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Java Exception: UnsupportedClassVersionError (Unsupported Major.Minor Version 51.0)

When attempting to run a Java program, you may encounter the following exception:

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

This error indicates that the Java Virtual Machine (JVM) is unable to recognize the class file format.

Root Cause:

The error occurs when the compiled class files target a newer version of Java than the JVM is capable of running. Typically, this happens when you have multiple versions of Java installed on your system and the default JVM is not configured to use the latest version.

Resolution:

To resolve this issue, you can either:

1. Set Java Version as Default:

Open the terminal or command prompt and type:

sudo update-alternatives --config java

This will list the available Java versions on your system. Select the latest version (e.g., "Java 7") by entering the corresponding number.

2. Compile with the Correct Java Version:

If you have installed multiple versions of Java, ensure that you compile your Java programs using the same version that you intend to run them with. Check the JAVA_HOME environment variable and make sure it points to the desired Java installation directory.

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