java-21 - Gopi Gorantala

Java NullPointerException: Cannot invoke ... because ... is null

Decode the helpful NullPointerException message — Cannot invoke "String.length()" because "s" is null — find the exact null reference, and fix it for good.

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.

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.