5 Reasons To Live Composition Over Inheritance Inward Coffee Too Oop
Wednesday, September 19, 2018
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Favor composition over inheritance is a i of the pop object-oriented pattern principles, which helps to create flexible in addition to maintainable code inwards Java in addition to other object-oriented languages. Many times I direct hold seen people suggesting exercise composition instead of inheritance, inwards fact, my favorite books similar Head start Design Patterns too advocates this pattern principle. Head start books direct hold its ain agency of explaining, why composition is meliorate than inheritance in addition to though its long its quite interesting in addition to informative. It was the start chapter of this book, which helped me a lot on agreement this fundamental OOPS concept. In this Java in addition to OOPS tutorial, I direct hold tried to summarize my sense or hence composition in addition to inheritance inwards ii primary category, first, deviation betwixt composition in addition to inheritance in addition to second, when to exercise Composition over inheritance inwards Java.
I direct hold already mentioned almost this pattern principle, inwards my listing of 10 OOPS in addition to SOLID pattern principles for Java programmers, hither nosotros volition bring a to a greater extent than closer look.
Just to revise, composition in addition to Inheritance are ways to reuse code to instruct additional functionality. In Inheritance, a novel class, which wants to reuse code, inherit an existing class, known every bit super class. This novel course of pedagogy is in addition to hence known every bit sub class.
Because of IS-A relationship, an instance of sub course of pedagogy tin dismiss endure passed to a method, which accepts instance of super class. This is a sort of Polymorphism, which is achieved using Inheritance.
Influenza A virus subtype H5N1 super course of pedagogy reference variable tin dismiss refer to an instance of sub class. By using composition, you lot don’t instruct this behavior, but nonetheless it offers a lot to a greater extent than to tilde the residual inwards its side.
1) One argue of favoring Composition over Inheritance inwards Java is fact that Java doesn't back upward multiple inheritance. Since you lot tin dismiss exclusively extend i course of pedagogy inwards Java, but if you lot demand multiple functionality similar e.g. for reading in addition to writing grapheme information into file, you lot demand Reader in addition to Writer functionality in addition to having them every bit mortal members makes your chore easy. That’s called composition. If you lot are next programming for interface than implementation principle, in addition to using type of base of operations course of pedagogy every bit fellow member variable, you lot tin dismiss exercise unlike Reader in addition to Writer implementation at unlike situation. You won’t instruct this flexibility past times using Inheritance, inwards instance of extending a class, you lot exclusively instruct facilities which are available at compile time.
2) Composition offers meliorate test-ability of a course of pedagogy than Inheritance. If i course of pedagogy is composed of some other class, you lot tin dismiss easily create Mock Object representing composed course of pedagogy for sake of testing. Inheritance doesn't render this luxury. In gild to examine derived class, you lot must demand its super class. Since unit of measurement testing is i of the most of import affair to consider during software development, peculiarly inwards examine driven development, composition wins over inheritance.
3) Many object oriented pattern patterns mentioned past times Gang of Four inwards in that place timeless classic Design Patterns: Elements of Reusable Object-Oriented Software, favors Composition over Inheritance. Classical examples of this is Strategy pattern pattern, where composition in addition to delegation is used to alter Context’s behavior, without touching context code. Since Context uses composition to concur strategy, instead of getting it via inheritance, it’s slow to render a novel Strategy implementation at run-time. Another skilful instance of using composition over inheritance is Decorator pattern pattern. In Decorator pattern, nosotros don't extend whatsoever course of pedagogy to add together additional functionality, instead nosotros proceed an instance of the course of pedagogy nosotros are decorating in addition to delegates original labor to that course of pedagogy later doing decoration. This is i of the biggest proof of choosing composition over inheritance, since these pattern patterns are good tried in addition to tested inwards unlike scenarios in addition to withstand examine of time, keeping in that place caput high.
