How to Determine the JDK Version Used to Compile a .class File in Java

How can I check the JDK version that compiled a .class file?

javap -verbose MyClass.class

When dealing with Java development, it's common to encounter issues related to version incompatibility. This often arises when a .class file is compiled with a newer version of the JDK than the version of the Java Runtime Environment (JRE) executing it. In this guide, we will outline how to check the JDK version used to compile a .class file, specifically using the `javap` tool.

javap -verbose MyClass.class // Replace MyClass.class with the actual class file name

Causes

• The .class file was compiled with a different JDK than the one used to run it.
• The project settings might not correctly specify the compatibility level, leading to unexpected compile-time behavior.

Solutions

• Use the `javap -verbose` command to check the 'major version' of the .class file.
• Ensure your Eclipse or IDE configuration matches the JDK version used for compiling.

Mistake: Not using the correct `javap` command to display the .class file version.

Solution: Always include the `-verbose` flag to get complete information.

Mistake: Assuming the JRE version can execute a .class file compiled with a newer JDK without compatibility settings.

Solution: Set your Eclipse project compatibility or use the `-source` and `-target` options during compilation.