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Object-oriented languages divide a program into modules called classes. Each class contains features, which consist of data (fields) and methods. (Not all languages use these terms, but they'll do for this.) Languages have various rules about what other classes can access the features of a class, these are often based on access modifiers that apply to a class.

13 May 2003



Flowing JSON data structures through a server is something I'm seeing more these days. JSON documents can be persisted directly, either by using an AggregateOrientedDatabase or a serialized LOB in a relational database. JSON documents can also be served directly to web browsers or used to transfer data to server-side page renderers. When JSON is being used in this way, I hear people saying that using an object-oriented language gets in the way because the JSON needs to be translated into objects only to be rendered out again - a waste of programming effort . I agree with the point about waste, but I argue that it's not a problem with objects but a failure to understand encapsulation.

4 June 2013



You can tell them by the twitch in the left hand side of the mouth when they see a getter method, there's swift pull on their battleaxe and a satisfied cry as another getter is hewn unmercifully from a class which immediately swoons in an ecstasy of gratefulness at the manly Getter Eradicator's feet.

22 February 2006



This is the opposite attitude to DesignedInheritance. Advocates of open inheritance do not look to disallow inheritance by Sealing their classes or doing anything else to stop people inheriting classes.



One of the most common ways to modularize an information-rich program is to separate it into three broad layers: presentation (UI), domain logic (aka business logic), and data access. So you often see web applications divided into a web layer that knows about handling http requests and rendering HTML, a business logic layer that contains validations and calculations, and a data access layer that sorts out how to manage persistant data in a database or remote services.

26 August 2015



When I first came across C# I liked the notion of properties right from the start. The getX and setX conventions of C++/Java always seems rather silly to me, it's much more natural to write obj.X = other.X. Providing a property with get and set methods turns a common convention into a naturally supported feature of the language.

4 February 2004



Sealing a method or a class prevents subclasses from overriding it.



Tell-Don't-Ask is a principle that helps people remember that object-orientation is about bundling data with the functions that operate on that data. It reminds us that rather than asking an object for data and acting on that data, we should instead tell an object what to do. This encourages to move behavior into an object to go with the data.

5 September 2013



One of the longest running arguments on object-oriented circles is the debate between OpenInheritance and Designed Inheritance. The principle of Designed Inheritance is probably best summed up by Josh Bloch: "Design and document for inheritance or else prohibit it". With this approach you take care to decide which methods can be inherited and Seal the others to stop them being overridden.

6 October 2006



If you learn about object-oriented design, you quickly learn that it's important to encapsulate your data. The simplest form of encapsulation is to use accessors (getting and setting methods) or properties - if your language supports it. (Some even do this within the class - SelfEncapsulation



What do people mean when they say a method doesn't change the observable state of an object?

5 January 2006



When you write tests against a web page, you need to refer to elements within that web page in order to click links and determine what's displayed. However, if you write tests that manipulate the HTML elements directly your tests will be brittle to changes in the UI. A page object wraps an HTML page, or fragment, with an application-specific API, allowing you to manipulate page elements without digging around in the HTML.

10 September 2013



Is it good OO design to have data in my classes with the protected AccessModifier?

14 May 2003



Published Interface is a term I used (first in Refactoring) to refer to a class interface that's used outside the code base that it's defined in. As such it means more than public in Java and indeed even more than a non-internal public in C#. In my column for IEEE Software I argued that the distinction between published and public is actually more important than that between public and private.

26 December 2003



Self Encapsulation is designing your classes so that all access to data, even from within the same class, goes through accessor methods.



All services offered by a module should be available through a uniform notation, which does not betray whether they are implemented through storage or through computation.

-- Bertrand Meyer

Bertrand Meyer coined this principle in his highly-influential book Object-Oriented Software Construction.

The essential point of the principle is that if you have a person object and you ask it for its age, you should use the same notation whether the age is a stored field of the object or a computed value. It effectively means that a client of the person should neither know nor care whether the age is stored or computed.

20 April 2011