Why Does My Java Project Throw an UnsupportedClassVersionError Despite Using the Same Java Version for Compilation and Execution?

Understanding the "java.lang.UnsupportedClassVersionError" Exception

When attempting to integrate an open-source library compiled from source, you may encounter the "java.lang.UnsupportedClassVersionError." This error typically suggests a mismatch between the version of the compiled code and the runtime environment.

However, in your case, you have verified that you are using the same version of Java for compilation (javac) and execution (java). This has led you to question the possibility of an internal issue within the class itself causing the exception.

Unveiling the Source of the Problem

Upon closer examination, it appears that the underlying cause of this error is not as you initially suspected. The root of the problem lies in the compilation settings within your Eclipse IDE.

Specifically, your project's Java compiler settings may be configured to generate bytecode for a more recent version of Java than your system's JRE can support. Even though you have only a Java 1.5 JRE installed, Eclipse's compiler allows you to compile for higher versions.

Resolving the Issue

To rectify this situation, navigate to the Project Properties for the affected project, then proceed to the Java Compiler section. Inspect the settings to ensure that the compilation level matches the version of your Java runtime environment.

In most cases, this entails unchecking any option that allows higher compilation levels. Once you have made the necessary adjustments, rebuild your project and attempt to run it again. This should resolve the "java.lang.UnsupportedClassVersionError" exception.

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