Object size in Java

Suppose I have:

Class A{
   int a;
}

A obj = new A();

Then what is the size of obj? Is it the same size as int, just like in C?

If I can figure this out, I can keep large hashmaps in ram without using a database

Thank you in advance

edit

Friends,

In fact, I have:

HashMap<Long,List<T>> map;

and

class T{
   private int a;
   private int b;
   private int c;
   // constructor,getters and setters
}

And the size of the map may increase to 10000000 keys. For each key, I will have a list of sizes of 100-1000

Will the whole map stay in the pile?

Edit 2

When I load a map with about 70000 keys, when I serialize it into a file, the file is about 18 MB, so is it 18 MB in my map heap?

Solution

It may depend on the JVM, but in fact, for a 32 - bit JVM, the object = 8 - byte header field The title contains an ID reference to the class

The field size depends on the field type, reference – 8 bytes, Boolean – 1 byte, etc

Except that the object is aligned to 8 bytes That's your A. It takes 16 bytes Minimum size = 8 bytes, no fields

More here http://www.javamex.com/tutorials/memory/object_memory_usage.shtml

If you are using an Oracle JVM, you can use sun misc. Unsafe investigates the object structure byte by byte