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This exception occurs when using an older version of java. Mainly when trying to run a jar compiled with a newer Java version in an older Java version. We can avoid this issue by executing the jar in the same version which it was compiled.
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java.lang.UnsupportedClassVersionError: HashMapTest (Unsupported major.minor version 49.0)
at java.lang.ClassLoader.defineClass0(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)