ArbitraryObject: A useful useless class

23 10 2009

I’ve recently introduced a trivial little class called “ArbitraryObject” into my Java test-case code. Here’s the full story…

When writing test cases in Java, every now and then one comes across a situation where an object is needed but its precise type doesn’t matter, and you just need to pick some arbitrary class to use as an example.

Sometimes any class at all will do; sometimes there are constraints on what the class must not be, but anything else will do (e.g. anything that isn’t class X or one of its subclasses or superclasses).

Most commonly this happens for tests of methods that take “Object” arguments – an obvious example is testing that an implementation of “equals(Object)” returns false for any argument that isn’t of the appropriate type.

Another common case is testing of generic classes and methods with “any class” generic parameters, where one needs to pick a class to be used as the generic parameter’s actual type.

In these situations, what class should one use?

Perhaps the simplest choices are Object or String. However, Object seems a poor choice for this in general – if you’re testing something that takes any Object, you probably want to test it with something more specific than Object itself (even if you do also want to test it with a basic Object). It’s also not going to work where you need something that isn’t inheritance-related to some particular class.

Similarly, although String can be very convenient for this, strings are so common as argument values and in test-case code that their use tends to blend into the background. So it’s hard to see when a string is being used for this purpose as opposed to being a “real” use of a string.

More generally, if you’re trying to show how some code handles any arbitrary type, neither Object nor String seem the most useful or convincing examples to pick.

What we’re really looking for is a class that meets the following criteria:

  • It shouldn’t be relevant in any way to the class being tested (isn’t the class being tested, doesn’t appear as a method argument anywhere in the class being tested, and isn’t a superclass or subclass of such types);
  • It shouldn’t be used otherwise in the test-case code (so as to avoid any confusion);
  • Ideally it ought to be somewhat out-of-the-ordinary (so that we can reasonably assume that the code being tested doesn’t give it any special treatment, and so that its use in the test-case code stands out as something unusual, and so as to emphasise that it’s just an arbitrary example representing any class you might happen to use);
  • It should be easy to construct instances of the class (it should have a public constructor that doesn’t require any non-trivial arguments or other set-up or configuration);
  • There shouldn’t be any significant side-effects or performance impact from creating and disposing of instances and using their Object methods such as equals/hashCode/toString (e.g. these shouldn’t do anything like thread creation, accessing of system or network resources etc).

Until now I’ve been picking classes for this fairly arbitrarily. Sometimes I just grab one of the primitive-wrapper classes like java.lang.Float or perhaps java.math.BigInteger if these aren’t otherwise involved in the code – even though they’re rather too widely used to be ideal for this. Otherwise I’ve picked something obscure but harmless from deep within the bowels of the JDK, such as

The problems with this approach are:

  • The intention and reason for using the chosen class aren’t obvious from the code;
  • The test-case ends up with an otherwise-unnecessary and rather misleading “import” and dependency on the chosen class (unless it’s a java.lang class, but the most suitable of those suffer the drawback of being too widely used);
  • Any searches for the chosen class will find these uses of it as well as its “genuine” uses;
  • There’s no easy way to find everywhere that this has been done (for example, if I ever want to change how I handle these situations).

So instead I’ve now started using a purpose-built “ArbitraryObject” class.

The only purpose of this class is to provide a suitably-named class that isn’t related to any other classes, isn’t otherwise relevant to either the test-case code or the code being tested, and isn’t used for any other purpose.

The main benefit is that this makes the intention of the test-case entirely explicit. Wherever ArbitraryObject is used, it’s clear that it represents the use of any class, at a point where a test needs this. In addition, the test-case code no longer has any dependencies on obscure classes that aren’t actually relevant; it’s easy to find all the places where this is being done; and searches for other classes aren’t going to find any “accidental” appearances of a class where it’s been used for this purpose.

ArbitraryObject must be the most trivial class I’ve ever written. Not even worth showing the code! It’s just a subclass of Object with a public no-argument constructor and nothing else.

Potentially one could argue for additional features, such as giving each instance a “name” to be shown in its “toString” representation, making it Serializable, and so forth. But none of that seems worth bothering with.

So this ArbitraryObject class is entirely trivial, and as a class it’s kind of useless, but the name in itself is useful to me.

Sometimes all you need is an explicit name.



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