What extraction cannot slim, and whether function injection can: a pre-registered study of 580 Java repositories

Daniel Sagenschneider

A method of 26 code lines or more reads as a Long Method, and Extract Method is the usual fix. Some long methods have no single-exit fix: a return cannot leave an extracted piece, a piece hands back one value, and some methods are knotted so that every cut is blocked. Function injection, as the OfficeFloor framework does it, cuts differently: each piece becomes a function the framework injects and sequences, an early return ends a function, and a function hands on several values. The framework's author pre-registered a test of whether it takes apart what extraction cannot. A stratified random sample of 580 open-source Java repositories, each pinned before AI coding assistants were in general use, was scanned by released tools whose cost models were fixed before the sample was drawn: 3,026,902 methods, 168,466 of them long. Single-exit extraction cannot slim 22.8% of the long methods. The registered hypothesis, that at least a quarter of those stuck methods are ones only function injection takes apart, is refuted: 22.2% (95% CI 20.3 to 24.3). Nearly half of the stuck methods are early-return methods the plan counts as conventionally fixable, and the split takes three in four of…