core-java - Gopi Gorantala

NoClassDefFoundError vs ClassNotFoundException in Java

NoClassDefFoundError and ClassNotFoundException look interchangeable and are not. Read the frames, find the real cause, and stop the repeat throws.

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.

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.