or the meaning – Ludwig Mies van der Rohe (1886–1969): “Less is more.” of error codes. – Albert Einstein (1879–1955): “Everything should be made as simple as possible, but not simpler.” 14 16 YAGNI (1) YAGNI (3) ● An acronym that stands for “You Aren't Gonna ● The costs of developing a feature that is not Need It”. need now (Martin Fowler): ● A principle of extreme programming (XP). 17 19 YAGNI (2) YAGNI (4) ● “Always implement things when you actually ● YAGNI only applies to capabilities built into the need them, never when you just foresee that software to support a presumptive feature, it you need them.” does not apply to effort to make the software – See: Ronald E. Jeffries. You’re NOT gonna need it! easier to modify. Apr 4, 1998. ● YAGNI is only a viable strategy if the code is https://ronjeffries.com/xprog/articles/practices/pracn otneed/ easy to change. ● See also: – Martin Fowler. Yagni. 26 May 2015. https://martinfowler.com/bliki/Yagni.html 18 20 Coupling (1) Coupling (3) ● Coupling: the degree to which a software ● Loose coupling: module depends on another software module. – It allows developers to write code that conforms to – In other words, coupling between software modules the open-closed principle (OCP), i.e., it makes code is a measure of how closely connected they are. extensible. – Coupling can either be loose or tight. – It makes code extensible, and extensibility makes it maintainable. ● Reference: – It allows parallel development. – Joseph Ingeno. Software Architect's Handbook. Packt Publishing, 2018. https://www.packtpub.com/application-development/ software-architects-handbook 21 23 Coupling (2) Law of Demeter (1) ● Tight coupling: ● The Law of Demeter was proposed by Ian Holland in 1987. – It increases complexity that makes modification of code more difficult, thus, it reduces maintainability. ● The law was named after the Demeter project in – It also reduces reusability. which Holland was working when he discovered it. ● Also known as: Don't Talk to Strangers – Craig Larman. Applying UML and Patterns: An Introduction to Object-Oriented Analysis and Design and Iterative Development. 3rd ed. Prentice Hall, 2005. 22 24 Law of Demeter (2) Law of Demeter (4) ● References: ● Class form: – Karl J. Lieberherr, Ian M. Holland, Arthur Joseph Riel. – A method M of class C may use only members Object-Oriented Programming: An Objective Sense of Style. (methods and data) of the following classes and their Proceedings on Object-oriented programming systems, base classes: languages and applications (OOPSLA), pp. 323– 334, 1988. https://doi.org/10.1145/62084.62113 ● C ● data-member classes of C – Ian M. Holland, Karl J. Lieberherr. Assuring Good Style for Object-Oriented Programs. IEEE Software, vol. 6, no 5, pp. ● argument classes of M 38– 48, 1989. https://doi.org/10.1109/52.35588 ● classes whose constructors are called in M – Karl Lieberherr. Law of Demeter: Principle of Least ● the classes of global variables used in M Knowledge. http://www.ccs.neu.edu/home/lieber/LoD.html – Can be checked at compile time. 25 27 Law of Demeter (3) Law of Demeter (5) ● Restricts the message-sending structure of ● Object form: methods. – A method M of object O may use only members – Informally, the law says that each method can send (methods and data) of the following objects: messages to only a limited set of objects. ● O ● The goal of the Law of Demeter is to organize ● data members of O and reduce the dependencies between classes. ● argument objects of M ● objects that are created/instantiated directly in M ● objects in global variables – Can be checked only at runtime. 26 28 Law of Demeter (6) Law of Demeter (8) ● The law promotes maintainability and ● Related PMD ruleset: comprehensibility. – Design (Java) – It effectively reduces the methods that can can be https://pmd.github.io/latest/pmd_rules_java_design. called inside a given method and therefore limits html the coupling of methods. ● See the LawOfDemeter rule: https://pmd.github.io/latest/pmd_rules_java_design.html#l – It enforces one kind of information hiding (structure awofdemeter hiding): the internal structure of an object is known only by the object itself. 29 31 Law of Demeter (7) Law of Demeter (9) ●