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In reference to : Calculate size of Object in Java

I tried to run this program and get the size of the object created

  //main 
 while(true){
        Integer x=new Integer(50);
        System.out.println("HashCode of this object="+x.hashCode()+" 
                  ,  with Size of "+ObjectSizeFetcher.getObjectSize(x));
 The output -- NullPointerException is thrown when getObjectSize() is invoked.
   new Integer() -->
should have created a new wrapper object. Instead it creates NPE. Why ?
java version "1.6.0_65"
Apple OSX - 10.9.1 
--Ignore Syntax errors
                That ObjectSizeFetcher requires that it be run as an agent. See docs.oracle.com/javase/6/docs/api/java/lang/instrument/… for details.
– SamYonnou
                Jan 22, 2014 at 14:52
Because the static field ObjectSizeFetcher.instrumentation was null when you called ObjectSizeFetcher.getObjectSize(x).
You should call method ObjectSizeFetcher.premain(String args, Instrumentation inst) before, with a non-null inst argument.
        
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