ClassFormatError: Unsupported Major.Minor Version

Why This Happens

Each Java version produces class files with a specific major version number. Java 17 classes have version 61, Java 11 has 55, Java 8 has 52. Running a class compiled with a newer Java on an older JRE produces this error.

The Problem

# Compiled with Java 17
javac --release 17 MyClass.java
# Running with Java 11
java -cp . MyClass  # UnsupportedClassVersionError

The Fix

# Option 1: Run with Java 17
export JAVA_HOME=/path/to/java17
java -cp . MyClass

# Option 2: Compile for Java 11 compatibility
javac --release 11 MyClass.java

Step-by-Step Fix

1. 1

   Identify the version mismatch

   Read the error for the class version number. Java 8=52, 11=55, 17=61, 21=65.

2. 2

   Check your JRE version

   Run java -version to see what version you are running.

3. 3

   Upgrade JRE or recompile

   Either upgrade to a JRE that supports the class version, or recompile with --release flag targeting the older version.