Primitives, Objects and Autoboxing

The type system in Java makes a difference between so called „primitives“, which are boolean, byte, char, int, long, float and double and Objects, which are anything derived from Object in object oriented philosophy, including the special case of arrays, which I will not discuss today.

Primitive types have many operations that are kind of natural to perform on them, like arithmetic. They behave as values, so they are actually copied, which is no big deal, because they are at most 64 bits in size, which is in modern java implementations the size of a pointer when using references. Now a major benefit of object orientation is arguable the polymorphism and this has been heavily used when implementing useful libraries like the collection classes, which were based mostly on Object and thus able to handle anything derived from Object. This has not changed with generics, they are just another way of writing this and adding some compile time checks and casts in a more readable way, as long as the complexity of the generics constructions remains simple and under control. Actually I like this approach and find it much more healthy than templates in C++, but this is a IT-theological discussion that is not too relevant for this article.

Now there is a necessity of using collections for numeric types. Even though I do recommend to thoroughly think about using types like BigInteger and BigDecimal, there are absolutely legitimate uses of long, int, boolean, double, char and less frequently short, byte and float. The only one that is really flawless of these is boolean, while the floating point numbers, the fixed size integral numbers (also this) and the Strings and chars in Java have serious flaws, some of which I have discussed in the linked articles.

Now we need to use the wrapper types Integer, Long, Double and Boolean instead of int, long, double and boolean to store them in collections. This comes with some overhead, because these wrappers use some additional memory and the wrapping and unwrapping costs some time. Usually this does not impose a problem and using these wrappers is often an acceptable approach. Now we would be tempted to just work with the wrappers, but that is impossible, because the natural operations for the underlying boolean and numeric types just do not work with the wrappers, so we have to unwrap (or unbox) them.

Now Java includes a feature called „autoboxing and autounboxing“ which tries to create a wrapper object around a primitive when in an object context and which extracts the primitve when in a primitive context. This can be enforced by casting, to be sure.

There are some dangers in using this feature. The most interesting case is the „==“-operator. For objects and also for the wrappers of the primitives this always compares object identity based on the pointer address. For primitives that is simply impossible and the comparison compares the value. I think that it was a mistake to define the „==“-operator like that and it should do a semantic comparison and there should be something else for object identity, but that cannot be changed any more for Java. So we get some confusion when comparing boxed primitives with == or even worse when comparing boxed and unboxed primitives. Another confusion occurs, when using autounboxing and the wrapper object is null. This creates off course a NullPointerException, but it is kind of hard to spot where it actually comes from.

So I do see some value in using explicit boxing and unboxing to make things clearer. It is a good thing to talk about this in the team and find a common way. Now the interesting question is how boxing and unboxing are done. We are tempted to use something like this:

int x = ...;
Integer xObj = new Integer(x);

This works, but it is not good, because it creates too many objects. We can reuse them and java provides for this and reuses them for some small numbers. The recommended way for explicit boxing is this:

int x = ...
Integer xObj = Integer.valueOf(x);

This can reuse values. If we are using this a lot and know that our range of commonly used numbers is reasonably small but still beyond what Java assumes, it is not too hard to write something like „IntegerUtil“ and use it:

int x = ...;
Integer xObj = IntegerUtil.valueOf(x);

Look if you can find an implementation that fits your needs, instead of writing it. But it is no pain to write it.
Unboxing is also easy:

Integer xObj = ....;
int x = xObj.intValue();

The methods intValue(), longValue(), doubleValue(),… are actually in the base class Number, so it is possible to unbox and cast in one step with these.

Decide how much readability you want.

It is useful to look at the static methods of the wrapper classes even for converting numbers to Strings and Strings to numbers. Avoid using constructors, they are rarely necessary and some neat optimizations that the Java libraries give us for free only work when we use the right methods. This does not make a huge difference, but doing it right does not hurt, but rather makes code more readable.

It is also interesting how the extended numeric types like BigInteger and BigDecimal work similar to the wrapper types and to use them right.

Another interesting issue is to use actually specific collection implementations for primitives. This may add to the complexity of our code, because it gives up another piece of polymorphism, but they can really save our day by giving a better performance. And in cases where we actually know for sure that the data is always belonging to a certain primitive type, I find this even idiomatic.

Other languages have solved the issues discussed here in a more elegant way by avoiding this two sided world of primitives and wrappers or by making the conversions less dangeerous and more natural. They have operator overloading for numeric types and they use a more consistent concept of equality than Java.

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