Upgrade JAVA

At Linux box:

sonnyyu@debian75386240:~/test$ java -version
java version "1.7.0_75"
OpenJDK Runtime Environment (IcedTea 2.5.4) (7u75-2.5.4-1~deb7u1)
OpenJDK Server VM (build 24.75-b04, mixed mode)
nano HelloWorld.java
class HelloWorld
{
   public static void main(String args[])
   {
      System.out.println("Hello World");
   }
}

Compile HelloWorld.class file:

javac HelloWorld.java

Copy HelloWorld.class into Openwrt Box:

root@Arduino:/mnt/sda1# jamvm HelloWorld
Hello World