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Changes in the API/non-API method call ratio with program size

Amount of code is the fundamental metric of software engineering. How do things change as the amount of code changes and often just as interestingly what does not change with code size?

Most languages include some kind of base library functionality. Languages such as Java and C++ not only include a very large library but also a huge, widely used, collection of third-party libraries.

Let’s count every method call in lots of Java programs and for each program divide these calls into two groups, calls to methods in well-known libraries (call these the API methods) and all other method calls (i.e., calls to methods written by the developers who wrote each of the programs measured; call these the non-API methods).

I would expect the ratio of API to non-API method calls to be independent of program size.

Yes, the number of possible different API calls is fixed while the number of possible non-API calls increases with program size, but I don’t see why a changing ratio of unique calls should change the ratio of total calls.

Yes, larger programs are likely to contain more architectural stuff whose code is more likely to contain calls to non-API methods, but the percentage of architectural code is very small and unlikely to have much impact on the overall numbers.

The authors of the paper: Large-scale, AST-based API-usage analysis of open-source Java projects made their data available and so I got to check out my thinking :-)

The plot below shows everything going to plan until around 10,000 method calls (about 50,000 lines of code). Why that sudden kink in the line (code and data)?

API vs. non-API method calls

One possibility is that once a program gets to a size of around 50,000 lines the developers decide to invest in one or more wrapper packages which create a purpose built interface to an API (programs often have their own requirements and needs that existing an existing API interface does not quite meet); this would cause API calls to decrease and non-API calls to increase. If this pattern of usage occurred there would be a permanent change in the API/non-API ratio, and in practice the ratio change appears to be temporary.

I’m a bit stumped by this behavior. Suggestions on possible mechanisms welcome.

I wish I had the time to investigate, but I have a book to finish.

  1. jakob
    October 16th, 2016 at 06:57 | #1

    Could it be because a program of a certain scale has to be about solving a more difficult problem? Until 50kloc you have some kind of “simple” programs that do faily generic stuff, while beyond that it gets into really interesting internals? I.e., as you kind of allude to, that the amount of internal code increases.

    On the other hand, since you mix both the OS API and basic datastructures and formatting into one “API”, it is a bit strange. Maybe programs at this level starts to have their own data structures instead of just using the basic ones in the library?

    I think you should also look at the ratio between libraries that are part of the language/operate internally to the program and those that involve IO to the underlying machine and OS.

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