Exception in thread "main" java.lang.UnsupportedClassVersionError: AdapterTest : Unsupported major.minor version 51.0

Exception in thread "main" java.lang.UnsupportedClassVersionError: AdapterTest : Unsupported major.minor version 51.0

Description and Solving:

First check the javac and java version:

rizvi@rizvi-pc:~/Desktop/Adapter$ javac -version

javac 1.7.0_65

rizvi@rizvi-pc:~/Desktop/Adapter$ java -version

java version "1.6.0_32"

Java(TM) SE Runtime Environment (build 1.6.0_32-b05)

Java HotSpot(TM) 64-Bit Server VM (build 20.7-b02, mixed mode)

rizvi@rizvi-pc:~/Desktop/Adapter$

So, a higher JDK version  is used to compile the source file and  a lower JDK version is used to run the program.

But its important to note is that vice-versa is not true "you can compile your program in J2SE 1.4 and run on J2SE 1.5 and you will not get any UnSupportedClassVersionError". When a higher JDK is used for compilation it creates class file with higher version and when a lower JDK is used to run the program it found that higher version of class file not supported at JVM level and results in java.lang.UnsupportedClassVersionError.

Read more: http://javarevisited.blogspot.com/2011/07/javalangunsupportedclassversionerror.html#ixzz3JOMYEkuF

First solution:

The version number "major.minor version 51.0" show which Java was used to run eclipse.

51.0
= Java J2SE 7

To fix your problem you should try to run eclise with Java 7. This can be done by updating your Java configuration:

sudo update-alternatives --config java

Then select a Java 7 version and retry.
If Java 7 isn't installed you can get it with apt-get:

sudo apt-get install default-jdk

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Second Solution:

http://crunchify.com/exception-in-thread-main-java-lang-unsupportedclassversionerror-comcrunchifymain-unsupported-major-minor-version-51-0/

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Third Solution: 

Java Cross-compilation:

rizvi@rizvi-pc:~/Desktop/Adapter$ javac -target 1.6 -source 1.6 -bootclasspath /usr/local/java/jdk1.6.0_32/jre/lib/rt.jar *.java

Here,
target and source must be same.

rizvi@rizvi-pc:~/Desktop/Adapter$ java AdapterTest;

Area
= 100

rizvi@rizvi-pc:~/Desktop/Adapter$

http://stackoverflow.com/questions/18320587/javac-cross-compilation-with-1-7