get java object size on your machine. execute this command on your folder and see what will hapen

javac ObjectSizeFetcher.java && jar cvfm test.jar manifest.mf ObjectSizeFetcher.class && java -javaagent:test.jar -cp test.jar ObjectSizeFetcher

on CentOS 64 server, java version:

java version "1.7.0_05"
Java(TM) SE Runtime Environment (build 1.7.0_05-b06)
Java HotSpot(TM) 64-Bit Server VM (build 23.1-b03, mixed mode)

And we can find some output info like this:

empyt hashmap size:48
empty hashset size:16
empty arraylist size:24
empty LinkedList size:32
empty String size:32
empty String[] size:16
String[1] size:24
int[0] size:16
int[1] size:24
int[2] size:24
int[3] size:32
int[4] size:32

So I find that the UseCompressedOops option is open by default on my server, relate post in Chinese