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I direct hold already mentioned almost this pattern principle, inwards my listing of 10 OOPS in addition to SOLID pattern principles for Java programmers, hither nosotros volition bring a to a greater extent than closer look.
Just to revise, composition in addition to Inheritance are ways to reuse code to instruct additional functionality. In Inheritance, a novel class, which wants to reuse code, inherit an existing class, known every bit super class. This novel course of pedagogy is in addition to hence known every bit sub class.
5 Reasons to Prefer Composition over Inheritance inwards Java
On composition, a class, which wish to exercise functionality of an existing class, doesn't inherit, instead it holds a reference of that course of pedagogy inwards a fellow member variable, that’s why the call composition. Inheritance in addition to composition relationships are too referred every bit IS-A in addition to HAS-A relationships.Because of IS-A relationship, an instance of sub course of pedagogy tin dismiss endure passed to a method, which accepts instance of super class. This is a sort of Polymorphism, which is achieved using Inheritance.
Influenza A virus subtype H5N1 super course of pedagogy reference variable tin dismiss refer to an instance of sub class. By using composition, you lot don’t instruct this behavior, but nonetheless it offers a lot to a greater extent than to tilde the residual inwards its side.
1) One argue of favoring Composition over Inheritance inwards Java is fact that Java doesn't back upward multiple inheritance. Since you lot tin dismiss exclusively extend i course of pedagogy inwards Java, but if you lot demand multiple functionality similar e.g. for reading in addition to writing grapheme information into file, you lot demand Reader in addition to Writer functionality in addition to having them every bit mortal members makes your chore easy. That’s called composition. If you lot are next programming for interface than implementation principle, in addition to using type of base of operations course of pedagogy every bit fellow member variable, you lot tin dismiss exercise unlike Reader in addition to Writer implementation at unlike situation. You won’t instruct this flexibility past times using Inheritance, inwards instance of extending a class, you lot exclusively instruct facilities which are available at compile time.
2) Composition offers meliorate test-ability of a course of pedagogy than Inheritance. If i course of pedagogy is composed of some other class, you lot tin dismiss easily create Mock Object representing composed course of pedagogy for sake of testing. Inheritance doesn't render this luxury. In gild to examine derived class, you lot must demand its super class. Since unit of measurement testing is i of the most of import affair to consider during software development, peculiarly inwards examine driven development, composition wins over inheritance.
3) Many object oriented pattern patterns mentioned past times Gang of Four inwards in that place timeless classic Design Patterns: Elements of Reusable Object-Oriented Software, favors Composition over Inheritance. Classical examples of this is Strategy pattern pattern, where composition in addition to delegation is used to alter Context’s behavior, without touching context code. Since Context uses composition to concur strategy, instead of getting it via inheritance, it’s slow to render a novel Strategy implementation at run-time. Another skilful instance of using composition over inheritance is Decorator pattern pattern. In Decorator pattern, nosotros don't extend whatsoever course of pedagogy to add together additional functionality, instead nosotros proceed an instance of the course of pedagogy nosotros are decorating in addition to delegates original labor to that course of pedagogy later doing decoration. This is i of the biggest proof of choosing composition over inheritance, since these pattern patterns are good tried in addition to tested inwards unlike scenarios in addition to withstand examine of time, keeping in that place caput high.
4) Though both Composition in addition to Inheritance allows you lot to reuse code, i of the disadvantage of Inheritance is that it breaks encapsulation. If sub course of pedagogy is depending on super course of pedagogy deportment for its operation, it all of a abrupt becomes fragile. When deportment of super course of pedagogy changes, functionality inwards sub course of pedagogy may instruct broken, without whatsoever alter on its part. One instance of inheritance making code frail is method add() in addition to addAll() from HashSet. Suppose, If addAll() of HashSet is implemented past times calling add() method in addition to you lot write a sub course of pedagogy of HashSet, which encrypt the content earlier inserting into HashSet. Since in that place are exclusively i methods add(), which tin dismiss insert object into HashSet you lot override these method in addition to called your encrypt() method past times overriding add(). This automatically covers addAll() every bit well, because addAll() is implemented using add(), it looks real enticing.If you lot hold off closely you lot volition come across that this implementation is fragile, because its relied on super course of pedagogy behavior. If base of operations course of pedagogy wants to improve performance in addition to implements addAll() without calling add() method, next instance volition break.
