Do Short, Random Field Names Really Speed Up Java? A Deep Dive

Background

The author encountered a viral claim that changing a field name from userName to usrNme boosted Java service latency by 47 ms. The story describes a late‑night refactor of a SpringBoot service, a cat walking on the keyboard, and a dramatic before‑and‑after latency drop from 127 ms to 80 ms (≈37 % improvement).

Benchmark Results

After rolling back the change, latency returned to the original level. The author ran 2 847 benchmark iterations, publishing a chart (see image) that supposedly supports the claim.

Original Claim

Longer strings make String.hashCode() slower.

Names that are similar increase hash collisions.

Heavy use of reflection causes many String.hashCode() calls, thus slowing the program.

The author of the original post concluded that “short, random field names make Java run faster.”

Technical Analysis

In reality, the cost of computing String.hashCode() is tiny—just a few nanoseconds per character. The JDK implementation is shown below:

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; 
}

Even with a 10‑character field name, the extra loop iterations add only a few CPU cycles, far too small to account for tens of milliseconds of latency.

Reflection does not rely on hash codes at all; it locates fields by iterating over class metadata and comparing names with equals(). Therefore, the “hash‑collision” argument is irrelevant.

Moreover, String.hashCode() results are cached after the first computation, eliminating repeated work.

Real Performance Bottlenecks

Typical sources of millisecond‑level latency in backend services include network latency, database queries, lock contention, garbage collection pauses, I/O delays, and thread scheduling. These factors can fluctuate by dozens of milliseconds, dwarfing any theoretical impact of field‑name length.

Conclusion

The claim that shorter or more random field names noticeably improve Java performance is unfounded. The observed latency changes were likely caused by unrelated factors, and the field‑name effect is negligible—essentially “a mosquito’s leg” compared to real performance concerns.