Object Oriented Design Principles are core of OOPS programming but I have seen most of Java programmer chasing design patterns like Singleton pattern, Decorator pattern or Observer pattern but not putting enough attention on Object oriented analysis and design or following these design principles. I have regularly seen Java programmers and developers of various experience level who either doesn't heard about these OOPS and SOLID design principle or simply doesn't know what benefits a particular design principle offers or how to use these design principle in coding.
Bottom line is always strive for highly cohesive and loosely couple solution, code or design and looking open source code from Apache and Sun are good examples of Java design principles or how design principles should be used in Java coding. Java Development Kit follows several design principle like Factory Pattern in BorderFactory class, Singleton pattern in Runtime class and if you interested more on Java code read Effective Java by Joshua Bloch , a gem by the guy who wrote Java API. My another personal favorite on object oriented design pattern is Head First Design Pattern by Kathy Sierra and others and Head First Object Oriented Analysis and Design.
Object oriented design principle 1 - DRY (Don't repeat yourself)
As name suggest DRY (don't repeat yourself) means don't write duplicate code, instead use abstraction to abstract common things in one place. if you use a hardcoded value more than one time consider making it public final constant, if you have block of code in more than two place consider making it a separate method. Benefit of this SOLID design principle is in maintenance. Its worth to note is don’t abuse it, duplicate is not for code but for functionality means if you used common code to validate OrderID and SSN it doesn’t mean they are same or they will remain same in future. By using common code for two different functionality or thing you closely couple them forever and when your OrderID changes its format, your SSN validation code will break. So be aware of such coupling and just don’t combine anything which uses similar code but are not related.
Object oriented design principle 2 - Encapsulate what varies
Only one thing is constant in software field and that is "Change", So encapsulate the code you expect or suspect to be changed in future. Benefit of this OOPS Design principle is that Its easy to test and maintain proper encapsulated code. If you are coding in Java then follow principle of making variable and methods private by default and increasing access step by step e.g. from private to protected and not public. Several of design pattern in Java uses Encapsulation, Factory design pattern is one example of Encapsulation which encapsulate object creation code and provides flexibility to introduce new product later with no impact on existing code.
Object oriented design principle 3 - Open Closed principle
Classes, methods or functions should be Open for extension (new functionality) and Closed for modification. This is another beautiful object oriented design principle which prevents some-one from changing already tried and tested code. Ideally if you are adding new functionality only than your code should be tested and that's the goal of Open Closed Design principle.
Object oriented design principle 4 - Single Responsibility Principle (SRP)
There should not be more than one reason for a class to change or a class should always handle single functionality. If you put more than one functionality in one Class in Java it introduce coupling between two functionality and even if you change one functionality there is chance you broke coupled functionality which require another round of testing to avoid any surprise on production environment.
Object oriented design principle 5 - Dependency Injection or Inversion principle
Don't ask for dependency it will be provided to you by framework. This has been very well implemented in Spring framework, beauty of this design principle is that any class which is injected by DI framework is easy to test with mock object and easier to maintain because object creation code is centralized in framework and client code is not littered with that.There are multiple ways to implemented Dependency injection like using byte code instrumentation which some AOP (Aspect Oriented programming) framework like AspectJ does or by using proxies just like used in Spring.
Object oriented design principle 6 - Favour Composition over Inheritance
Always favour composition over inheritance if possible. Some of you may argue this but I found that Composition is lot more flexible than Inheritance. Composition allows to change behaviour of a class at runtime by setting property during runtime and by using Interfaces to compose a class we use polymorphism which provides flexibility of to replace with better implementation any time. Even Effective Java advise to favor composition over inheritance.
Object oriented design principle 7 - Liskov Substitution Principle (LSP)
According to Liskov Substitution Principle Subtypes must be substitutable for super type i.e. methods or functions which uses super class type must be able to work with object of sub class without any issue". LSP is closely related to Single responsibility principle and Interface Segregation Principle. If a class has more functionality than subclass might not support some of the functionality and does violated LSP. In order to follow LSP design principle, derived class or sub class must enhance functionality not reducing it.
Object oriented design principle 8 - Interface Segregation principle (ISP)
Interface Segregation Principle stats that a client should not implement an interface if it doesn't use that. this happens mostly when one interface contains more than one functionality and client only need one functionality and not other.Interface design is tricky job because once you release your interface you can not change it without breaking all implementation. Another benefit of this desing principle in Java is, interface has disadvantage to implement all method before any class can use it so having single functionality means less method to implement.
Object oriented design principle 9 - Programming for Interface not implementation
Always program for interface and not for implementation this will lead to flexible code which can work with any new implementation of interface. So use interface type on variables, return types of method or argument type of methods in Java. This has been advised by many Java programmer including in Effective Java and head first design pattern book.
Object oriented design principle 10 - Delegation principle
Don't do all stuff by yourself, delegate it to respective class. Classical example of delegation design principle is equals() and hashCode() method in Java. In order to compare two object for equality we ask class itself to do comparison instead of Client class doing that check. Benefit of this design principle is no duplication of code and pretty easy to modify behaviour.
All these object oriented design principle helps you write flexible and better code by striving high cohesion and low coupling. Theory is first step but what is most important is to develop ability to find out when and to apply these design principle and find our whether we are violating any design principle and compromising flexibility of code. but again as nothing is perfect in this world, don't always try to solve problem with design patterns and design principle they are mostly for large enterprise project which has longer maintenance cycle.
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