Back to basics: Classes

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Let’s talk about classes! Classes in Java are super weird (okay they’re weird in any language), and I really struggled with them when I was learning Java, so don’t feel bad if classes/objects are hard for you.

When you write simple code where everything is in main and runs immediately, it’s way easier. With classes you can’t just write it and call it, you have to create an instance first. What’s even going on there?

A class in Java is just a blueprint, it doesn’t do anything until you create an object using the blueprint (there are exceptions but let’s go with that for now). That’s a big part of the weirdness, you write all this code and it just kinda sits there until you write even more code, and here’s the kicker: that code has to be somewhere else.

In a small project your probably have a Main class somewhere that has a main() method, that’s where you need to call your other class from. Why? Because Java says so, that’s why :) Or more precisely, when you run a Java project you have to tell the JVM where to start. Something, somewhere, has to have a main method no matter how you run your project. Even if you create an archive that you drop into a servlet container like Tomcat or Jetty, that servlet container has a main method somewhere that gets called when you start it up. It may be very stealthily called by a script that calls a script that starts a service, but there’s definitely a main method in there somewhere. No matter how complicated an application seems, it’s really just a much larger version of the tiny little projects you do in school. Programs get larger and larger but the basic principles always stay the same.

Back at basic principles, you create actual objects you can interact with using the class/blueprint. In Java you do that with the new keyword. new YourClassName() creates an object using the blueprint, you can assign that to a variable like any other value and then do what you like with it. Once you create an object it’s separate from any other object you create even if you use the same blueprint (class) again. Again, there are exceptions, but I’m trying to keep this simple.

It’s like building two houses from the same set of plans, they’re separate houses even if they start out the same. You can put completely different furniture in each of them, paint them different colours, make a bedroom into a home office, but whatever you do to one house doesn’t affect the other one. And you can put objects inside other objects, like a house object could contain a fridge object or a couch object or a CNC router object if you have an especially interesting garage.

You could also have an array of house objects – pretend you’re keeping track of one of those subdivisions where all the houses are the same :) Those houses could have a method like closeWindows (pretend it’s an automated house), so you could loop through your array of suspiciously similar houses and close all the windows if the weather report said it was likely to rain. But you would need to call the closeWindows method on every single house, if you close the windows of one house it doesn’t affect the others.

This gets much more complicated when you start thinking about multithreading, static methods, and static fields, but those are another blog post. For now all I want to get at is that Java needs you to spell out exactly where to start and that programs all work pretty much the same way no matter how complicated they are. Frameworks like Spring or Struts and containers like Tomcat and Jetty will hide details from you, but no matter how many other programs or libraries you use, the computer always needs to know where to start.

Code smell: feature envy

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Today’s code smell is feature envy. Feature envy is when one class uses the methods of another class to excess. It’s called feature envy because classes that use methods from another class excessively look envious of that class’s features.

The problem with that, as usual, is the way your logic gets spread around between multiple classes. If you have one method that’s using another class so much that it’s hardly even separate from it, you’ll probably have to change things in both places when you need to fix a bug or add a new feature, but the compiler can’t tell you that you need to do that so you risk creating a shiny new bug by missing a spot.

Feature envy probably makes more sense with an example. Let’s say you have an address class and a customer class, and when you call customer.getMailingLabel() your customer class calls address.getStreetNumber(), address.getStreetName(), address.getCity() and so on to build a complete address string for printing a mailing label. Let’s imagine that policy changed and you’re now able to ship to rural mailboxes, so you need to add route and box numbers to your address and to use those instead of street number and street name if they’re present. If your code has feature envy, you have to change both the address object and the getMailingLabel method to support rural mailboxes. If you moved the getMailingLabel method into the address class, then you would only have to change one class when you wanted to change how you build mailing labels.

As with just about all code smells, feature envy isn’t always a sign there’s a problem. Sometimes you want to separate your data from operations on it, especially if you’re using a strategy or visitor design pattern. If, for example, you want addresses formatted differently for different reports, the formatting is probably more the job of the reporting code than the address object. In that case it would make more sense for a visitor to contain the formatting code so the address object doesn’t have to care about all the different reports.

Code smell: switch statements

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Today’s code smell is switch statements. Long switch statements or complicated if statements often (but not always!) mean that something has gone wrong with your design. Objected oriented languages give you a lot of tools to avoid big complicated switches, if you aren’t using one of those there needs to be a good reason for it. If you’re not using an object oriented language, on the other hand, I’m honestly not sure if switch statements are a sign of a potential issue. If any of my readers could weigh in on that, that would be really helpful.

But why are switch statements a problem? Because you’re spreading around logic that should probably be in one place, and if you add a value to one switch statement you’re likely to need to hunt down all the other ones and change them too.

If, for example, you have a switch statement that does different calculations based on the type of the object passed in, that logic for how to do the calculation probably belongs inside the object passed in to the switch. If you change that object, you’ll have to hunt down every place it’s used and make sure nothing else needs to be changed, which is obviously asking for trouble :) If you give each object a doCalculation method and replace the switch statement with a simple call to doCalculation, then all of the logic for that object stays inside the object. With all your logic in one place, you don’t have to search for every thing you need to change and risk missing a spot, and your design just makes more sense when logic for one thing isn’t smeared across multiple objects.

Polymorphism isn’t the only way to fix a switchy smell, if your method switches on a parameter that’s passed in, you may want to change it to separate named methods to more directly do the thing. If you have a setProperty(String name, String value) method, you could replace it with named methods like setType, setModel, setYear, etc. You might end up with a lot of little methods, but it’s still clearer than one setProperty method with a giant switch. If you end up with a truly excessive number of methods, that’s another code smell that points to your object doing too much.

Just because switch statements are sometimes a problem doesn’t mean they always are. Sometimes what you’re doing is so simple that an interface and a set of objects that implement it (or changes to your existing interface and objects) makes your code more complicated rather than less. And sometimes there just isn’t a better way to do it: factory methods and abstract factories rely on switches or a series of ifs to figure out which implementation to return.

Lastpass tip of the day

LastPass tries to be helpful by filling in form fields for you, but sometimes it gets it wrong. If you see it fill in the wrong data or if you just mysteriously struggle to sign into services that use 2FA (for me LastPass was helpfully inserting an old 2FA code when I tried to sign into AWS, which meant AWS thought my authenticator app was wildly out of sync and would make me resync it every time I tried to log in), follow these steps and see if it helps.

In case that link ever stops working, you’ve got to find the problem site in your vault, click the little wrench icon to edit that site, then click the little wrench icon again in the edit popup to edit form fields, then delete all the fields with bad values. There may be quite a few of these.

Credit where it’s due, one of my coworkers told me about this. Thanks Logan, I don’t know if I ever would have found that on my own!

JUnit tip of the day

Fun fact about JUnit tests: if something throws an exception that prevents your test from completing normally, it can’t clean up after itself. Normally this isn’t a big deal but if, for example, your setup method adds any test data to your database or creates a whole new database, you’re going to need to clean that up manually. Turns out extra databases eat up a lot of hard drive space if you don’t tidy them up for, uh, months. Just fyi :)

Link of the day

Conveniently enough, I’m not the only blogger on a “be a better programmer while still having a life” kick. If you’re interested in more tips about becoming a better programmer, check out Itamar Turner-Trauring‘s excellent post about learning more tools and techniques while you’re at work.

While you’re at it you should check out Itamar’s Software Clown newsletter, it’s full of great mistakes you can learn from and maybe even avoid running into yourself :)