Java \"UnsupportedClassVersionError\": Why Can\'t My Program Run with JDK 7?

Troubleshooting "Exception in thread "main" java.lang.UnsupportedClassVersionError: a (Unsupported major.minor version 51.0)"

Question: I've installed JDK7, but when I run a simple Java program, I receive an "UnsupportedClassVersionError". How can I resolve this?

Answer:

This error indicates a mismatch between the Java version your program was compiled for and the version you're attempting to run it with. To resolve this:

1. Check Your Java Version: Verify that you have Java 7 installed by running java -version from the command line. Ensure it shows "java version 1.7".

2. Set Java Version: If multiple Java versions are installed, you may need to explicitly set which version to use by editing the $JAVA_HOME environment variable or using sudo update-alternatives --config java to select the desired version.

   • For example, run sudo update-alternatives --config java and select the path to your Java 7 installation.
3. Recompile Your Program: Once you've set the Java version, recompile your program using javac.
4. Run Your Program: Attempt to run your program again. If the issue persists, check for any other possible causes, such as incorrect classpath settings or library conflicts.

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