Java Memory Calculation

Question

Dear all,
I have class
Student{
private int id;
And my program, init array 10000000 student object.
size = 10000000;
Student[] sts = new Student[size];
for(int i = 0; i< size; i++){
sts[i] = new Student();
Learning from
http://www.javamex.com/tutorials/memory/object_memory_usage.shtml
and
http://www.javamex.com/tutorials/memory/array_memory_usage.shtml
I calculation the memory usage for my programe is
- 8: house keeping
- 4 leng
- size * 4 (object reference)
- Each Student object size take (8 byte house keeping + 4 byte for id) = 12, padding 4 => 16 byte
So the total memory is 8 + 4 + 4*size + 16*size = 20*size + 12 ~ 190M
But the momory usage for my program is 277MB. That is the large difference.
Could you give me some advice?
Thank a lot for support.

Answer 1

Helo..
Using Windows Task manager is not the proper way to measure a Java programs Memory usage.
Reason:When you run a java program ,it also requires the underlying JRE to run ,so the memory used for a java process in this case would be the memory for the Java programs Objects and also for the entire JRE.
The Best way to measure your application's performance is use either of the two free tools:
1.JConsole-its comes along with JDK. JDK_HOME/bin/jconsole
2.Any Profiler Programs:A profiler for Java is a program which allows you to trace a running java program and see the memory and CPU consumption for the Java application.Netbeans comes with its own profiler.you can chk this out.
The other option is to measure free memory before and after execution of the program using System Util Class.
Hope this clarifies your qn.
Thanks.