Java – Major .minor version 52.0 using ant is not supported

I’m trying to compile the ics-openconnect android app ( https://github.com/cernekee/ics-openconnect)。

I followed the “readme”. However, when I try to debug, I get an error.

Error:

-dex:
  [dex] input: /home/yj/ics-openconnect/bin/classes
  [dex] input: /home/yj/ics-openconnect/libs/openconnect-wrapper.jar
  [dex] input: /home/yj/ics-openconnect/libs/acra-4.5.0.jar
  [dex] input: /home/yj/ics-openconnect/libs/stoken-wrapper.jar
  [dex] Pre-Dexing /home/yj/ics-openconnect/libs/openconnect-wrapper.jar -> openconnect-wrapper-58de756051305e9ad3753d649df24013.jar
   [dx] Exception in thread "main" java.lang.UnsupportedClassVersionError: com/android/dx/command/Main : Unsupported major.minor version 52.0
   [dx]     at java.lang.ClassLoader.defineClass1(Native Method)
   [dx]     at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
   [dx]     at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
   [dx]     at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:141)
   [dx]     at java.net.URLClassLoader.defineClass(URLClassLoader.java:283)
   [dx]     at java.net.URLClassLoader.access$000(URLClassLoader.java:58)
   [dx]     at java.net.URLClassLoader$1.run(URLClassLoader.java:197)
   [dx]     at java.security.AccessController.doPrivileged(Native Method)
   [dx]     at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
   [dx]     at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
   [dx]     at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:301)
   [dx]     at java.lang.ClassLoader.loadClass(ClassLoader.java:247)
   [dx] Could not find the main class: com.android.dx.command.Main. Program will exit.

BUILD FAILED

Also, my java:

$java -version
java version "1.6.0_45"
Java(TM) SE Runtime Environment (build 1.6.0_45-b06)
Java HotSpot(TM) 64-Bit Server VM (build 20.45-b01, mixed mode)

javac:

$javac -version
javac 1.6.0_45

I know version 52.0 says “JDK 1.8”
But I only installed JDK 1.6

How do I fix it?