Java Comparator Bug Caused Intermittent Aircraft Stand Sorting Failures in Production

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An engineer reviving an abandoned flight-operations service discovered a Java IllegalArgumentException crashing aircraft stand sorting requests intermittently in production. The bug originated in a custom Comparator that violated Java's antisymmetry contract — when two parking-area codes both lacked numeric parts, the comparison returned 1 regardless of argument order, meaning compare('A','B') and compare('B','A') both returned positive. Java's TimSort algorithm, used in Arrays.sort since Java 7, detects such contract violations and throws an exception rather than silently return a misordered result, which is why the error only surfaced on certain real-world data sets and never in tests. The correct fix was to repair the comparator logic — not to use the legacy merge sort flag, which merely suppresses the warning without resolving the underlying flaw. The case illustrates that a missing exception is not proof of a working comparator, since a broken one may go undetected until a specific data pattern exposes it.

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