A programming idiom I often see in managed runtime environments, such as Java, is to return a collection or array from a method that is allocated within the method. Having originally come from a C/C++ language background this always struck me as a bit strange and lazy. In the C language world this idiom would likely be a source of memory leaks. I do however understand how convenient it can be in garbage collected environments. A good example is the split function on a String as below:
String str = “Some string that I may want to split”;
String[] values = str.split(“ ”);
For this a new array has to be allocated within the method. If the array is small it can be allocated in the young generation. Due to the round-robin nature of the young generation it will most likely involve a cache miss. The OS, in addition, must zero the memory for security before handing it over. What’s worse is the size of many arrays cannot be determined until the method completes, so temporary structures may have to be allocated before the final structure is allocated and copied into. Then after all this, the memory will eventually need to be garbage collected. This feels like the CPU is doing a lot of work to make this idiom work.
What is the alternative?
Another idiom is to let the caller allocate the structure and pass it into the method like in the example below:
String str = “Some string that I may want to split”;
Collection<String> values = new ArrayList<String>();
str.split(values, “ ”);
With this approach the caller has more control and flexibility. Not only can the caller reuse the collection across multiple calls, they can also choose the most appropriate structure for the end needs. For example, it could be a HashSet if finding distinct words, an ArrayList for cache friendly performance, or a LinkedList if memory is a scarce resource. The method can return the collection allowing a fluent coding pattern.
In the Java world this idiom becomes critical to performance when large arrays are involved. An array may be too large to be allocated in the young generation and therefore needs to be allocated in the old generation. Besides being less efficient and contended for large object allocation, the large array may cause a compaction of the old generation resulting in a multi-second stop-the-world pause. Now that is a good topic for another blog post...
I can hear folk cry out, “but short lived objects are cheap!” While this is true when compared to longer living objects, they are relatively cheap but certainly not free. I’d prefer to use those extra cycles solving the business problem and, in addition, have the flexibility to choose the most appropriate data structure for the task at hand.
