how to test the execution speed of code in android?

I want to test my code execution speed, my code :

public static String foo () {

     final long startTime = System.currentTimeMillis ();
     try {
         // my code here
         long final endtime = System.currentTimeMillis ();
         System.out.println ("Total execution time:" + (endtime - startTime));
         return msg;
     } Catch (GeneralSecurityException e) {
         return "ERROR";
     }
 }

but after execute this code multiple times (repeat test), I find the difference between total execution of same code. I assume it is because the systems loaded with background process. Is there a better way to do this?