java-17

6 Posts

NullPointerException in switch: null selector and case null

A switch on a null String, enum, Integer or sealed type throws NullPointerException, and default won't catch it. Why, how 8 to 25 differ, and the fix.

InvocationTargetException with a null message: finding the real cause

Method.invoke wraps everything its target throws in an InvocationTargetException with no message. Here's how to see the real cause, on JDK 8 through 25.

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 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 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.