Size of objects in java

We all know that there is no operator or method to calculate size of an object in java. But can't really understand the reason for it, i mean if c++ can have it then why not java?
Only way to calculate appox. size of an objects might be to find out all the primitive data types forming the object and then add up their byte size.

If anybody knws a better a way or knws more size calculations then comments are welcome ...