Can Renaming Java Fields Really Boost Performance? A Deep Dive into String.hashCode

A Medium post claimed that renaming the variable userName to usrNme cut API latency from 127 ms to 80 ms, a 37 % improvement. The author presented screenshots of the before‑and‑after code and reported running 2,847 benchmark iterations to obtain the result. Later it was clarified that the discussion actually concerned field names, not local variables.

Key takeaways from the original post:

Longer strings make String.hashCode() slower.

Names that are similar increase the chance of hash collisions.

Heavy use of reflection that repeatedly calls String.hashCode() can degrade performance.

The conclusion drawn was that “short, random field names make Java run faster.” The article treats this as a serious performance tip, but the author’s tone suggests a tongue‑in‑cheek approach.

Examining the actual String.hashCode() source reveals a simple algorithm: for each character it multiplies the current hash by 31 and adds the character code. Adding a few characters merely adds a few nanoseconds of work, far too small to explain tens of milliseconds of latency difference on modern CPUs.

public int hashCode() {
    int h = hash;
    if (h == 0 && value.length > 0) {
        char val[] = value;
        for (int i = 0; i < value.length; i++) {
            h = 31 * h + val[i];
        }
        hash = h;
    }
    return h;
}

Reflection does not rely on hash codes; it locates fields by iterating over class metadata and comparing names with equals(). Moreover, hashCode() results are cached after the first computation, so subsequent calls are virtually free.

Even assuming no caching, shaving a few characters reduces only a handful of multiplications, which cannot account for a 30‑plus‑millisecond gain. Real performance bottlenecks typically stem from network latency, database queries, lock contention, garbage collection, I/O, or thread scheduling—each of which can easily add dozens of milliseconds.

Thus, the impact of field‑name length or randomness is negligible, comparable to a “mosquito‑leg” effect. Ironically, the article’s author genuinely believes the claim, and many readers who saw the reposted version in China praised it as a valuable lesson.