Java programmers are served with numerous choices of collections, varying from simple sequential ordered lists to sophisticated hashtable implementations. These choices are well-known to have different characteristics in terms of performance, scalability, and thread-safety, and most of them are well studied. This paper analyzes an additional dimension, energy efficiency. We conducted an empirical investigation of 16 collection implementations (13 thread-safe, 3 non-thread-safe) grouped under 3 commonly used forms of collections (lists, sets, and mappings). Using micro-and real world-benchmarks (Tomcat and Xalan), we show that our results are meaningful and impactful. In general, we observed that simple design decisions can greatly impact energy consumption. In particular, we found that using a newer hashtable version can yield a 2.19x energy savings in the micro-benchmarks and up to 17% in the real world-benchmarks, when compared to the old associative implementation. Also, we observed that different implementations of the same thread-safe collection can have widely different energy consumption behaviors. This variation also applies to the different operations that each collection implements, e.g, a collection implementation that performs traversals very efficiently can be more than an order of magnitude less efficient than another implementation of the same collection when it comes to insertions.

Analyzing the energy consumption of application level software is an emerging direction. This artifact makes available all the toolset and raw data needed to reproduce the main findings of our research paper. The artifact consists of: ● The source code of the micro-benchmarks analyzed, ● The source code of the case study used, ● The jRAPL tool, ● The raw energy data generated by the jRAPL tool with the source code of the experiments, ● The plotting scripts used to create the figures of the paper, based on the raw energy data.