Java Is Fast. Your Code Might Not Be

A Java order-processing app was optimized from 1,198ms to 239ms (5x throughput, 87% less heap, 79% fewer GC pauses) by fixing eight common anti-patterns including string concatenation in loops, O(n²) stream iteration, autoboxing, exceptions for control flow, and virtual thread pinning. These patterns compile and pass code review but cause significant performance degradation that profiling tools like Java Flight Recording can reveal.