title: what is java.lang.UnsupportedClassVersionError?
body: <p>what is <code>java.lang.UnsupportedClassVersionError</code> ?</p>
  
  <p>I read from the <a href="http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/UnsupportedClassVersionError.html" rel="noreferrer">doc</a> :<em>Thrown when the Java Virtual Machine attempts to read a class file and determines that the major and minor version numbers in the file are not supported</em> , but couldn't understand what does that mean. </p>
  
  <p>What are the <em>major and minor versions</em> in the file ?</p>
  
  <p>I launched an application in someone else machine when the following exception was thrown :</p>
  
  <pre><code>Exception in thread "main" java.lang.UnsupportedClassVersionError: client (Unsupported major.minor version 51.0)
     at java.lang.ClassLoader.defineClass0(Native Method)
     at java.lang.ClassLoader.defineClass(ClassLoader.java:539)
     at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:123)
     at java.net.URLClassLoader.defineClass(URLClassLoader.java:251)
     at java.net.URLClassLoader.access$100(URLClassLoader.java:55)
     at java.net.URLClassLoader$1.run(URLClassLoader.java:194)
     at java.security.AccessController.doPrivileged(Native Method)
     at java.net.URLClassLoader.findClass(URLClassLoader.java:187)
     at java.lang.ClassLoader.loadClass(ClassLoader.java:289)
     at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:274)
     at java.lang.ClassLoader.loadClass(ClassLoader.java:235)
     at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:302)
  </code></pre>
  
  <p>though it worked fine on my machine.</p>