Singletons are Pathological Liars. By Miško Hevery So you join a new project, which has an extensive mature code base. Your new lead asks you to implement a new feature, and, as a good developer, you start by writing a test. But since you are new to the project, you do a lot of exploratory “What happens if I execute this method” tests. You start by writing this: testCreditCardCharge() { CreditCard c = new CreditCard( "1234 5678 9012 3456", 5, 2008); c.charge(100); } This code: Only works when you run as part of the suite.When run in isolation, throws NullPointerException.When you get your credit card bill, you are out $100 for every time the test runs. Now, I want to focus on the last point. But why do I get NullPointerException in isolation while the test works fine when run as part of the suite? TestCreditCardCharge() { CreditCardProcessor.init(); CreditCard c = new CreditCard( "1234 5678 9012 3456", 5, 2008); c.charge(100); } You run the test again; still no success, and you get a different exception.
But there is more! Where Have All the Singletons Gone? By Miško Hevery In Singletons are Pathological Liars we discussed the problems of having singletons in your code. Let’s build on that and answer the question “If I don’t have singletons how do I ensure there is only one instance of X and how do I get X to all of the places it is needed?” An OO application is a graph of objects. There are three different kinds of graphs I think of when I design an application Collaborator Graph: This is the graph of objects that would be emitted if you serialized your application.
If the new operators are mixed with application logic (see: How to Think About the new Operator) then the Constructor Graph and the Collaborator Graph tend to be one and the same. The above shows the application collaborator graph. Now, in order to have a testable codebase we have to make sure that we don’t mix the object construction with application logic. In our tests it is now easy to instantiate the graph of objects and substitute test-doubles for our collaborators.
Root Cause of Singletons. Since I have gotten lots of love/hate mail on the Singletons are Pathological Liars and Where Have All the Singletons Gone I feel obliged to to do some root cause analysis. Lets get the definition right. There is Singleton the design pattern (Notice the capital “S” as in name of something) and there is a singleton as in one of something (notice the lower case “s”).
There is nothing wrong with having a single instance of a class, lots of reasons why you may want to do that. However, when I complain about the Singletons, I complain about the design pattern. Specifically: (1) private constructor and (2) global instance variable which refers to the singleton. I would say that at this point most developers recognize that global state is harmful to your application design.
Someone pointed out that any design pattern can be abused. Now, there is one kind of Singleton which is OK. The other kind of Singletons, which are semi-acceptable are those which don’t effect the execution of your code. How to Write 3v1L, Untestable Code. July 24th, 2008 · 15 Comments · by Miško Hevery, Jonathan Wolter, Russ Ruffer, Brad Cross, and lots of other test infected Googlers This guide lists principles that will help you write impossible to tests code. Or, avoiding these techniques will help you write code that can be tested.
Make Your Own Dependencies – Instantiate objects using new in the middle of methods, don’t pass the object in. This is evil because whenever you new up an object inside a block of code and then use it, anyone that wants to test that code is also forced to use that concrete object you new’ed up. Java Specific Final Methods – Use final classes and methods all the time. C++ Specific Use non-virtual methods – Unless you need to override the method in a deep and scary inheritance hierarchy, just make the method non-virtual. Tags: Advice · Rant · Testability. Dependency Injection – Why Bother? « Chris Collins. Dependencies by jonathanvlarocca In this post we discuss the various mechanisms of Inversion of Control (IoC) including Dependency Lookup and Dependency Injection. We look at the differences between the various IoC approaches and present the advantages and disadvantages of each.
Take the following example Java code. The Target class needs to collaborate with a Dependency class to do its work. What is wrong with this code? Firstly, we are creating a hard link between the Target class and the Dependency class. This is known as tight coupling. Lets say you want to unit test the Target instance in isolation. Loose Coupling Is there a way we can satisfy this dependency, without the tight coupling? Inversion of Control, or IoC, attempts to solve this issue by providing services by which the Target class can satisfy its dependencies without the tight coupling between them. Dependency Pull Dependency Pull is a type of Dependency Lookup. Contextualised Dependency Lookup Setter Dependency Injection. Google Guice. Contributions Appearing in the Book. C#, Objective-C and JavaScript Move Up in TIOBE Index - ReadWriteCloud. GambleAware is set to shut down in a phased closure following the introduction of new gambling legislation in the United Kingdom.
The charity has been in operation since 2002, working to raise awareness of gambling-related harm and to provide essential support to those in need. The organization confirmed the end of its tenure in an official statement, which will see it succeeded by the appointment of three new commissioners, as well as the introduction of a new statutory levy to fund their work. Commissioners will be tasked to carry out work and development on research into gambling harm, prevention, and treatment measures. All of this comes after the long-awaited gambling reform policy was presented in May. In its statement, the body indicated “the work historically delivered by GambleAware will now transition to the UK government and new commissioners across England, Scotland and Wales”, with the charity’s closure expected by March next year. Image credit: GambleAware.
[Infographic] PHP vs. Python vs. Ruby. Wildcards (The Java™ Tutorials > Bonus > Generics) Consider the problem of writing a routine that prints out all the elements in a collection. Here's how you might write it in an older version of the language (i.e., a pre-5.0 release): void printCollection(Collection c) { Iterator i = c.iterator(); for (k = 0; k < c.size(); k++) { System.out.println(i.next()); } } And here is a naive attempt at writing it using generics (and the new for loop syntax): The problem is that this new version is much less useful than the old one.
Whereas the old code could be called with any kind of collection as a parameter, the new code only takes Collection<Object>, which, as we've just demonstrated, is not a supertype of all kinds of collections! So what is the supertype of all kinds of collections? And now, we can call it with any type of collection. Collection<? Since we don't know what the element type of c stands for, we cannot add objects to it. On the other hand, given a List<? These classes can be drawn on a canvas: public void drawAll(List<?
List<? Google’s Dart Programming Language Disappoints Hackers. As expected, Google announced its new programming language Dart at GOTO Conf. We previously reported on the language here. Dart is a new programming language that will run in its own virtual machine in Chrome and compile to JavaScript in other browsers. The stated goals are: Create a structured yet flexible language for web programming.Make Dart feel familiar and natural to programmers and thus easy to learn.Ensure that Dart delivers high performance on all modern web browsers and environments ranging from small handheld devices to server-side execution. The essential purpose seems to be to create a language better suited for creating large, complicated Web applications such as Gmail. Since the two presenters at GOTO, Gilad Bracha and Lars Bak, both worked on Smalltalk related projects – Newspeak and Strongtalk- so many of us expected Dart to be Smalltalk like.
The overall sentiment in places like Hacker News is that of disappointment, but some developers did find something to like. Evolution of Computer Languages [Infographics] Things You Should Never Do, Part I. By Joel Spolsky Thursday, April 06, 2000 Netscape 6.0 is finally going into its first public beta. There never was a version 5.0. The last major release, version 4.0, was released almost three years ago. Three years is an awfully long time in the Internet world. It's a bit smarmy of me to criticize them for waiting so long between releases.
Well, yes. They decided to rewrite the code from scratch. Netscape wasn't the first company to make this mistake. We're programmers. There's a subtle reason that programmers always want to throw away the code and start over. It’s harder to read code than to write it. This is why code reuse is so hard. As a corollary of this axiom, you can ask almost any programmer today about the code they are working on. Why is it a mess? "Well," they say, "look at this function. The idea that new code is better than old is patently absurd. Back to that two page function. Each of these bugs took weeks of real-world usage before they were found. Is there an alternative?