Cargo Cult

Getting it Wrong by Wanting it Right

Patrick Bucher

2018-10-05

Accepting interfaces instead of requiring specific implementation is a good practice. Take Java collections as an example. A method accepting an ArrayList argument will only be useful if the caller is already using an ArrayList. Otherwise he’d need to convert the implementation he’s using, say, LinkedList, to an ArrayList first. A method accepting the List or even the Collection interface is far more likely to accept what the caller already has.

Many programmers recite this practice as “program on interfaces, not on implementations”. However, this rule is misleading, because the somewhat blurry notion of “programming on interfaces” does not mean the same as “accepting interfaces”.

Once in a while, a unexperienced Java programmer tries to follow the “program on interfaces” rule and stumbles upon a problem. He wrote:

List<String> names = new ArrayList<>();

Where’s the problem? The unexperienced Java programmer objects that he’s not “programming on an interface”, because he wrote new ArrayList<>() and hence has a reference to a specific implementation.

Maybe he’ll try to write a ListFactory (he’s a Java Programmer, after all) that provides a method with the signature List<T> createList() that will do the “dirty work” for him. But guess what: this method will contain the instantiation of ArrayList as well. The problem is not solved, but wrapped in an additional layer of pointlessnes.

What will the unexperienced Java programmer do now? He might search the issue on the web. Maybe he finds an article about dependency injection. But then he’ll be wondering where to get that dependency injected from. So he’ll either give up and stick to his original solution, or post the “issue” on a chat or forum.

What’s wrong with the original code? Nothing. Interfaces are for references, classes are for instances. At one point, somebody has to decide on an implementation. One could wonder, if the compiler or a framework might figure out the right implementation, optimized for performance according to what the programmer uses the collection for. A linked list is faster than an array list if items have to be inserted at the beginning. An array list is faster than a linked list for random access. But the compiler is unable to figure that out, at least at this point.

The issue is not only about performance, it’s also about semantics. Whereas the programmer doesn’t have to care about whether a list is implemented using a linked list or an array, semantics matter a lot to him in other cases. Take maps for example. The Java interface Map<K, V> has the implementations TreeMap<K, V> and LinkedHashMap<K, V>, among others. When adding an element to a TreeMap, the element will be inserted at a specific position, so that the map’s element will be sorted in their natural order. A LinkedHashMap, however, preserves the insertion order of the elements.

Of course, methods dealing with maps should still use the Map interface. It’s entirely up to the caller, if and how the elements of the map passed in are sorted. Maybe he doesn’t care, maybe he cares a lot. But he has to take the decision. Such decisions not only affect the runtime properties of a program (performance, memory consumption), but possibly also its semantics and therefore its results.

So why the misunderstanding of the unexperienced Java programmer that insisted on “programming on interfaces”? He’s guilty of Cargo Cult Programming. He heard the general rule to “accept interfaces instead of implementations” and understood it as “use interfaces instead of implementations” or even interpreted it as “replace class names with interface names” (because he only used the former in his code). He translated the rule “use A instead of B in context C” and forgot about the context. He just remembered the “A instead of B” part.

So how could the unexperienced programmer avoid such misunderstandings? Instead of literally following some rules he heard of, or following the part of the rule he understood, he should ask himself, if there’s an actual problem in his code. If he’s unable to explain that problem in a couple of sentences, he either doesn’t understand the problem―or maybe there is no problem at all.