Fixing eight common Java anti-patterns reduced an order-processing app's elapsed time from 1,198ms to 239ms, throughput from 85,000 to 419,000 orders per second, heap from 1GB to 139MB, and GC pauses from 19 to 4.

The patterns include string concatenation in loops causing O(n²) copying, O(n²) stream iteration inside loops, String.format() in hot paths, autoboxing creating millions of wrapper objects, exceptions for control flow, too-broad synchronization, recreating reusable objects like ObjectMapper, and virtual thread pinning. Each includes benchmarks and code fixes like using StringBuilder, single-pass loops, and primitives.