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Gopi Gorantala

I'm Gopi — 15+ years in Java, building Kafka and Flink platforms for banks, where one lost event is a financial discrepancy. I write javahandbook.com because the guides I needed didn't exist. Everything here is tested against a real cluster first.

Java ConcurrentModificationException: Causes and Fixes

ConcurrentModificationException usually has nothing to do with threads. Here is exactly what modCount does, why it fires, and the four fixes that actually work.

Comparison Method Violates Its General Contract in Java

IllegalArgumentException: Comparison method violates its general contract! comes from TimSort, not your code. Here is why it fires, and how to fix the comparator.

Java InaccessibleObjectException: Causes and --add-opens Fix

InaccessibleObjectException: module java.base does not "opens java.util" to unnamed module. Why strong encapsulation breaks reflection and how to fix it properly.

Java IllegalStateException: Duplicate Key in Collectors.toMap

Collectors.toMap throws IllegalStateException: Duplicate key when two elements map to the same key. Exact causes, JDK 8-25 message differences, and the fixes.

Java UnsupportedClassVersionError: Class File Version F

java.lang.UnsupportedClassVersionError means your bytecode is newer than your JVM. Decode the class file version numbers and fix the mismatch for good.

Java OutOfMemoryError: Java Heap Space - Causes and Fixes

java.lang.OutOfMemoryError: Java heap space and GC overhead limit exceeded, explained: reproduce both, read the GC log, find the leak, and fix it properly.

Flink Heartbeat of TaskManager Timed Out: Causes and Fixes

Fix the Flink 'Heartbeat of TaskManager with id ... timed out' error. Real causes — GC pauses, OOMKilled pods, network saturation — and the exact config fixes.

Kafka TransactionalIdAuthorizationException: Fix the ACL

TransactionalIdAuthorizationException: Transactional Id authorization failed. Why your EOS/Spring transactional producer dies on a secured cluster and the exact ACLs that fix it.

Kafka CorruptRecordException: Record Is Corrupt — Fix It

Kafka CorruptRecordException: Record is corrupt (stored crc != computed crc). Why the CRC check fails, how to recover the partition, and how to stop it recurring.

Kafka's UNKNOWN_PRODUCER_ID and OutOfOrderSequenceException: Why Your Idempotent Producer Fails After a Restart

A deep dive into why idempotent Kafka producers throw UNKNOWN_PRODUCER_ID and OutOfOrderSequenceException in production, how broker-side producer state expiry causes it, and how to fix it with KIP-360 semantics and proper client configuration.