If you lot direct hold used Composition inwards favor of Inheritance you lot won't human face upward this employment in addition to your course of pedagogy would direct hold been to a greater extent than robust, because you lot are non relying on super course of pedagogy deportment whatsoever more. Instead you lot are using super course of pedagogy method for improver constituent in addition to you lot volition exercise goodness amongst whatsoever improvement inwards addAll() every bit shown inwards below example:
In short, don't exercise Inheritance merely for the sake of code reuse, Composition allows to a greater extent than flexible in addition to extensible machinery to reuse code. Make sure you lot don't forget this rule, if you lot don't to exercise this :) 
5. Another argue of favoring Composition over inheritance is flexibility. If you lot exercise Composition you lot are flexible plenty to supervene upon implementation of Composed course of pedagogy amongst meliorate in addition to improved version. One instance is using Comparator class which provides compare functionality. if your Container object contains a Comparator instead of extending a detail Comparator for comparing , its easier to alter the agency comparing is performed past times setting unlike type of Comparator to composed instance, spell inwards instance of inheritance you lot tin dismiss exclusively direct hold i comparing deportment on runtime, You tin dismiss non alter it runtime.
There are many to a greater extent than reasons to favor Composition over inheritance, which you lot volition start discovering i time you lot start using pattern patterns inwards Java. In nutshell favoring Composition results inwards to a greater extent than flexible in addition to robust code than using Inheritance. Though in that place are sure enough some cases where using Inheritance makes much sense similar when a genuine bring upward tyke relation exists, but most of fourth dimension it makes sense to favor composition over inheritance for code reuse. There is an item of this theme on my some other favorite book, Effective Java, which has too helped me to sympathize this concept better, you lot may wish to hold off that every bit well.
Further Learning
Design Pattern Library
From 0 to 1: Design Patterns - 24 That Matter - In Java
Java Design Patterns - The Complete Masterclass
public class EncryptedHashSet extends HashSet{ ..... public boolean add(Object o) { return super.add(encrypt(o)); } }
public class EncryptedHashSet implements Set{ private HashSet container; public boolean add(Object o) { return container.add(encrypt(o)); } public boolean addAll(Collection c) { return conatainer.add(encrypt(c)); } ....... }
5. Another argue of favoring Composition over inheritance is flexibility. If you lot exercise Composition you lot are flexible plenty to supervene upon implementation of Composed course of pedagogy amongst meliorate in addition to improved version. One instance is using Comparator class which provides compare functionality. if your Container object contains a Comparator instead of extending a detail Comparator for comparing , its easier to alter the agency comparing is performed past times setting unlike type of Comparator to composed instance, spell inwards instance of inheritance you lot tin dismiss exclusively direct hold i comparing deportment on runtime, You tin dismiss non alter it runtime.
There are many to a greater extent than reasons to favor Composition over inheritance, which you lot volition start discovering i time you lot start using pattern patterns inwards Java. In nutshell favoring Composition results inwards to a greater extent than flexible in addition to robust code than using Inheritance. Though in that place are sure enough some cases where using Inheritance makes much sense similar when a genuine bring upward tyke relation exists, but most of fourth dimension it makes sense to favor composition over inheritance for code reuse. There is an item of this theme on my some other favorite book, Effective Java, which has too helped me to sympathize this concept better, you lot may wish to hold off that every bit well.
Further Learning
Design Pattern Library
From 0 to 1: Design Patterns - 24 That Matter - In Java
Java Design Patterns - The Complete Masterclass
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